A 3rd ANCA Subtype: LAMP-2

We all know about the p-ANCA and the c-ANCA, which are antibodies against either myeloperoxidase (MPO) or proteinase-3 (PR3), respectively.  Now get ready for a 3rd ANCA subtype, an antibody against LAMP-2!

In a recent Nature Medicine article by Kain et al, the investigators show that in most patients with the disease focal necrotizing glomerulonephritis FNGN)--a severe form of pauci-immune glomerulonephritis with positive ANCA titers--there are antibodies against an epitope of human LAMP-2.  Evidence for its pathogenicity is convincing:  when either the patient's antibody OR a designed monoclonal antibody raised against LAMP-2 is injected into rats, they develop glomerulonephritis.

Interestingly, the LAMP-2 antigen is highly homologous to a bacterial antigen termed FimH.  The authors' hypothesis is that patients who develop bacterial infection with an organism containing FimH develop antibodies against this antigen which are also cross-reactive with LAMP-2, expressed on neutrophils.  This is another example of "molecular mimicry" which may also be found in conditions such as rheumatic heart disease.  

Urinary Exosomes

Did you know that there are tiny vesicles in your pee?

A 2004 PNAS article by Pisitkun et al demonstrates that this is the case; the vesicles are secreted by tubular epithelial cells at the apical surface and contain numerous membrane proteins, such as (for example) aquaporin. A significant percentage of the proteinuria detected in dipstick analysis is found in these exosomes.

Urinary exosomes are of interest for several reasons, with one potential application being the ability to generate an enriched population of exosomes from human urine, which is being used for the identification of new biomarkers for acute kidney injury.

Proliferation Requirement for Cystogenesis

I'm suffering from information overload after my first day at the 2008 ASN in Philadelphia, PA. I attended a series of great talks and have a whole list of possible blog topics in my notebook. For now, I'll briefly discuss one of the central themes of the polycystic kidney disease session; namely, that cyst development occurs in the context of cell proliferation.

To back up a bit: PKD1 and PKD 2 are the two main disease genes for autosomal dominant polycystic kidney disease. Complete knockout of PKD1 and PKD2 in mice results in massive cystic dilatation at birth. However, we know that human patients with this disease do not usually develop large cyst growth and advanced CKD often into their 5th decades. In order to better duplicate this physiology, researchers have developed "conditional knockout" mice in which the expression of Pkd1 can be switched OFF at various ages of mouse life by injecting it with tamoxifen. It turns out that turning off PKD1 after several months of life leads to much more delayed cyst growth, leading investigators to hypothesize that the reason cyst growth occurs so vigorously in younger kidneys is because the nephrons are still actively proliferating and are therefore "primed" to form cysts. Interestingly, subjecting conditional knockout mouse kidney to hypoxia/ischemia leads once again to more rapid cystogenesis, providing further evidence for the idea that cystogenesis is favored in an environment of rapid proliferation.

Tacrolimus Drug Interactions

I'm reporting LIVE from the American Society of Nephrology convention in Philadelphia, PA. I'll be blogging from here as time & internet connectivity permit. The conference starts in earnest tomorrow morning.

Until then: common drug interactions for TACROLIMUS (Prograf):

1. Drugs which lower tacrolimus levels:
-anti-TB drugs (e.g., rifampin, rifabutin)
-anti-convulsants (e.g., barbituates, phenytoin)
-antibiotics (e.g., nafcillin, ciprofloxacin, imipenem, certain cephalosporins)
-other (e.g., St. John's wort, antacids, ticlodipine, octreotide)

2. Drugs which raise tacrolimus levels:
-non-dihydropyridine calcium channel blockers (e.g., diltiazem, verapamil)
-antifungals (all azole derivatives)
-macrolide antibiotics (especially clarithromycin, erythromycin)
-protease inhibitors (for HIV)
-grapefruit juice

Also, the combination of either statins plus tacrolimus or fibrates + tacrolimus can cause an increased risk of rhabdomyolysis, so watch out for this. There is also an increased risk for hyperkalemia with the combination of ACE or ARB + tacrolimus.

Waiting Times for Potential Kidney Transplant Recipients With History of Cancer

I saw a patient in the renal transplant clinic today for evaluation to get onto the kidney transplant list, as he is just around the corner to starting dialysis (actually, he may actually start tonight since his routine labs came back with a K of 6.9...). 

His chronic kidney disease was thought secondary to (a) long-standing hypertension, and (b) a history of bilateral partial nephrectomies for a history of bilateral renal cell carcinomas which were diagnosed about 4 years ago.  

Given this patient's history of cancer and the possibility of recurrence, should this patient be recommended to go onto the kidney transplant list?  How long does he need to wait?  Could the administration of immunosuppresant medications really result in decreased immune surveillance of cancer cells which could potentially result in earlier recurrence of metastatic disease?

  

These are all good questions, and there seems to be a relative lack of data.  According to one seasoned renal transplant doctor I spoke with today, if there is any increased likelihood of cancer in these patients (other than skin cancers or PTLD), it is likely small and not a reason to postpone transplant; the main reason in his mind for waiting to ensure a lack of cancer recurrence is to minimize the likelihood that the patient would need to undergo toxic chemotherapy or a major surgery which could interfere with the graft function.  In addition, there is also the issue of wanting to preserve a scarce resource (donated kidneys) for patients who will benefit from them the longest.  

Nonetheless, the American Transplant Society (ATS) has come up with a list of guidelines for "waiting times" after being treated for various cancers before one should be considered for kidney transplant.  To generalize:  breast, colorectal and melanoma cancers have a recommended waiting time of 5 years unless they are early stage; most other cancers (e.g., renal cell cancer, leukemia, lung cancer, prostate cancer, etc) have a waiting time of only 2 years.  Non-melanoma skin cancers (basal cell, squamous cell) are generally not a reason for delaying kidney transplant as they are generally treatable.  

Middle Molecules

The term "Middle Molecules" is a little confusing, in part due to its evolving definition.  It was initially used to describe anything not dialyzed off by the older dialyzer membranes of the 1970s, which predominantly filtered out only small, water-soluble molecules such as potassium, for example.  The advent of newer, "high-flux" dialyzers has enabled the removal of higher molecular weight species, but to a variable degree.  

The European Uremic Toxin Work Group has defined the term middle molecule to be between 500 Daltons - 60 kD.   There is still a strong thought that as-yet-unidentified unfiltered middle molecules may be a major reason for the unusually high cardiovascular mortality rate of ESRD patients.  

Here is a helpful list of molecular weights of some common substances:

BUN (0.06 kD).  Obviously, not a middle molecule; small, cleared rapidly.

Creatinine (0.113kD).  Same as BUN:  cleared rapidly.

Vitamin B12 (1.355 kD).  A middle molecule of lower MW which is cleared by most modern dialyzers.

B2-microglobulin (11.8kD).  One of the few middle molecules whose accumulation has been shown to be associated with a real medical condition:  b2-amyloidosis.  Newer generation dialyzers provide good clearance of b2-microglobulin and have made the clinical entity of b2-amyloidosis much more rare.

light chain (25kD).  Light chains are small enough to get dialyzed--providing some rationale for dialyzing patients with paraproteinemias (though the clinical evidence for this being effective is not very good).

albumin (66kD).  Albumin is not a middle molecule--it is not appreciably dialyzed off, nor would it be desirable to do so.

Scleroderma & the Kidney

Scleroderma is a chronic autoimmune disease characterized by fibrosis and hardening of the skin and other organs--occasionally involving the kidney to such an extent that dialysis becomes necessary. There are two main flavors of scleroderma:

(1) the CREST syndrome, a localized form of the disease in which the kidney is NOT typically involved. This variant is typically anti-centromere antibody-positive. Hearkening back to your internal medicine days, you will recall that "CREST" stands for "Calcinosis, Reynaud's phenomenon, Esophageal dysmotility, Sclerodactyly, and Telangiectasias."

(2) Diffuse scleroderma, which is the type that may result in hypertension and renal involvement. These patients are more likely to be anti-SCL70 antibody-positive. The term "scleroderma renal crisis" is used to describe scleroderma patients with aggressively accelerating hypertension which may even result in renal failure. The mechanism of disease is a thrombotic microangiopathy, and urinalysis is generally bland. The mainstay of therapy for scleroderma renal crisis is blockade of the renin-angiotensin system.

PAN versus MPA

Polyarteritis nodosa (PAN) and Microscopic polyangiitis (MPA) are two vasculitides which were previously grouped together.

However, they are currently thought of as two separate entities.

PAN is a vasculitis of the medium-sized arteries, sparing the smaller vessels and therefore does NOT cause a glomerulonephritis. Any renal compromise which results is generally due to ischemia from inflammation of the medium-sized renal vessels. PAN is often associated with hepatitis B infection.

In contrast, MPA is a vasculitis of the small-sized vessels, and therefore results in a pauci-immune glomerulonephritis. It is very commonly ANCA-positive.

Epidural Abscesses in ESRD Patients

Infectious metastases (e.g., infective endocarditis, epidural abscess, liver & lung abscesses, etc) are unfortunately not uncommon in the ESRD population, particularly in those with in-dwelling catheters.  I have personally seen several cases of epidural abscesses as a fellow, a complication which can be neurologically devastating and nearly impossible to treat.  Clearly, prevention is the key issue.

A small but instructive case series from Kovalik et al in a 1996 issue of JASN reviewed all the cases of dialysis patients with epidural abscess over a 5-year period within the Duke health care system.  Of the 10 patients identified, the majority (8) had catheters, 5 of whom with known bacteremia with an attempt to treat the infection with antibiotics alone.  The lesson, if you haven't learned it by now:  catheter-associated bacteremia virtually always should be treated with a line change, as there are significant risks to doing otherwise.  

ATRA Syndrome

The M3 subtype of acute myelogenous leukemia--also known as acute promyelocytic leukemia--is one of the most responsive forms of leukemia, as the addition of all-trans retinoic acid (ATRA) is known to overcome the maturation block in affected lymphocytes. 

While this is generally good news for leukemia patients with M3 disease, about 5-25% of patients get an ATRA-induced systemic illness termed "ATRA Syndrome".  Symptoms are widely variable but can include fever, interstitial pulmonary infiltrates, pleural/pericardial effusions, and (the reason for its inclusion in this blog), acute renal failure.  The topic has been a relevant one for me as there are currently two patients on the Renal Consult service I saw today with ATRA-induced renal failure.  The mechanism is unknown but is posited to be due to mass release of cytokines by affected APL cells.  It can be treated by holding the ATRA and administering corticosteroids.  

Newly-Discovered Antigen for Membranous Nephropathy!

I went to a very memorable talk today at our Renal Grand Rounds during which fairly convincing data was presented that identifies the long-sought-after antigen which causes membranous nephropathy--still the most common cause of adult-onset nephrotic syndrome in whites.

The presenter (Larry Beck of Boston University Medical Center) described a fairly straightforward approach:  he took serum from patients with membranous nephropathy and used it on Western blots of normal human kidney tissue.  In about 70% of cases of idiopathic membranous nephropathy (but in 0% of cases of secondary membranous nephropathy and 0% of other nephrotic syndrome states such as FSGS and diabetic nephropathy), Western blotting reveals a 189kD band, which is identified as M-type phospholipase A2 receptor.  We don't know much about the protein's function, but it does appear to be present in podocytes, and IgG4 antibody against this protein can be eluted from membranous nephropathy biopsy specimens.  Interestingly, in some patients this 189kD band disappears after successful treatment with standard membranous agents, implying that an ELISA test may be an effective way of monitoring these patients (much like the ANCA is used for Wegener's granulomatosis, for instance).  

The data is not yet published but chances are good it will end up in a high-profile journal soon.

IgA Nephropathy versus Post-Strep Glomerulonephritis

IgA Nephropathy & Post-Strep Glomerulonephritis (PSGN) are both relatively common causes of glomerulonephritis which can both be associated with upper respiratory infections. How can you tell them apart?

One of the most important distinctions can be made in taking a good history: In PSGN, glomerulonephritis typically does not set in until several weeks after the initial infection. In contrast, IgA Nephropathy may present with so-called "synpharyngitic glomerulonephritis"--pharyngitis and glomerulonephritis at the same time.

In addition, PSGN classically presents with hypocomplementemia, and if the patient undergoes a renal biopsy there is evidence of an immune complex-mediated process. In contrast, IgA Nephropathy shows normal serologic values (though IgA levels may be elevated in about a third of patients) and the renal biopsy will show mesangial IgA deposition.

Review: The Juxtaglomerular Apparatus

The juxtaglomerular apparatus (JGA) is located between the afferent arteriole and the returning distal convoluted tubule of the same nephron. It is responsible for regulating both intrarenal (tubuloglomerular feedback) and extrarenal (renin-angiotensin-aldosterone) mechanisms necessary to maintain both renal and entire body volume status.

The three components of the JGA are the following:

(1) the juxtaglomerular cells of the afferent arteriole, synthesize and store renin, which is secreted in response to specific stimuli (e.g., low blood flow, decreased NaCl delivery). The juxtaglomerular cells could be considered the "effector arm" of the renin-angiotensin-aldosterone axis.

(2) the macula densa, a region of the distal convoluted tubule characterized by tubular epithelial cells which are more densely-packed than in other regions of the nephron (and thereby leading to its characteristic appearance on light microscopy). The macula densa can be considered the "sensory arm" of the renin-angiotensin-aldosterone axis in that these are the cells which sense decreased Na Cl delivery which determines downstream function. They are also involved in the mechanism of tubuloglomerular feedback.

(3) mesangial cells, which form connections via actin and microtubules which allow for selective vasoconstriction/vasodilation of the renal afferent and efferent arterioles with mesangial cell contraction.

WAGR

The WAGR Syndrome is a rare genetic syndrome whose acronym stands for Wilms tumor, Aniridia (an absence of the iris), Genitourinary abnormalities, and mental Retardation. It is caused by a deletion of chromosome 17p.

The genitourinary abnormalities may take the form of gonadoblastoma (a gonadal tumor), or in boys hypospadias or undescended testicles. Occasionally (though not commonly) renal failure can accompany the syndrome. The fact that these individuals often get Wilms tumor is because the WT1 (Wilms tumor) gene, a tumor suppressor, lies on the deleted portion of chromome 17. A subset of individuals with the WAGR syndrome also have obesity--potentially due to whether the gene BDNF (brain-derived neurotrophic factor) is deleted or not.

Ciliary Hypothesis

The "ciliary hypothesis" refers to the idea that primary cilia--single, non-motile, microtubule-based structures which are found on the apical surface of renal tubular epithelial cells--play an integral role in the pathogenesis of polycystic kidney disease.

Scientists have long known about the existence of renal cilia; however, they felt up until the past decade or so that they were a "vestigial organelle" with no function.  What changed?  First, with the cloning and characterization of the two main genes for autosomal dominant polycystic kidney disease (PKD1 & PKD2), it was determined that the proteins which they encode localize in part to the cilia. Furthermore, a number of other rare pediatric diseases of cystic kidney formation (e.g., Bardet-Biedl Syndrome, nephronophthisis, ARPKD, etc) are also caused by mutations in genes which play an important role in the cilia.  

Exactly how a loss of cilia function might result in cyst formation is still under investigation, but the current dogma states that as urine flows down the tubule, it bends the rigid cilia forward, thereby sending a signal to halt proliferation; if the cilia are not present, the signal is not sent, and cells continue to proliferate.  This is a gross simplification, but this is the ciliary hypothesis in a nutshell.  

Differential Diagnosis for Large Kidneys

The differential diagnosis for large kidneys (often demonstrated on renal ultrasound) is interesting, as there are a few conditions which are especially associated with this finding. Of course, with any type of chronic damage, the kidneys tend to scar down and atrophy.

1. HIV Nephropathy.
2. multiple myeloma.
3. the early stages of diabetic nephropathy.
4. any type of infiltrative process--classically amyloidosis or sarcoidosis of the kidney.
5. autosomal dominant polycystic kidney disease.
6. pyelonephritis.
7. any type of very acute nephritis (e.g., RPGN).

Double Antibody Disease

The incidence of ANCA-positive disease is in the ballpark of 1:10,000. The incidence of anti-GBM disease is even more rare, at about 1:1,000,000. One would expect if these 2 disease-causing antibodies were entirely independent of one another, the incidence of a "double positive" (e.g., both ANCA & anti-GBM-positive) would be predicted to be (1:10,000 x 1:1,000,000 =) 1 in 10,000,000,000, or one in 10 billion.

Double positive disease is much more common than this, however. One study by Levy et al in a 2006 Kidney International paper showed that 5% of all ANCA-positive serum samples were also positive for anti-GBM antibodies, and about 1/3 of all anti-GBM positive samples had detectable ANCA. Most of the time (82%) the double-positive patients had an anti-MPO (myeloperoxidase) specific antibody. Perhaps not surprisingly, the double antibody-positive patients have a worse outcome than single-positive patients.

One theory to explain the existence of double-positivity is that the disease starts off as an ANCA-mediated process and damages the glomerular basement membrane, thereby exposing antigens to which an anti-GBM can form. Treatment in the acute setting is generally aggressive (e.g., treat them as if they have anti-GBM disease) and chronically is often managed as an ANCA disease.

Ret & Kidney Development

The Ret gene plays an integral role in kidney development. As is covered in any basic embryology class, the cross-talk between the ureteric bud (destined to become the collecting system of the kidney) and the metanephrogenic mesenchyme (destined to become the renal parencyma and tubules) is a prime example of how reciprocal signals from two distinct tissue types can influence the development of the other. In the case of the kidney, GDNF (glial-derived neurotrophic factor) secreted by the mesenchyme interacts with the Ret receptor, a membrane tyrosine kinase, on the ureteric bud. This interaction is critical for branching morphogenesis, the process by which the kidney serially reiterates in order to create an organ with a million nephrons, as demonstrated by the fact that mice deficient in the Ret gene have a congenital absence of the kidney.

Ret is important other human diseases as well. It is a proto-oncogene, as gain-of-function mutations lead to various forms of multiple endocrine neoplasia (MEN). Loss-of-function mutations in contrast can lead to either Hirschprung's disease (a congenital absence of the enteric nerves) or renal hypoplasia.

The Brenner Hypothesis

The Brenner Hypothesis--developed by Barry Brenner of the Brigham and Women's Hospital, author of the well-known Brenner & Rector "The Kidney" textbook--states that individuals with a congenital reduction in nephron number have a much greater likelihood of developing adult hypertension and subsequent renal failure. This hypothesis is supported by the observation that there is a strong epidemiologic relationship between intrauterine growth retardation (IUGR)/low birth weight and adult hypertension. The mechanistic explanation for this phenomenon is that compensatory hyperfiltration by the remaining nephrons--similar to what happens in diabetic nephropathy--results in accelerated renal decline. While this theory is appealing at several levels, it does not explain everything: for instance, why most kidney transplant donors, who instantaneously lose 50% of their nephron mass, for the most part do well post-transplant without developing hypertension or renal disease.

The Brenner Hypothesis is part of a larger body of work which posits that many common adult diseases (including diabetes and hypertension) are caused by factors which are established during early growth and development, the "fetals origins of adult disease" hypothesis.

Fistula First Controversy

In 2003, the Fistula First Initiative was enacted, in which an attempt is actively being made to get as many dialysis patients as possible to get dialyzed via an AV fistula. As AV fistulas have been perceived to be the optimal form of dialysis access (with the longest life and fewest infectious complications), this policy seemed to make sense.

And while the initiative has achieved important benchmarks (>50% of dialysis patients using AV fistulas as of July 2008 according to the Fistula First website, with a recently revised goal to achieve 65% of prevalent patients using an AVF by 2009), there is some controversy regarding the project. The main issue is that while the # of patients with AV fistulas has increased, the # of patients with tunneled catheters (and its accompanying risks of infection) have also increased, as shown in this graph from Lok in a 2007 C-JASN review.

The reason for the increased use of catheters likely stems from the fact that there is an increasing rate of primary fistula failure--typically caused by failure to mature or thrombosis, possibly related to a sicker, more vasculopathic, aging population--and the decreased use of AV grafts. A recent KDOQI workgroup in 2006 re-emphasized that the goal of the Fistula First campaign should be a functional AV fistula, not simply the insertion of an AV fistula with a poor chance at maturing. As one vasscular surgeon at our hospital points out, the initiative should be "Patient First" rather than simply "Fistula First." In other words, the vascular access strategy should be individualized to the patient--meaning that for elderly, vasculopaths with poor native vessels who are anticipated to require dialysis in the near future, it may be appropriate to begin with a AV graft rather than attempting an AV fistula with little chance of success.

Vancomycin & Red Man Syndrome

Once a frequent culprit of acute interstitial nephritis, vancomycin is a very rare cause of renal failure with newer formulations which use a different vehicle for its iv administration. It still however has the potential to cause the relatively common side effect of red man syndrome, a histamine-induced phenomenon in which there is the appearance of flushing, erythema, and potentially even hypotension and angioedema which occur during an infusion of this gram-positive antibiotic. This phenotype is due to mast cell (see accompanying electron micrograph) degranulation and is not an IgE-mediated allergic reaction. It can be treated by slowing down the rate of infusion (it is recommended to give a full dose over at least a 60-minute time period) and administering antihistamines.

Interestingly, red man syndrome appears to occur more frequently in healthy individuals (who are volunteering for drug studies) than in those with active infection; the thought here is that elevated baseline levels of serum histamine (as might be seen in active infection, which is an overall inflammatory state) are paradoxically desensitizing to the effects of vancomycin-induced histamine release from mast cells.

Vancomycin was originally isolated from a soil sample from the jungles of Borneo.

My friend from graduate school writes about a real-life personal experience with vancomycin-induced red man syndrome on her ABC Blog.

Neonatal Lupus

Neonatal lupus occurs when autoantibodies are transmitted from the mother to the fetus; it usually manifests as a transient and mild lupoid rash which lasts 3-6 months, but can also result in more serious manifestations such as congenital heart block, immune thrombocytopenia, cholestatic liver disease, or autoimmune hemolytic anemia.  The autoantibodies anti-Ro (SSA) and anti-La (SSB) (also seen in Sjogren's Syndrome) are the culprits here; they are present in about 30% of SLE patients.  

Treating Lupus Nephritis During Pregnancy

It is somewhat controversial whether or not pregnant women with lupus experience flares more frequently than their non-pregnant counterparts.  Regardless, however, the issue of a lupus flare in a pregnant patient is not uncommon, especially considering that the disease tends to affect young females.

In general, the mainstays of treatment for lupus flares during pregnancy are AZATHIOPRINE and STEROIDS; both have had a pretty good track record for safety in pregnancy.  Low-dose cyclosporin A may also be a consideration, and for patients in whom there is evidence of anti-phospholipid antibody syndrome, clot prophylaxis should be initiated with aspirin or low-molecular weight heparin.  

Drugs specifically to be avoided in lupus pregnancies are CYCLOPHOSPHAMIDE (which unequivocally causes birth defects, especially during the 1st trimester) and MYCOPHENOLATE MOFETIL (which recently has been shown to result in ear and other facial malformations such as cleft lip and cleft palate).  

Sporadic Hypocalcemia and Gadolinium

It turns out that not only do gadolinium-based contrast agents result in nephrogenic systemic fibrosis (NSF), but certain ones can also interfere with some of the common assays for quantifying serum calcium, leading to sporadic hypocalcemia. The two widely used gado agents associated with this phenomenon are gadodiamide and gadoversetamide. It is especially true in patients with underlying renal insufficiency, as the gadolinium will hang around longer.

Proof That Kidneys Are Important

As a 2nd year nephrology fellow I am now doing some work in a lab which studies zebrafish kidney development. Here is an example of a control (wild-type) zebrafish at 54 hours post-fertilization.

If you use a morpholino (a form of antisense technology) to knock down a gene important for kidney function, you get massive edema after 3-4 days--as shown in the photos below. This is because zebrafish (a type of minnow native to India) live in freshwater, where the osmolarity is much lower than the intravascular osmolarity; with loss of renal salt and water homeostasis, volume overload ensues. This condition can be mitigated by putting the fish in saline water approximating the fish's endogenous osmolarity.

New Mechanism for Cyclosporin in Proteinuric Disease

Cyclosporin A (CsA) has been a mainstay of therapy for nephrologists: it (along with tacrolimus) has become the cornerstone of immunosuppressant therapy for kidney transplant patients; in addition, it has also seen use as an effective agent in proteinuric disease states such as membranous nephropathy, FSGS, and minimal change disease.

The mechanism by which CsA acts as an immunosuppressant is well worked out: it binds to and inhibits calcineurin, an intracellular phosphatase, and thereby prevents the transcription factor NFAT from translocating to the nucleus and upregulating IL-2 transcription.

However, new data presented in a recent Nature Medicine paper suggests an entirely new mechanism by which CsA acts in the treatment of nephrotic syndrome: it tightens up the actin cytoskeleton of the podocyte which is required for maintaining a working slit diaphragm. In this paper, the authors demonstrate that CsA blocks the calcineurin-mediated dephosphorylation of synaptopodin, a protein in podocytes which regulates the actin cytoskeleton. As mentioned recently in this blog, we know already know that the actin cytoskeleton is critical for maintaining the filtration barrier as genes such as alpha-actinin 4 when mutated result in congenital FSGS.

Cadmium Toxicity

Heavy metals & the kidney are an interesting topic all-around. Aberrant metabolism of heavy metals such as iron, copper, lead all contribute to significant renal pathology. And you can add the heavy metal cadmium to that list as well.

Cadmium toxicity is most notably manifest as renal tubular impairment, specifically in the proximal convoluted tubule, though it may also result in hepatotoxicity and osteoporosis.

Normal cadmium handling begins in the liver, where it binds to the small molecular weight protein metallotheionein. The metallotheionein-cadmium complexes are freely filtered at the glomerulus, then gets taken up at the proximal tubule by pinocytosis. The metallotheionein-cadmium complexes are degraded in lysosomes and excess cadmium is excreted into the tubular lumen via a specific transporter. Large amounts of cadmium can overwhelm this system and lead to proximal tubular damage. Not surprisingly, this results in a Fanconi's Syndrome which can include a proximal (type II) RTA, glucosuria, phosphaturia, and amino aciduria. One of the ways in which subtle proximal tubular damage via cadmium can be monitored is to look for urine b-2 microglobulin levels, which are handled in the PCT via a similar mechanism as metallotheionein.

Case of Hyperammonemia & Fanconi's Syndrome

Quick case summary as presented in Renal Grand Rounds today:  A 54 year-old man presents with altered mental status, a sky-high ammonia level (>200) and anemia requiring blood transfusion.  Other unusual lab values include hypophosphatemia and hypokalemia with a significant non-anion gap metabolic acidosis.  Subsequent workup reveals 3+ glucosuria and aminoaciduria.  What's the underlying diagnosis?

This patient had multiple myeloma, as revealed by an M spike on serum protein electrophoresis. The diagnosis can explain all the major aspects of his presentation:

-myeloma is the cause of his anemia.

-myeloma is a cause of hyperammonemia (plasma cells for whatever reason have the ability to produce large amount of ammonia)

-myeloma and other paraproteinemias are a major cause of adult-onset Fanconi's Syndrome, which this patient has, as demonstrated by his aminoaciduria, glucosuria, phosphaturia, proximal renal tubular acidosis.  

Adynamic bone disease

Adynamic bone disease is being increasingly recognized as the most common form of renal osteodystrophy. It is characterized by the reduced synthesis of bone matrix due to decreased osteoblastic and osteoclastic activity. Adynamic bone disease is distinct from osteomalacia, in which osteoid (the bone protein matrix, composed primarily of type I collagen) accumulates due to a lack of osteoblast activity or defective osteoid mineralization, as opposed to the simple decreased rate of turnover seen in adynamic bone disease.

Adynamic bone disease is particularly common in the peritoneal dialysis population in that the constant exposure to calcium in the dialysate fluid leads to episodic hypercalcemia and suppression of PTH levels which results in adynamic bone. A 2006 study by Haris et al in Kidney International randomized PD patients with adynamic bone disease (as assessed by bone biopsy) to normal Ca (1.62mM) versus low Ca (1.0mM) dialysate. The low-Ca dialysate group developed a higher PTH and bone turnover rates within the normal range, suggesting that the low Ca dialysate strategy is a good one for PD patients in order to avoid adynamic bone.

Water as therapy for PKD?

There are several clinical trials regarding the slowing of progression of polycystic kidney disease currently being conducted; amongst the drugs being studied are V2 receptor antagonists. The rationale for this is that cyclic AMP (cAMP) is known to be required for cyst growth; as ADH signaling involves an elevation of intracellular cAMP levels in the collecting duct, V2 receptor antagonists (e.g., the "vaptans") would be predicted to lower intracellular cAMP concentration and thereby lead to decreased cyst growth.

An even more economic treatment logically presents itself: drink more water! It is not known whether or not this would have a long-term benefit, but the recommendation to drink 3L of water a day (a recommendation which is fairly routine for stone-formers) would be predicted to decrease ADH levels by approximately 50%.

PKD patients also should be advised to avoid health products which contain forskolin--a potent stimulator of cAMP, which would therefore be predicted to accelerate cyst growth. Forskolin is an ingredient in various bodybuilding supplements and other non-FDA-regulated health care products.

Measuring Adequacy

Dialysis adequacy is traditionally measured as either Kt/V (most popular in outpatient dialysis units) or urea reduction ratio (URR, most popular with acute dialysis/inpatient dialysis units). Both methods use the degree to which urea is cleared--by comparing the urea concentration before and after dialysis--as the central means by which adequacy is determined. Regardless of which method is chosen, the means by which these values are obtained is vitally important.

The "pre-dialysis" BUN should be drawn before dialysis is actually begun. It's okay to hook the patient up to the machine, but the blood sample should be drawn before the actual treatment is begun.

The "post-dialysis" BUN must be drawn in a manner described in the KDOQI Guidelines. The blood is drawn from the arterial line at the end of the treatment; one of the main risks here is that there is always some degree of access recirculation, in which some percentage of just-dialyzed blood makes it back into the arterial line. This will have the effect of artificially lowering the post-dialysis BUN, and can lead to falsely high URR or Kt/V readings. To avoid access circulation problems, the dialysis nurse should slow down the blood flow through the dialyzer to a rate at which access recirculation is negligible. Generally, a blood flow of 100 mL/min for 15 seconds is enough to clear the blood occupying the "dead space" of access plus tubing.

Congenital Nephrotic Syndrome

Congenital nephrotic syndrome only makes up between 2-8% of all forms of nephrotic syndrome, but is a hot topic in that there are numerous genes continuously being identified which are helping to advance our understanding of podocyte biology.

A list of nephrotic syndrome-causing genes identified thus far with a brief description of each is listed below:

1. nephrin (NPHS1): this was the first gene identified which causes congenital nephrotic syndrome of the Finnish type. As shown in the cartoon above, nephrin is a major component of the slit diaphragm.

2. podocin (NPHS2): this was the next gene identified and is the most commonly mutated gene causing steroid-resistant nephrotic syndrome.

3. WT1: this is the Wilms Tumor gene which acts as a transcriptional activator or repressor and is essential for proper development of the genitourinary system.

4. LAMB2: this is a gene for laminin, one of the components of the glomerular basement membrane.

5. PLCE1: this surprising gene--which unlike the others does not play an important structural role in the slit diaphragm--encodes an isoform of phospholipase epsilon gene. Individuals with this mutation actually show an increased responsiveness towards steroid therapy.

6. alpha-actinin-4: this gene encodes an actin filament-crosslinking protein which can cause congenital FSGS.

7. TRPC6: this gene encodes a calcium channel expressed in podocytes, and mutations in it can result in congenital FSGS.

This list of genes causing congenital nephrotic syndrome is likely to expand as there are families with this condition which do not link to any of the above loci.

Toluene & The Kidney

Toluene--a hydrocarbon which is present in gasoline, paint thinners, and many industrial solvents--can have a variety of effects on acid-base metabolism by the kidney:  it can cause both a non-anion gap OR an anion gap metabolic acidosis, depending on whether or not the exposure is acute or chronic.

The most well-described with chronic or repeated exposure--as might be seen in a patient who has the unhealthy habit of glue sniffing, or "huffing"--is a renal tubular acidosis, resulting in a non-anion gap metabolic acidosis and hypokalemia.  Multiple case reports support the existence of both distal and proximal RTAs with toluene exposure.

However, acute toluene exposure can also lead to an anion gap.  This is because the major byproduct of  toluene breakdown by the liver's cytochrome p-450 system is hippuric acid, which is an (unmeasured) organic acid.   

This is why your 1st-grade teacher told you to stop sniffing the glue.

"Full House" Immunostaining

Renal pathologists may throw out the term "full house" immunostaining...what does this mean?

This means quite simply that all five major immunofluorescent stains on a renal biopsy (IgM, IgG, IgA, C3, and C1q) are all positive.  The analogy holds true in that the "full house" in cards (3 of a kind plus a pair, to the left) is similar to the 3 types of immunoglobulins plus 2 types of complement which show positive staining.  

The classic situation for "full house" immunostaining occurs in lupus nephritis, in which there is immune complex deposition in the subendothelium and subepithelium.

Hypertension/Brachydactyly Syndrome

One of the as-yet unresolved major controversies in nephrology is whether or not the entity "hypertensive nephropathy" really exists.  Certainly there are a large percentage of the dialysis population who do not have diabetic nephropathy, glomerulonephritis, nephrotic syndrome, obstruction, or other obvious explanations for their renal failure other than the risk factor of hypertension.  Hence, the term "hypertensive nephrosclerosis" which we use frequently.  However, there is now a HUGE percentage (in fact, a majority) of the adult population in the U.S. and the vast majority of these folks do not ever get kidney disease, suggesting that hypertension alone is not sufficient to cause renal damage.

One additional piece of evidence against the "hypetensive nephrosclerosis" hypothesis is the existence of a rare, autosomal recessive disorder termed "Hypertension/Brachydactyly Syndrome", the gene for which has yet to be identified.  These individuals suffer brachydactyly a relative shortening of the fingers and toes, see picture) and severe hypertension, with systolic blood pressures often well greater than 200.  The mechanism of their hypertension is not entirely clear, but what is interesting in this small cohort of patients is that even after decades of exposure to blood pressures in the "malignant" range, very few develop renal damage.  They do have stroke commonly, but not renal disease.  

MYH9

Any nephrologist practicing in the U.S. does not need to be told that there is an increased incidence of end-stage renal disease in blacks. Furthermore, certain diseases--such as collapsing focal segmental glomerulosclerosis (FSGS), occur almost exclusively in black populations.

In the most recent issue of Nature Genetics, two different groups (Kopp et al; Kao et al) have identified a gene which may help to explain some of the increased prevalence of renal disease and FSGS in black populations. Using various genetic techniques, both groups independently identified polymorphisms in the gene MYH9 (a myosin heavy chain isoform on chromosome 22 which is expressed in podocytes) which predispose to (a) FSGS in blacks, and (b) non-diabetic causes of end-stage renal disease.

Perhaps in the future, genotyping of MYH9 and similar genes could allow an identification of which patients are most at-risk for developing ESRD.

The Declaration of Istanbul

In this month's C-JASN, "The Declaration of Istanbul" is presented.  

Recently a group of world leaders of the transplant community held a meeting in Istanbul, Turkey during which the general topic of discussion was the ethics of organic trafficking and transplant tourism--that is, the practice of a patient from one country visiting another country with the express purpose of obtaining an organ transplant (which for all practical purposes means a kidney transplant most of the time).  

The Declaration of Istanbul states that the poor who sell their organs are being exploited--either by more wealthy individuals within their own country, or by those abroad--and therefore that such practices be banned.  

The only country where individuals are allowed to sell their own organs is Iran--though certainly "transplant tourism" occurs with considerable frequency in many other countries illegally, and there are undoubtedly some physicians who are willingly participating in the practice.

gemcitabine-induced thrombotic microangiopathy

Another oncologic medication with a specific renal side effect is gemcitabine (Gemzar), a nucleoside analogue used with frequency to treat cancer of the lung, pancreas, breast and colon to name a few.  

Gemcitabine can cause a thrombotic microangiopathy, having been reported at an incidence of 0.31% according to a paper by Humphreys et al (in Cancer 2004).  It should be considered in any cancer patient receiving this medication with new onset renal failure, hypertension, or thrombocytopenia.

ACE DD Polymorphism

The human angiotensin coverting enzyme (ACE) gene is located on chromosome 17q23 and is comprised of 26 exons and 25 introns. Importantly, there is an insertion/deletion polymorphism in which a 278-bp fragment is either present or absent within intron 16. The deletion "D allele" has been shown to result in increased expression of the ACE gene, and interestingly the DD genotype is associated with an increased risk of hypertension, diabetic renal disease, and cardiovascular complications. The DD genotype turns out to be much more common in black populations than in white populations, and perhaps can account for some of the increased susceptibility towards renal disease observed in blacks compared to whites.

What are Randall's Plaques?

The latest theories on nephrolithiasis involve an anatomical lesion termed "Randall's Plaques", which are calcified, whitish plaques in the renal papillae which may be visualized during endoscopy (see arrows in Figure a, taken from a JCI paper by Evan et al, 2003).  They were first described by the American urologist Alexander Randall, and are now though to be the nidus upon which the majority of calcium oxalate kidney stones form.  

As an aside--remember the theory where urine urate serves as a nidus for calcium oxalate stone formation that you probably learned in medical school?  Well, it's probably not correct.  While elevated urine uric acid levels do correlate with probability of uric acid stone formation (which is relatively uncommon), it does NOT correlate at all with the probability of calcium oxalate stone formation (the most common form of nephrolithiasis).  

In any case, Randall's plaques are calcium phosphate deposits which are thought to occur intraparenchymally, originating at the thin ascending limb of the loop of Henle.  These deposits likely grow slowly over long periods of time, and eventually they are thought to erode through the parencyma and into the papilla, where upon contact with the urine they may predispose to stone formation.  That's the going theory, at least. 

Oxalate and Risk for Nephrolithiasis

Hyperoxaluria is one of the well-established risk factors for stone formation; this certainly makes sense as the most common type of kidney stone is of the calcium oxalate variety.  Interestingly, having high levels of serum oxalate does not appear to have as strong an effect as hyperoxaluria on the rate of stone formation in epidemiologic studies.  However, a low oxalate diet may still be recommended to stone formers with hyperoxaluria, defined as greater than roughly 45 mg oxalate/day.  The classic oxalate-rich food is spinach, though oxalate is also found in high abundance in most nuts (e.g., peanuts, almonds, cashews), baked potatoes, rhubarb, beets, and oranges.  

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Hyperoxaluria may occur via several scenarios.  Various mutations in genes necessary for oxalate metabolism can cause primary hyperoxaluria, in which there may be recurrent stone formation at a pediatric age which may lead to end-stage renal disease.  GI malabsorptive syndromes (e.g., chronic diarrhea, gastric bypass surgeries) also commonly lead to hyperoxaluria.  One hypothesis states that individuals likely to form oxalate stones due to hyperoxaluria have a relative deficiency of the gut bacterium Oxalobacter formigenes, part of the normal GI flora which naturally digest oxalate.

Maladie de Berger

The history of IgA Nephropathy is notable, in large part due to the many famous names of medicine which are associated with it.

The disease was first described in 1801 by Heberden (of "Heberden's nodes" fame) in a 5-year old child with abdominal pain, hematuria, and purpura of the legs. Then in 1837, Johann Schonlein and Eduard Henoch built upon these discoveries by further characterizing the disease in pediatric populations. It was not until 1968 that the pathogenic role of IgA depositions was revealed, based on the work of French pathologist Jean Berger (left). Prior to his discovery that mesangial IgA deposits are present in these patients, many had believed IgG to be the sole nephritogenic immunoglobulin. For quite some time, the disease was referred to as "Berger's Disease" (not to be confused with Buerger's Disease, or thromboangiitis obliterans), though it has largely been replaced by IgA Nephropathy in recent times.

K Channels

Potassium is obviously a key ion in nephrologists' constant battle against reduced renal clearance. What about the potassium channels which are responsible for the trafficking of this ion in & out of the cell?

There is a marked diversity of K channels--amazingly, there are over 80 different K channel genes in the human genome? Although there are many different types of K channels categorized based on their mechanism (e.g., calcium-activated K channels; inwardly-rectifying K channels; voltage-gated K channels; tandem pore domain K channels), they all contain the same basic structure, consisting of 4 identical or near-identical subunits forming a tetramer with a central ion conducting pore. The Chemistry Nobel Prize for discovering the structure of K channels was given to Rod McKinnon in 2003.

It's also important to make the distinction between CHANNELS and TRANSPORTERS, both of which are important in the kidney. CHANNELS are pores which allow a very rapid transport of ions into or out of the cell following an electochemical gradient. An example of a potassium channel in the nephron is the ROMK channel, found in the thick ascending limb. This channel allows a "backleak" of K+ ions taken into the cell which is important for maintaining the negative intracellular potential at about -70mV. In contrast TRANSPORTERS involve the binding of specific ions, often in a precise stoichiometry, which shuttles these ions either with or against their concentration gradient. Not surprisingly, ions move across the membrane much slower with transporters than with channels, but transporters offer the advantage of being able to transport against a concentration gradient and by regulating the correct ratio of ions. An example of an important renal K transporter (there are many) is the Na/K/2Cl cotransporter, also in the thick ascending limb.

ENAC Mutations

The ENAC channel in the distal tubule is responsible for sodium reabsorption. It is comprised of 2 alpha, 1 beta, and 1 gamma subunit with each encoded by a separate gene. Interestingly, mutations in ENAC can lead to two different and largely opposite phenotypes.

Gain-of-function mutations in the ENAC result in Liddle's syndrome, which is not surprisingly characterized by metabolic alkalosis, hypertension, hypokalemia. Despite behaving physiologically like patients with hyperaldosteronism, their aldosterone levels are suppressed. This condition can generally be treated with amiloride or triamterene (which inhibit the Enac channel some) and a Na restricted diet.

In contrast, loss-of-function mutations--usually in either the alpha or beta subunits--can result in type I pseudohypoaldosteronism. This is often (but not always) autosomal dominant in nature and causes severe salt-wasting in children which is unresponsive to mineralocorticoid treatment. It can be treated with a high Na diet.

Major Plug for MDIBL Origins of Renal Physiology Course

Today wrapped up the inaugural Origins of Renal Physiology course at Mount Desert Island Biological Laboratory. It's the first time this has been tried, and though there was significant uncertainty in my mind about what the course would entail, I am really glad that I had participated and I feel most consider it a definite success. I would highly recommend this course to other renal fellows--both those who are interested in a career in research as well as those with a more clinical bent.

In summary, the course included 6 modules on core topics in renal physiology, which included The Glomerulus, the Proximal Tubule, the Thick Ascending Limb, Understanding ENac, Water Metabolism, and (my personal favorite) a module on Salt Secretion using the shark rectal gland as a model system (allowing me to wrestle with an actual shark, as shown above). Each module is headed by a scientist (some nephrologists, some PhDs) with a special expertise in that area, and small groups are given the opportunity to design and carry out their own experiments. Each module was well-organized and in most cases, post-docs or lab technicians were readily available to help carry out experiments on a practical level. Every other day there is a large group meeting in which each module's renal fellows describes their results in the form of brief presentations.

We worked pretty hard on many of the experiments, staying up well past midnight on some nights, but there is also adequate time allotted for exploring the amazing Maine outdoors. The lab is situated within a few miles of Acadia National Forest, and every other day there is some type of group outing planned, such as hiking, biking, or kayaking. This was a fantastic experience.