Differentiating FHH from Primary Hyperparathyroidism

One of the diagnostic dilemmas which frequently comes up in the outpatient evaluation of hypercalcemia (and also, I'm told, on boards-type exams) is how to differentiate familial hypocalciuric hypercalcemia versus primary hyperparathroidism--both of which are common causes of hypercalcemia. The short answer: check the urine calcium--it should be low in FHH but normal to high in primary hyperparathyroidism.

Primary hyperparathyroidism, as we are all aware, usually results from either a parathyroid gland adenoma or 4-gland parathyroid hyperplasia. The PTH level is either high or inappropriately normal, and as a result there is constant high urinary calcium reabsorption, constantly high Ca reabsorption from bone, and increased 1,25-OH vitamin D synthesis leading to increased Ca uptake in the gut. The urine calcium concentration is high; hence, the increased risk of nephrolithiasis in these patients. Also, the urine concentrating ability may be diminished in primary hyperparathyroidism.

In contrast, the defect in familial hypocalciuric hypercalcemia is genetic--an autosomal-dominant-inherited mutation in the calcium-sensing receptor gene. Despite hypercalcemia, PTH levels are normal to only mildly elevated. The urine calcium concentration is low (often measured as the urine calcium:creatinine ratio, which in FHH should be <0.01). The urine concentration ability is usually intact in FHH. It is important to make the distinction between primary hyperparathyroidism and FHH because parathyroidectomy surgery is usually highly effective in the former but not the latter.

Zebrafish Model of Cystic Kidney Disease

For the research component of my Nephrology fellowship, I've chosen an unorthodox route:  I am using the zebrafish as a model organism for the study of kidney disease.  

The zebrafish system has a number of advantages.  First, a single male/female pair can produce hundreds of eggs overnight, thus opening the door for great genetic studies.  It's akin to the fruit fly in this respect, but because the zebrafish is a vertebrate and the fruit fly is not, the zebrafish genome is much more similar to the human genome.  Zebrafish embryos are completely transparent, and thus you can watch their kidneys develop in real-time.  The early zebrafish kidney (called the pronephros) first appears within 24-48 hours, so experiments can be done with a rapidity unmatched in mice.  Furthermore, the zebrafish pronephros is much more simple than the mammalian kidney:  in fact, it consists of a single nephron!!  Despite this simplicity, the nephron contains discrete proximal, distal, and collecting duct domains which appear to be very similar to their mammalian counterparts.  

I'm presently using the zebrafish system as a model of cystic kidney disease.  For example, knocking down the genes for PKD1 and PKD2 (mutations in which result in ADPKD in humans) result in glomerular cyst formation within the single nephron of the zebrafish.  I am presently using this system in order to identify new genes which may be part of the PKD1/PKD2 pathway.  For instance,  here is an example of a gene I knocked down which interestingly results in dilatation of the pronephric ducts (shown by arrows) and cysts which form in the single glomerulus (arrowheads, looks kind of like a big bubble). 

Potassium-Sparing Diuretics

The potassium-sparing diuretics consist of the following 4 medications--only some of which (spironolactone & eplerenone in the list below) are structurally similar.  

Spironolactone is an antagonist of the aldosterone receptor, which like other steroid receptors is an intracellular receptor.  The binding of spironolactone to the aldosterone receptor within collecting duct cells decreases the activity of apical ENac, thereby preventing Na reabsorption and K excretion.  Although spironolactone on its own has a weak diuretic effect, when used in combination with other diuretics may be clinically useful in reducing edema.  It also has well-documented beneficial effects in patients with congestive heart failure (see the 1999 RALES trial for details).  Aside from having the obvious side effect of hyperkalemia, spironolactone also has anti-androgen activity based on its interaction with the androgen receptor, which prevents binding of dihydrotestosterone.  Thus spironolactone is associated with the often unpleasant side effects of gynecomastia, decreased libido, testicular atrophy, and menstrual irregularities.

The drug eplerenone is structurally related to spironolactone, but has greater specificity for the aldosterone receptor (as opposed to the androgen receptor), thus allowing for its use as an effective K-sparing diuretic without the sex hormone side effects. 

Both amiloride and triamterine work by direct inhibition of ENac.   

The importance of residual renal function

A recent review in this month's AJKD by Perl and Bargman describes the increasingly recognized importance of residual renal function in dialysis patients' prognoses.  Although the benefits of residual renal function are most obvious in patients undergoing peritoneal dialysis (see the CANUSA and ADEMEX studies), apparently there are also convincing associations between residual renal function and mortality in hemodialysis populations.  For instance, in this 2001 AJKD study by Shemin et al, the ability to produce >100 cc of urine a day in 114 chronic hemodialysis patients was independently associated with a 65% decrease in the risk of death.  The mechanism of how such residual renal function gives a survival advantage is unclear, but one favored explanation is that the kidney's ability to clear "middle molecules" (something which is lacking in current hemo- and peritoneal dialysis therapies) helps remove toxins which are critical for the pathogenesis of cardiovascular disease. 

This brings up a whole host of questions which challenges our current management of ESRD patients.  Should all ESRD patients be put on ACE-inhibitors or ARBs in order to retain whatever minimal renal function they have?  Should we be less cavalier about prescribing prolonged courses of gentamicin for ESRD patients with gram-negative infections?  Should I avoid giving iv contrast to ESRD patients in order to avoid contrast nephropathy, or should I specifically arrange to have dialysis immediately after iv contrast is given in order to dialyze it off?  I find this latter point to be a great source of confusion, and sometimes contention, between house staff and nephrologists regarding when to schedule dialysis in reference to a cardiac catheterization or other heavy iv contrast-requiring procedure.  I don't know if there are any clear answers dictating what we should do in these situations, but perhaps there will be more conclusive guidelines in the future. 

How to lose a kidney

Couldn't figure out how to embed the video so I've just posted the link of a daredevil who "exploded his kidney" (according to the video at least) during a bike jump. Just think, somewhere out there there's a one-kidneyed daredevil--only one additional accident away from dialysis.

The link is here.

Minnesota Vikings Players' Illegal Bumex Use

Having grown up in Minnesota and being a rabid sports fan, I have an obligation to closely follow the Minnesota Vikings, no matter how much trouble with the law the team has encountered in recent years. Here's a story on Vikes' players Pat and Kevin Williams, who were recently suspended for having a urine sample test positive for a banned substance: bumetanide!

Why would these professional football players be taking bumetanide? Did they have refractory anasarca with a history of congestive heart failure? Of course not. Diuretics are frequently used as a masking agent for illicit anabolic steroid use, as they may dilute the urine enough to reduce the concentration of substances in the urine.

Actually, it's somewhat controversial as to why these players (not just the Williamses) actually took bumetanide, as no steroids were detected in these urine samples. The use of diuretics as a masking agent for steroids is less effective now that more sensitive techniques such as HPLC are used. And as this informative article on "The Steroid Report" blog points out, several non-FDA-approved dietary supplements marketed for weight loss have trace amounts of bumetanide in them.

One thing is for sure: with the Vikes' heading into the pre-season with two terrible choices for their starting QB (Tarvaris Jackson & Sage Rosenfels), Minnesota will need much more than a strong loop diuretic to compete this season...

Betel Nut Chewing

Following up on the discussion of milk-alkali syndrome, a similar syndrome (hypercalcemia, metabolic alkalosis, and acute renal failure) may also occur due to betel nut chewing. Betel nut chewing is a common habit and cultural practice amongst Indian and Southeast Asian populations. Generally, betel leaves are used to wrap a type of nut (the areca nut) and then chewed. Because of its bitter taste, alkaline calcium salts--mineral-slaked lime in the form of calcium hydroxide--are traditionally added to the mixture. Because of the presence of calcium & base, a small fraction of individuals who frequently participate in betel nut chewing can develop milk-alkali syndrome. So add "Chew any betel nuts recently?" to your list of bizarre questions to ask patients with unexplained hypercalcemia.

In Taiwan, "betel nut beauties" refer to scantily-clad young women who sell betel nuts and cigarettes at small kiosks, apparently a common site in big cities.

History of the Milk-Alkali Syndrome

Heard about a case of milk-alkali syndrome at Renal Grand Rounds yesterday. This is an interesting condition whose pathophysiologic mechanism really tests your knowledge of calcium and acid-base homeostasis. I won't attempt to chart the pathway but a really good overview can be found in this 2006 CJASN review by Felsenfeld and Levine. Briefly, patients who have large intakes of calcium and bicarbonate develop hypercalcemia, metabolic alkalosis, and acute renal failure.

The milk-alkali syndrome first rose to prominence when Bertram Sippy in 1915 developed a regimen for the treatment of peptic ulcer disease that involved drinking large volume of milk on an hourly basis along with "Sippy powders" that contained significant amounts of sodium bicarbonate. This provides really an ideal setup of a situation in which hypercalcemia, metabolic alkalosis, and acute renal failure can occur simultaneously, the hallmark of the milk-alkali syndrome.

It was not until 1936 that the toxicities associated with the Sippy protocol were tied to hypercalcemia by a Dr. Cope, who determined that in patients with milk-alkali syndrome the hypercalcemia and metabolic alkalosis could be relatively rapidly reversed by stopping calcium/base input and giving fluids. Although it is seldom used, the moniker "Cope's Syndrome" is occasionally used to refer to the acute- or subacute- forms of milk-alkali syndrome.

In 1949, Burnett et al described a chronic form of milk-alkali syndrome which involved the long-term toxicities of these metabolic derangements, including manifestations such as renal failure secondary to nephrocalcinosis and band keratopathy, in which calcium deposition on the cornea occurs. Fittingly the chronic form of milk-alkali syndrome is sometimes called "Burnett's Syndrome."

With the advent of H2-blockers to more effectively deal with peptic ulcer disease, the milk-alkali syndrome became quite rare for several decades as the general population's intake of popular antacids such as TUMS was greatly reduced. However, there has been a more recent increase in milk-alkali syndrome, particularly in women, with the increased attention to osteoporosis prophylaxis with calcium supplements. Several recent reviews cite milk-alkali syndrome as the third-most-common reason for hypercalcemia requiring hospitalization, after malignancy and primary hyperparathyroidism.

Acetate-Based Dialysate: The Middle Ages of Dialysis

Us renal fellows are likely to take for granted the fact that virtually all dialysis buffers used today are bicarbonate-based buffer systems. The bicarbonate-CO2 buffer system is the dominant buffer system in the human body, patients without renal function are often slightly acidotic, and therefore it makes sense that dialyzing a patient with a bicarbonate-based buffer system to restore bicarbonate levels to normal would be a naturally good idea.

However, bicarbonate-based dialysate has not always been the case! Up until the early 90s, the dominant form of dialysate buffer was an acetate-based dialysate. The reason that bicarbonate-based systems were not used at this time was because when mixed with any calcium, it would form an insoluble calcium bicarbonate compound; now there are systems which separate the acid (also containing the calcium) from the base (bicarbonate) and mix them just before use. According to multiple anecdotes from the more senior nephrology attendings & nurses at our hospital, most of the healthier patients would do relatively well with the acetate-based buffer systems--but those with liver failure, sepsis, or other severe illnesses would often feel extremely ill about an hour or so into acetate dialysis. This was in large part due to a defect in the ability to metabolically convert acetate to bicarbonate, and because bicarbonate was essentially being dialyzed off during dialysis, these patients could develop dangerous acidosis as a result. The effect is not unlike patients undergoing CVVH with a citrate-based replacement solution who have liver failure and are therefore unable to convert citrate to bicarbonate.

Another routine aspect of the dialysate which has changed is the calcium concentration of the dialysate. Previously, the tendency was to use a higher Ca concentration in the bath, between 3 or 3.5 mg/dL, in order to bring the post-dialysis serum calcium into the normal range. However now the current practice is to use a dialysate bath concentration of 2.25 or 2.5 g/dL, allowing the serum Ca concentration to remain slightly lower than the normal range with the idea that there will be less vascular calcification and (presumably) less cardiovascular mortality.

Genome-Wide Association Study for CKD

A recent article just published in Nature Genetics by Kottgen et al identifies several loci which appear to confer susceptibility to the development of CKD.  How did researchers identify these genes?  By a genome-wide association study.  Briefly, researchers obtained genetic samples from some of the largest existing public health databases--such as the Framingham Heart Study, just to name one--and performed a genome-wide associated study in order to identify single-nucleotide polymorphisms (SNPs) that are associated with a low estimated GFR (using either creatinine or cystatin as the measurement of GFR).  They identified significant SNP associations with low GFR in the genes UMOD, SHROOM3, GATM-SPATA5L1, CST, and STC1. 

Of these genes, perhaps the most convincing association was seen with UMOD, the only one of these genes which was associated with an estimated GFR low enough to be categorized as CKD, which they defined as Tamm-Horsfall protein, which is the most abundant protein in normal urine.  Interestingly, mutations in UMOD do account for some rare forms of kidney disease, including juvenile hyperuricemic nephropathy, glomerulocystic kidney disease, and medullary cystic kidney disease type 2.  Also, knockout of the UMOD gene in mice is associated with a decrease in their eGFR.  UMOD gene expression occurs primarily in the thick ascending limb of the loop of Henle, and therefore these new data imply that perhaps some of the pathogenesis of CKD dervies from the loop of Henle--and perhaps not all the action is in the glomeruli as has long been the focus.  The UMOD story for CKD--as well as the other genes identified in this major association study--may be an interesting one to follow in the years to come.

Kidney-Themed 30 Rock Episode

The NBC show "30 Rock" featured a humorous, kidney-themed season finale last Thursday. If you don't feel like watching the full 30-minute episode, you can purchase the full music video for the song "We Need A Kidney" at the I-Tunes web site for only 99-cents, the proceeds of which are being donated to the NKF!

GBM Collagens & Alport's Syndrome

Alport's Syndrome is a genetic disorder characterized by glomerulonephritis, progression to ESRD, and hearing loss. Intriguingly, it can be inherited in either an X-linked, autosomal recessive, or autosomal dominant manner. Why is the inheritance pattern so complex and what does it tell us about the biology of this interesting disease?

To answer this question we need to understand the composition of the glomerular basement membrane: in addition to being comprised of laminin, nidogen, and sulfated proteoglycans, a major component is collagen IV. There are 6 genes in humans which encode different alpha chains:

COL4A1 encodes alpha-1 collagen (Type IV).

COL4A2 encodes alpha-2 collagen (Type IV).

COL4A3 encodes alpha-3 collagen (Type IV).

COL4A4 encodes alpha-4 collagen (Type IV).

COL4A5 encodes alpha-5 collagen (Type V).

COL4A6 encodes alpha-6 collagen (Type VI).

Three different alpha-chains combine to form a triple helix called a "protomer" which is the fundamental structural unit of the collagen network. The particular collagen protomer which is the major player in the adult kidney is the alpha-3-alpha-4-alpha-5 promoter. Not surprisingly, then, mutations in any one of the three genes can cause Alport Syndrome.

The most common inheritance pattern is X-linked, which accounts for 85% of all cases of Alport Syndrome, and occurs due to mutations in the COL4A5 gene on the X-chromosome.

It is also possible to have autosomal recessive inheritance due to inherited mutations from both patients of either the COL4A3 or COL4A4 genes, both on somatic chromosomes.

Finally, there are rare cases of autosomal dominant inheritance of Alport Syndrome when dominant-negative mutations in either the COL4A3 or COL4A4 genes are inherited.

Big Pharma Nephrology

The following is a list of major pharmaceutical companies, ranked by total revenue. The numbers are staggeringly high. #1 overall is Johnson & Johnson ($ 63,747,000,000 in 2008 total revenue), followed by Pfizer and Bayer. A few of the notable nephrology-relevant pharmaceutical companies include Novartis (#5, valsartan & many other common drugs), Amgen (#14, Epogen, cinacalcet), Baxter (#17, dialysis products), Genentech (#19, Rituxan), Genzyme (not ranked, Renagel, hectorol, Fabrazyme), and Fresenius (not ranked; Phoslo and dialysis products).

Stranded on a Lifeboat

We all know that if you are stranded on a lifeboat in the middle of the ocean, you are not supposed to drink the salt water.  Why is this the case--shouldn't the kidney be smart enough to retain the water and excrete the salt?  

It turns out that the high osmolarity of seawater (usually >1000mosm/L) either approaches or exceeds the concentrating capacity of the kidney--and therefore you can't expect to retain any free water.  In addition, seawater contains high concentration of magnesium and sulfate-containing minerals which can result in an osmotic diarrhea when large quantities are ingested; this can exacerbate free water loss which may likely already be high due to high insensible losses from wind and sun.  

Some animals have impressive adaptive mechanisms to maintain homeostasis of osmolarity in a high salt environment;for instance,  the albatross' nasal gland excretes a highly-concentrated salt solution, and the shark rectal gland has the same ability as detailed in another post.  

 

Plus one fun (well, not so fun for the patient involved) link for the hell of it.  The headline says its all:  Tainted Castor Oil Used for Cosmetic Augmentation Causes Kidney Failure, but you can click the link for more juicy details.

alpha & beta intercalated cells of the collecting duct

Heard a great talk today from the 2007 recipient of the Homer Smith Award & editor of Kidney International: Qais Al-Awqati of Columbia. The full details of much of the talk can be found in this JASN article describing his acceptance talk at the ASN, but I will attempt to paraphrase.

The collecting duct has two types of cells: principal cells (the aldosterone-responsive, ENac-expressing cells which also mediate water reabsorption via ADH) and the intercalated cells. The intercalated cells come in two different varieties: alpha-intercalated cells (which secrete acid) and beta-intercalated cells (which secrete base). The alpha-intercalated cells (on the left in the figure) are tall, columnar epithelial cells which contain apical H+ ATPase, explaining its ability to secrete protons. The beta-intercalated cells (on the right) in contrast are shorter, flatter cells which contain an apical chloride-bicarbonate exchanger (called pendrin) which enables it to secrete base.

The interesting thing is that inducing metabolic acidosis results in the conversion of beta-intercalated cells to alpha-intercalated cells--giving the kidney a greater ability to secrete protons and return pH to the normal range. This conversion event is regulated by a secreted extracellular matrix molecule called hensin. Since these cells evidently retain the ability to switch between markedly different cellular phenotypes, it has become of interest to the ever-evolving field of stem cell biology.

Transvaginal Nephrectomy

Recently two separate groups have reported the successful carrying out of the first-ever human transvaginal nephrectomy--one report coming from the Cleveland Clinic and another from Venezuela. The surgery is being developed as an even less invasive surgical approach than laparoscopy to nephrectomy--in the Cleveland Clinic case, for instance, the patient was able to be discharged within 24 hours following the procedure. Transvaginal nephrectomy is one of several procedures included in the evolving and acronymed field of NOTES (natural orifice translumenal endoscopy surgery).

Chronic Pain in ADPKD

About 60% of all ADPKD patients are estimated to have a chronic pain syndrome, and often this is the initially presenting symptom of an ADPKD patient that brings them to medical attention. An excellent review of this field can be found in a 2001 Kidney Internal article by Bajwa et al.

There are many potential reasons for ADPKD patients to get pain syndromes:
1. Cyst Growth: Obviously the growing cysts themselves can cause pain by compressing on other structures or stretching out the renal capsule.
2. Cyst hemorrhage: Hemorrhage into a cyst can also occur, causing acute pain. Sometimes this is associated with gross hematuria, though only if the cyst is in communication with the rest of the nephron.
3. UTI's/recurrent pyelonephritis: not surprisingly, this is more common in women than in men. Again, because not all cysts communicate with the urine, it’s possible that you can have a cyst infection even with a sterile urine. If there is an infection, it’s also important what antibiotic you chose, since not all antibiotics have good penetration into the cysts. Penicillins, for example, are not very good at penetrating cysts; fortunately, both quinolones or Bactrim are quite good at cyst penetration.
4. Nephrolithiasis: up to 25% of ADPKD patients have nephrolithiasis; the most common type of stone in ADPKD is uric acid stones.
5. Chronic back pain: enlarging cysts can cause increased abdominal girth, leading to increased lumbar lordosis.
6. Polycystic liver disease: the same process which causes cyst formation in nephrons commonly causes cyst formation in bile ductules, though this is usually asymptomatic. Occasionally, severe liver cyst growth can cause severe abdominal distension and pain.

Two-Hit Hypothesis & ADPKD

One of the really interesting aspects of autosomal dominant polycystic kidney disease (ADPKD) is that only about 1% of all nephrons form cysts. You can do the math: if you have about 1 million nephrons per kidney, and 1% of those nephrons are affected, then you will develop about 10,000 cysts per kidney. The reason these patients often go on to develop ESRD is because the growing cysts gradually expand enough to crowd out the remainder of functioning nephrons in the parenchyma.

But why are only 1% of nephrons involved? What's different about the affected nephrons compared to the unaffected ones? One current thought (though this remains controversial) is the "two-hit hypothesis." As described in this excellent 1996 Cell paper, researchers managed to isolate epithelial cells from individual cysts in ADPKD individuals. First, they demonstrated that individual cysts are monoclonal, and therefore derive from a single renal tubular epithelial cell. Then, they demonstrated that within the majority of each monoclonal population, there was loss of heterozygosity at the PKD1 locus. In other words, cysts arise from cells in which a "2nd hit" of the remaining good copy of PKD1 suffers a somatic mutation. This mechanism is much like how tumor suppressor genes (such as APC in colon cancer). The fact that the PKD1 gene is so large (46 exons) makes it especially susceptible to the possibility of mutation. Thus, although the disease is inherited in an autosomal dominant fashion, at a molecular level the cysts themselves are autosomal recessive. This "two-hit hypothesis" is one possible explanation as to why only a small fraction of nephrons ultimately form cysts.

Kidney Transplant News Item

This is a little old but still funny if you haven't seen it already.


Anonymous Philanthropist Donates 200 Human Kidneys To Hospital

A better way for CKD staging?

Presently the KDOQI Staging system for CKD (see figure) is what is used most commonly to classify the severity of reduced renal function. This is based purely on an estimation of the GFR, using equations such as the MDRD to determine GFR. While this is helpful in identifying potential patients who will go on to develop ESRD--current recommendations state that individuals with CKD Stage 3 and above be referred to a nephrologist for evaluation--there is a substantial fraction of individuals who meet the criteria for CKD Stage 3 or 4 who will never go on to develop ESRD.

An interesting paper in this month's JASN by Hallan et al suggests an alternative staging system: instead of using only eGFR, the authors define a strategy whereby eGFR is combined with the degree of albuminuria to arrive at a new classification system. They arrived at this strategy by looking at over 65,000 adults from an older large-scale health screening study (HUNT2) and determining what factors best predicted the development of ESRD. The authors argue that their newer CKD staging system is much more efficient at identifying patients truly at risk for developing ESRD compared to the traditional KDOQI system.

• They calculate that referring all patients with CKD Stage 3 or above to a nephrologist would lead to working up 4.7% of the general population
• Which would identify 69% of all patients who progress to ESRD.
  

Using their own classification scheme, however, they claim that they would need to refer only about 1.4% of the general population without sacrificing sensitivity: they estimate that they would still identify about 66% of individuals who would go on to develop ESRD.

The Erythropoietin Receptor

As we all know, erythropoietin is secreted predominantly by the kidney (85% from the kidney, 15% from the liver) in response to hypoxia and its function is to stimulate erythropoiesis in the bone marrow.  What  is the receptor on which erythropoietin works?  

The Epo receptor is a member of the cytokine receptor family expressed on the cell surface of erythroid precursors.  Although the Epo receptor itself does not have kinase activity, it is bound by the tyrosine kinase Jak2, also called Janus kinase 2.  The activation of downstream transcription factors by Jak2 (e.g., the Stat family of transcription factors) results in accelerated erythrocyte maturation.

Interestingly, a common mutation in the Jak2 gene (V617F) accounts for the majority of cases of polycythemia vera--the hematologic condition in which there is a primary elevation in the hematocrit which can result in thrombotic complications.  These patients have a low circulating EPO level, but as the EPO receptor is constitutively active, there is always accelerated erythropoeisis.  

Fibromuscular Dysplasia

Fibromuscular dysplasia (FMD) accounts for a significant fraction of renal artery stenosis. The distinction between fibromuscular dysplasia and atherosclerotic renal artery stenosis is important as the former tends to respond much more readily to interventional therapy than the latter. More specifically, FMD tends to respond favorably to balloon angioplasty.

There is some suggestion that FMD has a genetic component to it, though no gene has yet been identified. The diagnosis is usually made by angiography showing a "beads on a string" pattern in the affected artery. FMD can affect arteries other than just the renal arteries (e.g. the carotid arteries), but the disease is most famous for its effect on the renal arteries where it may cause secondary hypertension due to activation of the renin-angiotensin system.