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Podocytes and the Big Story

Last Updated on Monday, December 05, 2011

Currently there's no way to know for certain whether preeclampsia will develop during any given pregnancy. This leaves pregnant women and their care providers with little choice but to wait for symptoms to appear... dangerous symptoms that mean the disease has progressed to the point where mother and baby are critically ill and will need intensive monitoring and carefully timed delivery to protect their health and lives. The only screening method to date is to measure those symptoms when they appear.

Early detection wouldn't be a treatment. But what if a screening test could let us know, weeks or even months in advance, that we'd probably be getting ill? Knowing might change the way we seek care - possibly choosing specialist care providers with the education and experience to manage medically complicated pregnancies. Women in parts of the world (like Wyoming in the winter) where such care is in short supply might be moved to bigger cities where NICUs and maternal-fetal medicine (MFM) specialists are more accessible. Neonatal specialists could be brought into the consulting team, and steroid shots to accelerate fetal lung development could be planned. All of these interventions together have the potential to lower the rates of fetal and maternal death and severe complications dramatically. (By the way, you may agree or disagree with this assessment; either way, we'd really like to hear your perspective in this important survey we recently launched.)

Since many experts consider such a test preferable to our current screening methods, recent research has tried to find a workable test with high levels of both specificity and sensitivity. An ideal test would be one that picked up *all* cases of preeclampsia - it would be sensitive - and *only* cases of preeclampsia, with no false positives - it would be specific. In practice this is all but impossible to achieve, but it's quite possible to find tests with very high levels of sensitivity and specificity, with one measure performing slightly better than the other.

So for researchers this creates a choice between creating tests that would produce false positives, and tests that would miss some cases. The worst-case scenario of a false positive is likely to be unnecessary close monitoring; it would not be an indication for immediate delivery. It would provide a reason to follow the pregnancy more closely for symptoms that do indicate that the acute phase of the illness has developed. Since the worst-case scenario of a false negative is a missed case of preeclampsia leading to death of mother and/or baby, public health researchers would prefer to develop tests that will overdetect cases of preeclampsia, but with a low rate of false positives.

At a meeting of the American Society of Nephrology (ASN) in November, Dr. Vesna Garovic (Mayo Clinic) reported on a study into the use of urinary podocytes as a screening test for preeclampsia, a test that may turn out to have both high sensitivity and specificity for preeclampsia. Podocytes are cells which line the blood vessels in the kidneys and act as filters which keep protein in the bloodstream. Their loss allows protein to spill into the urine, one of the primary signs of preeclampsia.

Garovic's research team used a population of 267 women and collected their urine between 25-28 weeks gestation. The samples were examined for podocytes. The 15 women who went on to develop preeclampsia all had podocytes in their urine at that gestational age. The 15 women who developed gestational hypertension did not. The control group of women who had normal pregnancies also did not.

Previous research from the same group has indicated that podocytes are present in the urine when preeclampsia symptoms first appear. When they shed into the urine, they cause disruptions in the filtration barrier in the kidney, resulting in proteinuria. The "classic lesion" of preeclampsia - glomerular endotheliosis - may include the loss of these podocytes from the glomeruli in the kidneys.

To confirm that this test works well, it will need to be repeated in multiple centers serving broader populations of pregnant women, and to make it available for general use, it will need to be turned into a commercially available testing product. That said, these are very hopeful results - unlike the soluble factors researchers are also pursuing, podocytes appear to only be elevated in women who go on to develop preeclampsia. The soluble factors are elevated in all pregnant women, and just unusually elevated in preeclamptics, which makes it very difficult to develop a screening test with high sensitivity and specificity.

Interestingly, it's likely that the podocytes are damaged because of the increased level of sFlt-1, which binds VEGF, which impairs repair of the podocytes and increases the rate at which they die off. It seems likely, given this new research, that it will be easier to detect the damage to the podocytes themselves than it will be to detect an increase of the soluble factors above the point where damage is likely.

If it's possible to develop a test similar to a standard pregnancy test, this might be a very effective method which wouldn't require laboratory work, and would make screening much easier and more rapid. Pregnant women who will likely go on to develop preeclampsia could be moved to the care teams that manage complications before they are critically ill, allowing both them and their babies to receive appropriate medical care that would probably reduce poor outcomes substantially.

A special thanks to Dr. Vesna Garovic at Mayo Clinic, for her expertise and technical input.

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