Monday, June 18, 2007

The genetics of racial differences in hypertension susceptibility   posted by p-ter @ 6/18/2007 04:47:00 PM

Part three of the "genetics and race" debate in the pages of The New Republic is up. One point the debaters seem to have oddly fixated on is the racial disparity in the prevalence of hypertension-- for those of you unaware, African-Americans have higher rates and more extreme forms of the disease.

The debaters at TNR seem to be aware of two hypotheses for why this disparity could exist: first, the "slavery hypothesis", according to which the slaves that survived the long trip from West Africa in sub-human conditions were those that were able to retain sodium and resist dehydration (sweating causes the loss of salt, which must then be replaced), leading to a massive selective event for improved salt retention that today manifests itself as hypertension. The alternative, they seem to think, is that discrimination and an abundance of unhealthy fast food restaurants in African-American neighborhoods in conjunction lead to the differential susceptibility.

There's no doubt that environmental factors play a role in hypertension, and it's likely that differential exposures to those factors play a role in this phenomenon. But in terms of genetics, it's certainly not the slavery hypothesis or nothing. I actually see no reason to invoke it-- the key observation (perhaps not a groundbreaking one) is that African slaves came from Africa, which is quite different from North America. One obvious difference between the two continents is climate. Is it possible that West African populations were adapted to an West African climate; perhaps in such a way that increases hypertension susceptibility in the US today? Absolutely.

I present as evidence this (open access) paper, which states (in the title) that "Differential Susceptibility to Hypertension is Due to Selection during the Out-of-Africa Expansion". I strongly recommend at least the introduction for anyone interested in this topic.

The authors look at a number of alleles involved in salt retention and blood pressure, and perform analyses simlar to those done in the paper on tonal languages and two brain-expressed genes-- they examine the correlations of the allele frequencies with distance from the equator. When they compare these correlations with those from randomly chosen loci, they find the salt-retention alleles are outliers, suggesting they've undergone selection in reponse to latitude (which they interpret as a proxy for climate). While in the tonal languages case I urged skepticism about causation, these alleles are already known to be involved in hypertension. This just presents evidence that the differences in allele frequencies between populations is due to natural selection, as opposed to some neutral process.

So African populations (and thus, African-American populations as well) have different allele frequencies of at genes known to be involved in hypertension. There is no doubt that this divergence plays a role in the different susceptibilities of African-American and European-American populations to this disease.

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