Nir Barzilai, a prominent researcher from the Albert Einstein College of Medicine in New York, read my story today on how a group of researchers from the Rush University Medical Center in Chicago had failed to replicate an earlier result from Barzilai's team that linked a particular gene variant to protection against dementia. (The same variant had also turned up more often in extremely long-lived Ashkenazi Jews.)

The Rush study, however, found that the version of the gene in question, known as CETP I405V, might actually predispose its bearers to a higher risk of Alzheimer's.

Barzilai had been quoted in my story. But after reading it, he had more to say. His explanation reveals some of the subtleties involved in drawing associations between genes and complex mediators of health and disease. Here's a link to my original article, and Barzilai's reply is below and also at the end of my story in the comments section.

"The validation of a single genetic variant is dependent on the genetic structure of the population. For example, Asians do not have this variant [CETP I405V], but have another CETP variant that is associated with the phenotype [of high HDLs]. In the Rush study the frequency of the VV variant [two copies of CETP I405V] was ~6 percent while it was ~20 percent in the Einstein Aging Study (Bronx) population, suggesting different genetic backgrounds (or survival). Thus, association of genotype without the phenotype is not very revealing. The question to both Rush and [our] EAS study is whether low CETP levels, high HDL levels (and large lipoprotein sizes) protect against cognitive decline? Analysis of genotypes alone may never reveal the whole truth, and geneticists need to get behind such limitations and the urge to focus on one variant at a time in a diverse populations."

Image source: Einstein Magazine