AIDS used to be considered a male-specific health problem — but now, the majority of its victims are female.

Those numbers aside, some studies have shown that certain females take much longer than males to progress to AIDS after being infected with HIV. Now, researchers have now found a mechanism that might explain this difference.

A two-stage study by Siddiqui et al. identified a genetic variant on the X chromosome that may slow progression from HIV-1 to AIDS in women. The study analyzed both primates and humans, demonstrating that genetic association findings can successfully translate across species, even over an evolutionary distance of 25 million years.

The researchers first analyzed 136 rhesus monkeys infected with SIV (simian immunodeficiency virus). They found a common genetic marker on the X chromosome among those monkeys that progressed slowly to AIDS-related disease. Researchers then studied 303 HIV-infected humans and found an analogous marker on the X chromosome —the SNP rs5968255 — that may affect progression to AIDS in human females.

The results, published in the online version of The American Journal of Human Genetics, indicate that in humans, females with one copy of the C version of rs5968255 progressed to AIDS more slowly than either females with TT or males with a C or T. (Note that females have two X chromosomes, while males only have one.)

Females with one copy of the C version of the SNP took an average of eight years to progress from HIV infection to AIDS, nearly four times less than females with no copies or males. Women with two copies of C are so rare that none were found in the study.

Previous research has linked some degree of HIV resistance to a mutation in SNP i3003626, which is in a gene that encodes the CCR5 receptor.

(23andMe customers can check their data for rs5968255 using the Browse Raw Data feature. Information on i3003626 is available in the Resistance to HIV/AIDS Clinical Report.)

The CT genotype of rs5968255 is more frequent among Asians than Europeans or Africans, suggesting that future research may find that HIV-1 positive Asian females have a slower progression to AIDS. Future studies may also be able to use this X chromosomal variant as a clue toward new drug development for both genders.

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Our SNPwatch posts here at The Spittoon are one of our most exciting features. They give our customers the opportunity to connect their genetic data to the newest discoveries, often within just hours of a study’s publication.

Looking ahead to 2009, we can only begin to imagine the exciting discoveries that will be made in genetics. In the meantime, here are a few of our favorite SNPwatches from 2008:

SNPs That Affect Drug Response
We reported on several studies this year that showed the importance of genetic variations in determining how different people react to certain medications.

• A report in Nature Genetics showed that some women with a particular version of a SNP in the NQO1 are less likely to survive breast cancer after treatment with the commonly used chemotherapeutic epirubicin.
• A study by the SEARCH Collaborative Group found that a version of one SNP is associated with an increased risk for myopathy (muscle pain and/or weakness) in people taking cholesterol-lowering drugs called statins.
• Mayo clinic researchers found that the weight loss drug sibutramine (Meridia) is effective only in people with specific versions of three different genes.
• And just this month we brought you news of three studies that showed that a genetic variant known to affect the metabolism of the anti-clotting drug clopidogrel (Plavix) also affects heart attack patients’ risk of a second major cardiovascular event.

Shared SNPs
Sometimes multiple conditions strike the same person or run in families. Several studies published this year showed that shared genetic risk factors may be part of the reason why.

• Obesity is a known risk factor for many cancers. Researchers found that a variant of adiponectin, a hormone released by fat cells, can increase the risk of developing colorectal cancer.
• Other researchers found variants that affect the risk of developing both type 1 diabetes and celiac disease, two autoimmune diseases that tend to cluster together. One of these shared variants is also associated with HIV resistance.
• Finally, a report published this month in the Proceedings of the National Academy of Sciences showed that a single genetic variant can make a person prone to greater indulgence in both smoking and drinking.

SNPs Associated with Risk Factors for Disease
Several studies this year looked beyond disease itself and instead found associations between SNPs and traits known to be risk factors for disease.

• One study found an association between several SNPs and fasting plasma glucose, a measure of how well a person’s body can control blood sugar levels – a process that goes awry in diabetes.
• Another research group reported SNPs associated with glycated hemoglobin levels, a measure of long-term blood sugar control and another factor associated with the risk of developing diabetes.
• The findings of three papers published in Nature Genetics roughly doubled the number of SNPs associated with blood levels of cholesterol and triglycerides, important risk factors for cardiovascular disease.
• And finally, in a study that looked at behavior instead of metabolic markers, researchers found that a variant in the FTO gene known to increase the risk for obesity affects food choices in children, pushing them towards foods denser in calories.

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