SNPwatch: Large Studies Find New Associations for Colorectal Cancer and Type 2 Diabetes

SNPwatch gives you the latest news about research linking various traits and conditions to individual genetic variations. These studies are exciting because they offer a glimpse into how genetics may affect our bodies and health; but in most cases, more work is needed before this research can provide information of value to individuals. For that reason it is important to remember that like all information we provide, the studies we describe in SNPwatch are for research and educational purposes only. SNPwatch is not intended to be a substitute for professional medical advice; you should always seek the advice of your physician or other appropriate healthcare professional with any questions you may have regarding diagnosis, cure, treatment or prevention of any disease or other medical condition.

Three studies published Monday in Nature Genetics report new associations between SNPs and common diseases. Two of the studies find new SNP associations for colorectal cancer, while the third addresses Type 2 Diabetes.

All three studies relied upon very large sample sizes, which increased the chances that the researchers would find SNPs that individually have a very modest effect on the risk of disease, but in the right combinations may greatly increase the odds a person would be affected.

Colorectal Cancer
Colorectal cancer is the third most common cancer (excluding skin cancers) and the second leading cause of cancer-related deaths in the United States. The average lifetime risk of developing colorectal cancer is about 5% in Western European and North American populations.

In Tomlinson et al, researchers from several large consortiums used a multi-phase approach to narrow in on SNPs significantly associated with colorectal cancer. Using three phases that examined about 18,800 people with colorectal cancer and 18,400 controls (all of European ancestry) the scientists identified two new SNPs associated with colorectal cancer – rs10795668 and rs16892766.

When the researchers split the individuals with colorectal cancer into more specific groups, they found some evidence that the riskier version of rs10795668 increased the odds of developing rectal cancer more compared to the odds of developing colon cancer. The effect of rs16892766 was significantly stronger in cases of colorectal cancer that arose before the age of 60.

The second study, by Tenesa et al, also took a multi-phase approach for finding SNPs associated with colorectal cancer. In four phases of testing that examined about 17,500 people with colorectal cancer and 16,400 controls (all of European ancestry except for one group from Japan) they found one new SNP associated with colorectal cancer and replicated two other associations that had been found in previous studies.

In people with European ancestry, the C version of SNP rs3802842 increased the odds of colorectal cancer. In the Japanese population, however, the SNP was not significantly related to the odds of developing colorectal cancer.

But if the researchers took into account whether a patient had rectal or colon cancer (instead of combining the two types of cancer into one category), they found that the C version of rs3802842 did increase the risk for developing rectal cancer in the Japanese population.

The A version rs7014346 and the C version rs4939827 were also found to be associated with an increased risk for colorectal cancer. These two SNPs had been found in previous studies. The riskier versions of both of these SNPs increased the risk for colorectal cancer in both the Japanese and European populations.

Tenesa et al calculated that a person with two copies of the riskier version for each of the three SNPs (rs3802842, rs7014346, and rs4939827) – a situation only expected to happen in 0.5% of the population – would have 2.6 times the odds of developing colorectal cancer as someone with two copies of the less risky version at all three SNPS.

Summary of New Colorectal Cancer Data:
23andMe customers can check out their data at each of the SNPs newly associated with colorectal cancer in the Genome Explorer (now called Browse Raw Data).

In the chart below, each SNP’s rsid number is followed by the version of the SNP that is less prevalent in the population. The two columns labeled “effect” show the change in odds for developing colorectal cancer when either one or two copies of the less prevalent version of the SNP are present compared with the odds of developing the disease when two copies of the other version of the SNP are present. The data in this chart only apply to people with European ancestry.

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(Note that for rs10795668, the less prevalent version actually protects against colorectal cancer – that’s why the change in odds is less than one. For rs4939827 the two versions are about equal in European populations.)

Type 2 Diabetes
Type 2 Diabetes occurs when chronically high blood sugar levels cause a breakdown of the body’s natural response to sugar. It can result in kidney failure, blindness, and circulatory problems that increase the risk of heart attack or stroke. In the United States, almost 21 million children and adults have diabetes, but the rate of new diagnoses is increasing.

Several SNPs have already been associated with Type 2 Diabetes (check out the Gene Journal (now called Health and Traits) article), but the established associations only explain a small proportion of the genetic component of the disease. Studies done to date may have been too small to find SNPs with very modest effects. SNPs with small effects, however, may be key to capturing the contribution that genes make to Type 2 Diabetes and many other common diseases.

Zeggini et al pooled the data from three large studies that previously examined SNPs and Type 2 Diabetes (more than 10,000 people total). The researchers then followed up on their findings in another 57,000 people (about 14,000 with Type 2 Diabetes).

The research replicated several SNPs that were found to be associated with Type 2 Diabetes in previous, smaller studies. But this new study also found six new associations. Two of these are available to 23andMe customers through the Genome Explorer (Browse Raw Data): rs7578597 and rs10923931, which is not included in the 23andMe Personal Genome Service but is equivalent to rs2793831, which is.

In the chart, each SNP’s rsid number is followed by the version of the SNP that is less prevalent in the population. The two columns labeled “effect” show the change in odds for developing Type 2 Diabetes when either one or two copies of the less prevalent version of the SNP are present compared with the odds of developing the disease when two copies of the other version of the SNP are present. So far these data apply only to people of European ancestry.

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