The two SNPs, located on chromosomes 9q22 and 14q13, are the first common variants replicated in multiple populations ever associated with increased risk of the disease. They were found through the analysis of the genomes of a total of more than 40,000 patients and control subjects from Iceland, the United States and Spain. Approximately 4 percent of people of European descent carry two copies of the at-risk versions of both SNPs, putting them at a 5.7-times greater risk of thyroid cancer than individuals who carry no copies of either. These variants contribute to an estimated 57 percent of all cases of the disease, and they associate with altered levels of key thyroid hormones.

"This is an important discovery with a clear medical utility. Thyroid cancer is the most common endocrine cancer, and about 1 percent of the general population will develop it at some point in their lifetime. But if detected early enough it is in general a treatable disease. Screening for the at-risk SNPs may therefore provide a new means of identifying those who are at highest risk, enabling closer monitoring of those individuals with an emphasis on addressing other risk factors and promoting early intervention if cancer is detected. We are already including these risk variants in our deCODEme full genome scan and deCODEme Cancer scan, and are analyzing the possibility of putting them into a reference laboratory diagnostic test," said Kari Stefansson, CEO of deCODE and senior author on the study.

The paper, "Common variants on 9q22.33 and 14q13.3 predispose to thyroid cancer in European populations," is published today in the online edition of Nature Genetics, at nature/ng.

deCODE would like to thank all those who participated in this study, as well as the collaborating clinicians and scientists from the National- University Hospital in Reykjavik, Ohio State University and the University of Zaragoza. This project was funded in part by the US National Institutes of Health under contracts CA16058 and CA124570.

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The authors hypothesise that this gene-environment interaction may affect the ability of the thymus, a key component of the immune system, to perform its regular tasks. The thymus produces an army of T cells, which identify invading pathogens, such as bacteria and viruses, and attack and destroy them. There are millions of different T cells, each designed to recognise a specific pathogen, but there is a risk that one type might mistakenly identify one of the body's own cells or proteins.

Ordinarily, the thymus will regulate the T cells and delete those that pose the greatest risk of attacking the body's own cells and proteins. However, the researchers believe that in people who carry the variant, a lack of vitamin D during early life might impair the ability of the thymus to delete these T cells, which then go on to attack the body, leading to a loss of myelin on the nerve fibres.

"Our study implies that taking vitamin D supplements during pregnancy and the early years may reduce the risk of a child developing MS in later life," says lead author Dr Sreeram Ramagopalan. "Vitamin D is a safe and relatively cheap supplement with substantial potential health benefits. There is accumulating evidence that it can reduce the risk of developing cancer and offer protection from other autoimmune diseases."

The research has been welcomed by Simon Gillespie, Chief Executive of the MS Society (UK).

"These remarkable results tie together leading theories about the environment, genes and MS but they are only part of the jigsaw," says Mr Gillespie. "This discovery opens up new avenues of MS research and future experiments will help put the pieces together."

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