Mar 22 2004
International research teams studying two distinct populations have found variants in a gene that may predispose people to type 2 diabetes, the most common form of the disease. The researchers, who collaborated extensively in their work, report their findings in companion articles in the April issue of Diabetes.
"This is an outstanding example of how scientists are using the tools of modern biology to understand the causes of our nation's most common - and most devastating - diseases," said Dr. Elias A. Zerhouni, director of the National Institutes of Health (NIH). "As researchers continue to build upon the foundation laid by the Human Genome Project, we can expect even swifter progress in our effort to understand, treat and eventually prevent many complex conditions such as diabetes, heart disease and mental illness."
Homing in on a wide stretch of chromosome 20 - flagged by earlier studies as a likely location for a type 2 diabetes susceptibility gene - the teams identified four genetic variants, called single nucleotide polymorphisms (SNPs), which are strongly associated with type 2 diabetes in Finnish and Ashkenazi Jewish populations.
All four SNPs cluster in the regulatory region of a single gene, hepatocyte nuclear factor 4 alpha (HNF4A), a transcription factor that acts as a "master switch" regulating the expression of hundreds of other genes. HNF4A turns genes on and off in many tissues, including the liver and pancreas. In the beta cells of the pancreas, it influences the secretion of insulin in response to glucose.
"It's a nice coalescence of findings," said Dr. Francis S. Collins, director of the National Human Genome Research Institute (NHGRI) and senior author of the article describing the Finnish study results. "What we found is a common variation in this gene. If you have this variation, it appears to raise your risk of type 2 diabetes about 30 percent. The variation isn't going to cause diabetes unless you have it in combination with other, yet-to-be-identified genetic susceptibility factors, together with certain environmental influences such as obesity and lack of physical exercise."
Translating the discovery into a treatment that benefits people with diabetes or those at risk is still years away. "We need to learn much more about this gene and how to modulate its function," Dr. Collins cautioned.
The Finland-United States Investigation of NIDDM Genetics (FUSION) study is led by Dr. Michael Boehnke at the University of Michigan; Drs. Jaakko Tuomilehto and Timo T. Valle at the National Public Health Institute in Helsinki; Dr. Richard N. Bergman at the University of Southern California; and Dr. Collins at NHGRI. Drs. Kaisa Silander and Karen L. Mohlke, both of NHGRI, were co-lead authors on the study. The team examined polymorphisms in 793 Finnish adults diagnosed with typical type 2 diabetes - formerly known as adult-onset or non-insulin dependent diabetes mellitus(NIDDM)) - and 413 non-diabetic controls. The researchers identified a total of 10 SNPs within and near the HNF4A gene that are associated with type 2 diabetes in the Finnish population. The most significant results were found in a region of DNA (called the "promoter") that regulates the gene's expression in the insulin-secreting cells of the pancreas. Individuals who inherited the risk variant tended to have higher levels of blood glucose at fasting and two hours after a glucose challenge.
The other international team of researchers, led by Dr. M. Alan Permutt of the Washington University School of Medicine in St. Louis, studied 100 SNPs in 275 Ashkenazi Jewish adults in Israel with type 2 diabetes and 342 non-diabetic controls. They found diabetes-related associations for SNPs in the same region of HNF4A.
"We believe these four variants are marking a regulatory region that determines the level of expression of HNF4A," Dr. Permutt said. "We're now looking to see if this region of DNA is affecting gene expression in some way."
An NIH-funded study in the Feb. 27, 2004, issue of Science suggests how polymorphisms in the HNF4A promoter might confer susceptibility to type 2 diabetes. In that work, Dr. Richard A. Young and coworkers at the Whitehead Institute for Biomedical Research in Cambridge, Mass., examined the genes regulated by several HNF transcription factors. They found HNF4A to be a highly active transcription factor, regulating a surprising number of beta cell and liver cell genes in humans. A misstep in the binding site for other transcription factors in the HNF4A promoter, they conclude, could result in "misregulation of HNF4A expression and thus its downstream targets, leading to beta cell malfunction and diabetes."
"The observations made by these groups mark a real leap forward in our understanding of the genetics of diabetes and may provide a blueprint for finding genes in other complex diseases," said Dr. Catherine McKeon of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), which funded the three studies. "The work builds on a solid foundation of advances in basic cell biology, studies in human populations and new gene-scanning technologies."
Other groups still need to confirm their results, Drs. Collins and Permutt stress. In fact, scientists are already looking for the variants in other populations. "We've e-mailed our findings in precise detail to scientists in the International Type 2 Diabetes Linkage Analysis Consortium," said Dr. Boehnke, who co-leads the NIDDK-funded consortium, a group of scientists around the world who are mapping type 2 diabetes susceptibility genes. "This approach will substantially increase the speed with which we see our results confirmed or contradicted," he added. Other studies will see if people with the risk variants have signs of beta cell impairment and will look at HNF4A's function in animals.
For years, scientists have known that single-gene mutations, most affecting beta cell function, contribute to rare forms of diabetes, including the six types of Maturity Onset Diabetes of the Young or MODY. Such mutations account for about 2 to 3 percent of all diabetes cases. A mutation in the coding region of HNF4A causes MODY type 1, a rare form of diabetes that begins before age 25 in people of normal weight.
Unlike MODY, however, type 2 diabetes usually begins after age 40 in overweight, inactive people and is more common in those with a family history of diabetes. In the United States, type 2 diabetes disproportionately affects African Americans, Hispanic/Latino Americans, and American Indians. Affecting about 17 million people nationwide, this form of diabetes accounts for 90 to 95 percent of all diabetes cases in the United States. Its prevalence has been steadily rising in the past 30 years, and it is increasingly being seen in younger people, even in children. Hallmarks of the disease are insulin resistance - the inability of target tissues to respond to insulin - and a gradual failure of beta cells to produce enough insulin.
Scientists have come a long way in understanding the basis for diseases arising from single-gene mutations. Understanding the genetic basis of the more common, polygenic diseases such as diabetes has been much more difficult.
"We could have scoured the coding region of every gene on chromosome 20 by sequencing it in many patients and not come up with anything," Dr. Permutt noted. "That's because the problem isn't in the gene itself but in a regulatory region far from the gene. This study suggests that perhaps the level of expression of candidate genes will expand our understanding of genetic risk factors. It also shows us that if we scrutinize the gene itself from beginning to end, we haven't done a complete job. We need to look at polymorphic markers surrounding the gene for differences in those markers in cases and controls." A major new research project, the International HapMap Project, is developing a map of human variation across all of the chromosomes, and will significantly accelerate the achievement of this goal.
The FUSION study was conducted by researchers from the National Human Genome Research Institute; University of Michigan; University of Southern California; National Public Health Institute in Finland; and The Wellcome Trust Sanger Institute in the United Kingdom. The study was funded by NHGRI and NIDDK. The Academy of Finland, Burroughs Wellcome and the American Diabetes Association provided additional support.
Dr. Latisha D. Love-Gregory from Washington University was the lead author on the study in the Ashkenazi Jewish population, which was conducted by researchers from Washington University and The Hadassah-Hebrew University in Israel. This study was funded by the NIDDK.
The study in Science by Dr. Duncan T. Odom and coworkers was funded by NHGRI, NIDDK and the National Institute of General Medical Sciences (NIGMS), all institutes of the National Institutes of Health under the Department of Health and Human Services. The study also received support from the Howard Hughes Medical Institute, the Juvenile Diabetes Research Foundation International, and Sloan Foundation/U.S. Department of Energy Program for Computational Molecular Biology.
The American Diabetes Association, which publishes the journal Diabetes, lifted the normal embargo on the papers. All other reports in the April issue of the journal remain under the normal embargo. Full text of the papers can be found at the Web site for the journal Diabetes.