Restless Leg Linked to Chromosome Six -- At Least

REYKJAVIK, Iceland, July 18 -- A major symptom of restless legs syndrome has been associated with a genetic variation on chromosome six, two research groups reported.

The variation has been linked to about half the risk of one of the major symptoms of restless legs syndrome, defined as the periodic limb movements in sleep that are commonly but not exclusively seen in RLS, according to Kari Stefansson, M.D., Ph.D., of deCODE Genetics here.

Using genome-wide association scanning to study patients with RLS, Dr. Stefansson and colleagues found that a genetic variation called rs3923809 accounts for about half the population attributable risk of having periodic limb movements in sleep.

On the other hand, it was not linked to RLS if patients did not also have the periodic motions when they were asleep, he and colleagues reported online in the New England Journal of Medicine.

The study is one of three reports in the journal and in Nature Genetics that detailed results of genome-wide association scanning as a method of linking genetics to disease.

Researchers led by Thomas Meitinger, M.D., of the Institute of Human genetics in Munch, Germany, reported in Nature Genetics that they had used the same method to study RLS and come up with complementary results.

But in addition to the chromosome six result, Dr. Metinger's group linked two other genetic regions to the risk of RLS.

In genome-wide association scanning, researchers use data on genetic variation - so-called single nucleotide polymorphisms, or SNPs -- amassed by the Human Genome Project and the international HapMap collaboration.

Then, using microarrays that recognize up to 500,000 SNPs, they look for polymorphisms that are more common in people with a disease than in healthy controls.

In a discovery phase look at RLS, Dr. Stefansson and colleagues first tested 306 Icelanders who had RLS with periodic limb movements in sleep, and compared their genetics with 15,664 healthy controls.

That analysis led to a single genetic variation in an intron of the BTB (POZ) domain-containing 9 (BTBD9) gene, which is expressed in many tissues, but whose functions are not well known.

The researchers then replicated the findings using two other cohorts - one in Iceland with 123 cases and 1,233 controls and one in the U.S. with 188 cases subjects and 662 controls.

All the cases had RLS with periodic limb movements in sleep, and carrying the variant rs3923809 was significantly associated with the condition, Dr. Stefansson and colleagues found.

Indeed, for the groups combined, the odds ratio for the condition was 1.7 for those carrying the variant, which was significant at P=3X10^-14.

But when the researchers looked at people who had RLS without the movements in sleep, there was no association with the variant. On the other hand, the risk remained for those with movements but without the other symptoms of RLS, the researchers said.

In other words, they said, "we have identified a genetic determinant of periodic limb movements in sleep" but further research is needed.

Dr. Meitinger's group, on the other hand, did not make the same distinction, but also found a link between RLS and chromosome six. In addition, they found links between the condition and variation in regions on chromosomes two and 15.

The region they cited on chromosome six contains the variant rs3923809, as well as several other polymorphisms in the BTDB9 gene.

On chromosome two, variations in the homeobox gene MEIS1 were associated with RLS, while on chromosome 15, the variation was found in the genes encoding mitogen-activated protein kinase MAP2K5 and the transcription factor LBXCOR1.

As in the other study, the researchers began with a discovery case-control cohort. There were 401 patients with familial RLS and 1,644 healthy controls.

Analysis of their genetics showed 28 single nucleotide polymorphisms in six regions that were possibly linked to the condition, Dr. Meitinger and colleagues said.

The researchers then tried to replicate the findings in two separate cohorts - one in Germany with 901 patients with either familial or sporadic RLS and 891 controls and the other in Canada with 255 cases and 287 controls.

Analysis of the first group reduced the significant regions to the final three, which was confirmed in the second group, the researchers said.

"A major proportion of the risk for RLS is explained by variants in the loci identified," the researchers said.

The Icelandic finding is "exciting and important," according to sleep researcher John Winkelman, M.D., Ph.D., of Brigham and Women's Hospital, and Harvard Medical School, while the Nature Genetics report "makes this finding even more secure."

Writing in an accompanying editorial, Dr. Winkelman noted that the Icelanders appear to have found a gene for periodic limb movements in sleep, rather than RLS itself.

But because the German researchers did not make that distinction "it is unclear whether the other sequence variants they found are truly for RLS, for periodic limb movements in sleep, or for some other RLS marker," Dr. Winkelman said.

But he added that the finding "offers hope" that better understanding of the syndrome's pathophysiology will lead to better treatment.

The Nature Genetics study was supported by the National Genome Research Network, the German Federal Ministry of Education and Research, the State of Bavaria, the German RLS patient organization, the Bavarian Ministry of Science, Culture and Art, and the Canadian Institutes of Health Research. Several of the authors, including Dr. Meitinger, have filed a patent application.

Dr. Winkelman reports financial links with Boehringer Ingelheim, GlaxoSmithKline, and Schwarz Pharma.