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A genome wide association study with two replication studies in populations from Iceland and the United States found a genetic sequence variant which is associated with periodic limb movements—a disorder in which recurrent dorsiflexion of the foot and lower leg during sleep occurs. The disorder often accompanies restless legs syndrome, a condition characterised by an intensely uncomfortable urge to move the legs, which is a major disruptor of sleepsleep.
A sequence variant in the intron of BTBD9 on chromosome 6p21.2 was found more often in people with restless legs syndrome and their relatives than in controls without the disorder. However, the association was strongest in people with periodic limb movements who didn't have restless legs syndrome, while no association was found in people with restless legs syndrome and no periodic limb movements.
A linked editorial (p 703) suggests this may identify periodic limb movements as a heritable biomarker, or endophenotype, for restless legs syndrome. The concept is increasingly used to help in understanding conditions with complex heritability, by dissecting them into simpler phenomena. The simpler pathophysiological and genetic characteristics of endophenotypes should help to understand the aetiology of the wider disorders of which they are markers and help find effective treatments.