FOunder effect in patients with Unverricht-Lundborg disease on reunion island

Unverricht-Lundborg disease (ULD) is the most frequent form of progressive myoclonus epilepsy. ULD is caused mostly by a homozygous expansion of a dodecamer repeat in the cystatin B gene (CSTB) promoter. We present here a clinical and molecular study of 14 ULD patients originating from Reunion Island, a French island in the Indian Ocean.

These ULD patients were clinically evaluated, and the diagnosis of ULD was confirmed molecularly. We analyzed 12 microsatellites flanking CSTB and estimated the date of introduction of the ULD mutation on Reunion Island.

These cases were clinically very similar, with the typical myoclonus syndrome associated with generalized tonic-clonic seizures, cerebellar involvement and, in some cases, mild mental deterioration. The mean age at onset was 9.6 years (range, 5-14 years), and the mean disease duration was 27 years (range, 5-47 years). The 14 patients harbored the typical ULD mutation, with variable degrees of expansion (mean of 56.3 repeats; range, 49-63). A founder effect was detected, with all but one of the Reunion ULD chromosomes displaying expansions belonging to the same haplotype, 1-1-1-2-6-4-3. We estimated the date of arrival of the most recent common ancestor (MRCA) of these patients on Reunion Island to the middle of the eighteenth century.

These Reunion ULD patients displayed a homogeneous phenotype. Our molecular results are compatible with the instability of the repeat expansion and revealed a founder effect in Reunion ULD patients and the existence of a MRCA about 12 generations ago.