Abstracts

Rationale: Progressive Myoclonus Epilepsies (PMEs) are severe intractable neurodegenerative epilepsies that afflict patients of all ages, but more commonly adolescents, and are the main differential diagnosis of Juvenile Myoclonic Epilepsy. Genetic diagnosis is available for many teenage-onset PMEs, such as EPM1, EPM2A and EPM2B, NEU1, SCARB2 and several mitochondrial genes. However, some forms of PME may only be diagnosed through brain biopsy. We report a multiplex family with autosomal recessive teenage-onset PME that had remained undiagnosed despite extensive genetic and pathological testing. Methods: The proband is a 24 year-old Pashtun Afghani man. Spontaneous and stimulus-induced myoclonus began at age 15. Generalized tonic-clonic convulsions are controlled with valproate. Cognitive decline appeared. Examination shows generalized hyperreflexia, mild ataxia, and stimulus-sensitive multifocal myoclonus. Two older cousins (Fig. 1A) had had the same disease. Both had died at around age 30. None of the three patients suffered visual decline. Investigations in all three showed cortical myoclonus and multifocal epileptiform discharges, and generalized brain atrophy. There were no urinary oligosaccharides (sialidosis). Skin and muscle biopsy and genetic testing for Lafora disease, Unverricht-Lundborg disease, MERRF and NCL were negative. Rectal and brain biopsies or autopsy were declined. We used whole exome sequencing (WES) combined with homozygosity mapping using a microarray-based SNP assay and genotyped over 600,000 SNPs in the affected proband and his male affected cousin. Results: SNP microarray studies revealed 2 regions of shared homozygozity in the patients' two genomes: one in chromosome 10 and another in chromosome 15. Of the 41,597 variants found in the WES only two were exonic, novel, nonsynonymous, and located within the shared homozygous regions (Fig. 1C). One of these variants was in the gene encoding the NYD-SP5 testis development protein, which we did not pursue further. The other was a homozygous sequence change (c.768C>G) in the CLN6 gene, which substitutes the protein product's aspartic acid residue at position 256 with glutamate (D256E). Aspartate in this position is conserved throughout studied evolution (Fig. 1D). The CLN6 (c.768C>G) variant segregates with the disease in the pedigree (Fig. 1A). Sequencing for it in 268 control individuals, including 176 Pashtun, revealed it to be present in a single Pashtun individual, in heterozygote state. Conclusions: The affected gene is the CLN6 gene previously known to underlie variant late-infantile and adult-onset neuronal ceroid lipofuscinoses. Combined with other recent work (1) our results add CLN6 to the genes mutations of which cause teenage-onset PME, expand the group of teenage-onset PME patients who can be diagnosed by genetic testing, and extend the clinical spectrum of CLN6 mutations to include teenage-onset PME. This work also exemplifies the potentiality of next generation sequencing in gene identification and in the diagnosis of patients with neurological diseases of unknown cause.