Complicated paroxysmal kinesigenic dyskinesia associated with SACS mutations

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is caused by pathogenic variants in the SACS gene and is characterized by ataxia, peripheral neuropathy, pyramidal impairment and episodic conditions such as epilepsy. Paroxysmal kinesigenic dyskinesia (PKD) had not been previously described in ARSACS.

We analyzed clinical manifestations and performed whole-exome sequencing (WES) in two independent patients with ARSACS and PKD. Both patients' parents were unaffected. Genetic data were filtered for potential pathogenic variants, searching for de novo mutations suggestive of a dominant disease model or homozygous and compound heterozygous variants of a recessive model. Potential mutations that existed in both patients were generated and subjected to Sanger sequencing. The WES results of 163 PKD patients without additional symptoms from previous experiments were also reviewed.

Novel compound heterozygous mutations in the SACS gene were identified in Patient 1 (p.P3007S and p.H3392fs), and a novel homozygous truncating mutation (p.W1376X) was identified in Patient 2. In both patients, each mutant allele was inherited from one of his or her unaffected parents. All 3 mutations were absent in 196 ethnic-matched control chromosomes or in data from the 1000 Genomes Project. No pathogenic variants associated with paroxysmal diseases, especially PKD and episodic ataxia, were identified. In PKD patients without additional symptoms, no homozygous or compound heterozygous variants in the SACS gene were detected.

This study expands the clinical phenotype of ARSACS and suggests the inclusion of SACS screening in patients with PKD plus ARSACS.