Identification of a Novel Homozygous Splice-Site Mutation in SCARB2 that Causes Progressive Myoclonus Epilepsy with or without Renal Failure

Abstract

Background: Progressive myoclonus epilepsies (PMEs) comprise a group of rare genetic disorders characterized by action myoclonus, epileptic seizures, and ataxia with progressive neurologic decline. Due to clinical and genetic heterogeneity of PMEs, it is difficult to decide which genes are affected. The aim of this study was to report an action myoclonus with or without renal failure syndrome (EPM4) family and summarize the clinical and genetic characteristics of all reported EPM4 patients.

Methods: In the present study, targeted next-generation sequencing (NGS) was applied to screen causative genes in a Chinese PME family. The candidate variant was further confirmed by cosegregation analysis and further functional analysis, including the reverse transcription polymerase chain reaction and Western blot of the proband's muscle. Moreover, literature data on the clinical and mutational features of all reported EPM4 patients were reviewed.

Results: The gene analysis revealed a novel homozygous splicing mutation (c.995-1G>A) of the SCARB2 gene in two brothers. Further functional analysis revealed that this mutation led to loss function of the SCARB2 protein. The classification of the candidate variant, according to the American College of Medical Genetics and Genomics standards and guidelines and functional analysis, was pathogenic. Therefore, these two brothers were finally diagnostically confirmed as EPM4.

Conclusions: These present results suggest the potential for targeted NGS to conduct a more rapid and precise diagnosis for PME patients. A literature review revealed that mutations in the different functional domains of SCARB2 appear to be associated with the phenotype of EPM4.

Keywords: Gene; Progressive Myoclonus Epilepsies; Progressive Myoclonus Epilepsy with or without Renal Failure; SCARB2; Targeted Next-Generation Sequencing.

Figure 1

The auxiliary examinations of the proband. (a) EEG revealed multifocal spike and wave complexes, especially in the left parietal lobe, occipital lobe, and temporal lobe. (b) Axial brain MRI revealed mild cerebellar atrophy. (c) Transverse MRI scan revealed mild cerebellar atrophy. (d) Muscle biopsy presented with certain muscle atrophy and other less obvious signs (H and E, original magnification ×100). MRI: Magnetic resonance imaging; EEG: Electroencephalography.

Figure 2

Sanger sequencing of the family with progressive myoclonus epilepsies. The two brothers were tested for the homozygous splice mutation (c.995-1G>A) of the SCARB2 gene. Then, their parents were tested for heterozygous mutations of the SCARB2 gene. The arrow indicated the homozygous splice mutation (c.995-1G>A).

Figure 3

SCARB2 gene expression and protein expression analysis of the proband. (a) Sanger sequencing to the cDNA of the SCARB2 gene: a c. 995-1036del42 mutation was observed in the proband. (b) Western blot analysis of the protein obtained from the muscle of proband and controls. Compared to the full length of the 72,000 of SCARB2 protein in healthy controls, the expression of the truncated SCARB2 protein that weighted from 43,000 to 55,000 significantly decreased (t = 2.887, P = 0.0447). The arrow indicated a 43,000-protein band appeared in patients but a 72,000 band in control.

Figure 4

The structure of the SCARB2 gene and the reported mutation of SCARB2. GBA: Beta-glucocerebrosidase; TM: Transmembrane.