A novel recessive hyperekplexia allele GLRA1 (S231R):: genotyping by MALDI-TOF mass spectrometry and functional characterisation as a determinant of cellular glycine receptor trafficking

Hyperekplexia or startle disease (stiff baby syndrome, STHE) is a hereditary neurological disorder characterised by an exaggerated startle response and infantile muscle hypertonia. Several autosomal dominant and recessive forms of the disorder have been associated with point mutations in GLRA1, the human gene encoding the alpha1 subunit of the inhibitory glycine receptor. Here, we describe a recessive point mutation (C1073G) in exon 7 of GLRA1 leading to an amino acid exchange of serine 231 to arginine in transmembrane region TM1. The mutation was detectable by restriction digest analysis of genomic PCR amplimers by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF-MS). Genotyping of family members was performed using an allele specific primer extension assay in combination with MALDI-TOF-MS and confirmed by conventional DNA sequencing. These studies demonstrate the broad applicability of MALDI-TOF-MS as a comparative screening tool applicable to the analysis of allelic gene variants. In comparison to the wild type all subunit, biochemical, electrophysiological, and confocal microscopy data indicate a reduced integration of functional alpha1(S231R) glycine receptors into the cell surface membrane upon recombinant expression. Apparently, the amino acid exchange S231R influences glycine receptor biogenesis and cellular trafficking by introducing a positive charge into transmembrane region TM1.