Friedreich's ataxia: a defect in signal transduction?

We have previously assigned the mutation causing Friedreich's ataxia (FRDA) to 9q13 by genetic linkage and fluorescent in situ hybridization analysis, and identified recombination events which position the gene centromeric to D9S5. We report here the extension of a yeast artificial chromosome contig to span the 860 kb interval immediately proximal to this marker, which includes the D9S886 and D9S887/888 loci reported to flank the FRDA locus, and the construction of a high resolution cosmid contig initiated from the D9S888 locus. Exon trapping and cDNA library screening strategies have resulted in the isolation of a candidate gene which traverses the centromeric boundary of the FRDA critical region. The gene spans a genomic interval greater than 220 kb with at least two of the coding exons located proximal to the D9S887/888 loci. Expression is complex, with multiple transcripts detected in a variety of tissues and evidence of alternative splicing and developmental control. The predicted amino acid sequence for the 2.7 kb transcript reported here shows a marked homology to the deduced amino acid sequence of the Saccharomyces cerevisiae MSS4 protein, proposed to function within the phosphoinositide cycle, suggesting a potential role for the human homologue in signal transduction. Whilst no evidence for mutation has been detected in this transcript, the sequence represents only one of the shorter alternatively spliced species identified by Northern analysis and direct sequencing. This gene remains a strong candidate for FRDA.