Identification of the first gene (FRG1) from the FSHD region on human chromosome 4q35.

Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant, neuromuscular disorder characterized by progressive weakness of muscles in the face, shoulder and upper arm. Deletion of integral copies of a 3.3 kb repeated unit from the subtelomeric region on chromosome 4q35 has been shown to be associated with FSHD. These repeated units which are apparently not transcribed, map very close to the 4q telomere and belong to a 3.3 kb repeat family dispersed over heterochromatic regions of the genome. Hence, position effect variegation (PEV), inducing allele-specific transcriptional repression of a gene located more centromeric, has been postulated as the underlying genetic mechanism of FSHD. This hypothesis has directed the search for the FSHD gene to the region centromeric to the repeated units. A CpG island was identified and found to be associated with the 5' untranslated region of a novel human gene, FRG1 (FSHD Region Gene 1). This evolutionary conserved gene is located about 100 kb proximal to the repeated units and belongs to a multigene family with FRG1 related sequences on multiple chromosomes. The mature chromosome 4 FRG1 transcript is 1042 bp in length and contains nine exons which encode a putative protein of 258 amino acid residues. Transcription of FRG1 was detected in several human tissues including placenta, lymphocytes, brain and muscle. To investigate a possible PEV mechanism, allele-specific FRG1 steady-state transcript levels were determined using RNA-based single-strand conformation polymorphism (SSCP) analysis. A polymorphic fragment contained within the first exon of FRG1 was amplified from reverse transcribed RNA from lymphocytes and muscle biopsies of patients and controls. No evidence for PEV mediated repression of allelic transcription was obtained in these tissues. However, detection of PEV in FSHD patients may require analysis of more specific cell types at particular developmental stages.