Human and murine FMR-1: alternative splicing and translational initiation downstream of the CGG-repeat.

Fragile X syndrome is associated with massive expansion of a CGG trinucleotide repeat within the FMR-1 gene and transcriptional silencing of the gene due to abnormal methylation. Partial cDNA sequence of the human FMR-1 has been reported. We report here the isolation and characterization of cDNA clones encoding the murine homologue, fmr-1, which exhibit marked sequence identity with the human gene, including the conservation of the CGG repeat. A conserved ATG downstream of the CGG repeat in human and mouse and an in-frame stop codon in other human 5' cDNA sequences demarcate the FMR-1 coding region and confine the CGG repeat to the 5' untranslated region. We also present evidence for alternative splicing of the FMR-1 gene in mouse and human brain and show that one of these splicing events alters the FMR-1 reading frame, predicting isoforms with novel carboxy termini.

Human genes containing polymorphic trinucleotide repeats.

Expansions of trinucleotide repeats within gene transcripts are responsible for fragile X syndrome, myotonic dystrophy and spinal and bulbar muscular atrophy. To identify other human genes with similar features as candidates for triplet repeat expansion mutations, we screened human cDNA libraries with repeat probes and searched databases for transcribed genes with repeats. From both strategies, 40 genes were identified and 14 characterized. Five were found to contain repeats which are highly polymorphic including the N-cadherin, BCR, glutathione-S-transferase and Na+/K+ ATPase (beta-subunit) genes. These data demonstrate the occurrence of other human loci which may undergo this novel mechanism of mutagenesis giving rise to genetic disease.