Characterization of a novel candidate gene in Xp22.3 with homology to Drosophila ms13

Abstract

MLS syndrome and orofaciodigital syndrome type I (OFD1), two X-linked dominant, male-lethal disorders featuring brain, eye, kidney and limb malformations, are caused by deletions of Xp22.3 (MLS) or mutations in a closely linked gene (OFD1). Three loci for non-syndromic mental retardation (MRX) were also mapped to the same region. These observations suggest that Xp22.3 may contain gene(s) which are essential for brain, eye, kidney and limb development. In an effort to identify candidate genes in Xp22.3, we have analyzed more than 1 Mb of genomic sequence. Here we report the characterization of a novel gene, named homolog of msl3 (HMSL3), which was identified by sequence analysis of BAC GS-590J6 (GenBank AC004554). BLAST homology searches with this sequence led to the identification of 25 overlapping human EST clones and 2 mouse EST clones. We derived a consensus cDNA sequence containing a 1257 bp ORF, which encodes a 462 amino acid protein. Two distinct regions share 40% amino acid identity with the Drosophila male-specific lethal 3 (Msl3p) protein: a chromo domain and a putative leucine zipper motif at the C terminus. HMSL3 spans 20 Kb of genomic DNA and contains 12 exons transcribed from telomere to centromere. It is expressed as a ubiquitous 2.5 Kb transcript with a 2.8 Kb isoform in skeletal muscle. Drosophila msl3 participates in a sex-specific dosage compensation pathway as part of a multiprotein complex, which binds to and activates transcription from the single X chromosome in males. As a putative chromo domain transcription factor. HMSL3 is an excellent candidate to cause the developmental defects in OFD1 patients, and in patients with MLS-like features who do not have large deletions, as in Aicardi syndrome and Goltz syndrome. Primers were designed to amplify each exon from genomic DNA. 24 Aicardi, 3 Goltz and 2 OFD1 patients were screened for mutations in HMSL3 using heteroduplex analysis and sequencing. To date no mutations have been found. In an effort to obtain full-length cDNAs for HMSL3, 57 positive clones were identified from a human fetal kidney library. Several cDNAs include a novel alternatively spliced exon (1a) with additional homology to Drosophila msl3. Further characterization of the isoforms and additional mutation analysis are in progress.