New mutations in MID1 provide support for loss of function as the cause of X-linked Opitz syndrome

• Published in: Human molecular genetics 2000-10, Vol.9 (17), p.2553-2562
• Main Authors: COX, Timothy C, ALLEN, Lillian R, COX, Liza L, HOPWOOD, Blair, GOODWIN, Bruce, HAAN, Eric, SUTHERS, Graeme K
• Format: Article
• Language: English
• Subjects: Abnormalities, Multiple - genetics; Amino Acid Motifs; Biological and medical sciences; Cell Nucleus - metabolism; Codon, Nonsense; Complex syndromes; Cytoplasm - metabolism; DNA, Complementary; Exons; Female; Genetic Linkage; Humans; Male; Medical genetics; Medical sciences; Microtubule Proteins; Microtubules - metabolism; MID1 gene; MID1 protein; Mutation; Mutation, Missense; Nuclear Proteins; Open Reading Frames; Opitz syndrome; Pedigree; Recombinant Fusion Proteins - metabolism; Syndrome; Transcription Factors - genetics; Transcription Factors - metabolism; Transcription Factors - physiology; Ubiquitin-Protein Ligases; X Chromosome; Zinc Fingers
• ISSN: 0964-6906; 1460-2083; 1460-2083
• DOI: 10.1093/hmg/9.17.2553

Opitz syndrome (OS) is a genetically heterogeneous malformation disorder. Patients with OS may present with a variable array of malformations that are indicative of a disturbance of the primary midline developmental field. Mutations in the C-terminal half of MID1, an RBCC (RING, B-box and coiled-coil) protein, have recently been shown to underlie the X-linked form of OS. Here we show that the MID1 gene spans at least 400 kb, almost twice the distance originally reported and has a minimum of six mRNA isoforms as a result of the alternative use of 5' untranslated exons. In addition, our detailed mutational analysis of MID1 in a cohort of 15 patients with OS has resulted in the identification of seven novel mutations, two of which disrupt the N-terminus of the protein. The most severe of these (E115X) is predicted to truncate the protein before the B-box motifs. In a separate patient, a missense change (L626P) was found that also represents the most C-terminal alteration reported to date. As noted with other C-terminal mutations, GFP fusion constructs demonstrated that the L626P mutant formed cytoplasmic clumps in contrast to the microtubular distribution seen with the wild-type sequence. Notably, however, both N-terminal mutants showed no evidence of cytoplasmic aggregation, inferring that this feature is not pathognomonic for X-linked OS. These new data and the finding of linkage to MID1 in the absence of a demonstrable open reading frame mutation in a further family support the conclusion that X-linked OS results from loss of function of MID1.