New mutations in MID1 provide support for loss of function as the cause of X-linked Opitz syndrome.
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.
Cox
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Department of Molecular Biosciences and ARC Special Research Centre for the Molecular Genetics of Development, Adelaide University, North Terrace, Adelaide, South Australia, Australia 5005. timothy.cox@adelaide.edu.au
Allen
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Cox
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Hopwood
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Goodwin
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Haan
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Suthers
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D000015:Abnormalities, Multiple / Q000235:genetics*; D020816:Amino Acid Motifs; D002467:Cell Nucleus / Q000378:metabolism; D018389:Codon, Nonsense; D003593:Cytoplasm / Q000378:metabolism; D018076:DNA, Complementary; D005091:Exons; D005260:Female; D008040:Genetic Linkage; D006801:Humans; D008297:Male; D008868:Microtubule Proteins; D008870:Microtubules / Q000378:metabolism; D009154:Mutation; D020125:Mutation, Missense; D009687:Nuclear Proteins; D016366:Open Reading Frames; D010375:Pedigree; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D013577:Syndrome; D014157:Transcription Factors / Q000235:genetics* / Q000378:metabolism / Q000502:physiology*; D044767:Ubiquitin-Protein Ligases; D014960:X Chromosome; D016335:Zinc Fingers
D018389:Codon, Nonsense; D018076:DNA, Complementary; D008868:Microtubule Proteins; D009687:Nuclear Proteins; D011993:Recombinant Fusion Proteins; D014157:Transcription Factors; C109052:MID1 protein, human; D044767:Ubiquitin-Protein Ligases