Genomic deletions of OFD1 account for 23% of oral-facial-digital type 1 syndrome after negative DNA sequencing.

Oral-facial-digital type I syndrome (OFDI) is characterised by an X-linked dominant mode of inheritance with lethality in males. Clinical features include facial dysmorphism with oral, dental and distal abnormalities, polycystic kidney disease and central nervous system malformations. Considerable allelic heterogeneity has been reported within the OFD1 gene, but DNA bi-directional sequencing of the exons and intron-exon boundaries of the OFD1 gene remains negative in more than 20% of cases. We hypothesized that genomic rearrangements could account for the majority of the remaining undiagnosed cases. Thus, we took advantage of two independent available series of patients with OFDI syndrome and negative DNA bi-directional sequencing of the exons and intron-exon boundaries of the OFD1 gene from two different European labs: 13/36 cases from the French lab; 13/95 from the Italian lab. All patients were screened by a semiquantitative fluorescent multiplex method (QFMPSF) and relative quantification by real-time PCR (qPCR). Six OFD1 genomic deletions (exon 5, exons 1-8, exons 1-14, exons 10-11, exons 13-23 and exon 17) were identified, accounting for 5% of OFDI patients and for 23% of patients with negative mutation screening by DNA sequencing. The association of DNA direct sequencing, QFMPSF and qPCR detects OFD1 alteration in up to 85% of patients with a phenotype suggestive of OFDI syndrome. Given the average percentage of large genomic rearrangements (5%), we suggest that dosage methods should be performed in addition to DNA direct sequencing analysis to exclude the involvement of the OFD1 transcript when there are genetic counselling issues.

Novel mutation in the gene encoding c-Abl-binding protein SH3BP2 causes cherubism.

Cherubism is a rare autosomal dominant inherited condition caused by mutations in the c-Abl-binding protein SH3BP2. It is characterized by multiple cystic giant cell lesions of the jaw appearing in early childhood with stabilization and remission after puberty. In the present study, we used direct sequence analysis of the SH3BP2 gene of several individuals from a family with cherubism to search for additional SH3BP2 mutations resulting in cherubism. In affected relatives, we found a previously unreported G to A transition in exon 9 leading to a Gly to Arg substitution at amino acid position 420. G420R has been reported previously with a G to C transversion. To date there have been no disease causing mutations outside exon 9. Therefore, the amino acid sequence from positions 415 to 420 may represent a specific protein domain which, when disrupted, leads to the cherubism phenotype.

Mutations in capillary morphogenesis gene-2 result in the allelic disorders juvenile hyaline fibromatosis and infantile systemic hyalinosis.

Juvenile hyaline fibromatosis (JHF) and infantile systemic hyalinosis (ISH) are autosomal recessive syndromes of unknown etiology characterized by multiple, recurring subcutaneous tumors, gingival hypertrophy, joint contractures, osteolysis, and osteoporosis. Both are believed to be allelic disorders; ISH is distinguished from JHF by its more severe phenotype, which includes hyaline deposits in multiple organs, recurrent infections, and death within the first 2 years of life. Using the previously reported chromosome 4q21 JHF disease locus as a guide for candidate-gene identification, we identified and characterized JHF and ISH disease-causing mutations in the capillary morphogenesis factor-2 gene (CMG2). Although CMG2 encodes a protein upregulated in endothelial cells during capillary formation and was recently shown to function as an anthrax-toxin receptor, its physiologic role is unclear. Two ISH family-specific truncating mutations, E220X and the 1-bp insertion P357insC that results in translation of an out-of-frame stop codon, were generated by site-directed mutagenesis and were shown to delete the CMG-2 transmembrane and/or cytosolic domains, respectively. An ISH compound mutation, I189T, is predicted to create a novel and destabilizing internal cavity within the protein. The JHF family-specific homoallelic missense mutation G105D destabilizes a von Willebrand factor A extracellular domain alpha-helix, whereas the other mutation, L329R, occurs within the transmembrane domain of the protein. Finally, and possibly providing insight into the pathophysiology of these diseases, analysis of fibroblasts derived from patients with JHF or ISH suggests that CMG2 mutations abrogate normal cell interactions with the extracellular matrix.

Charcot-Marie-Tooth disease and related neuropathies: mutation distribution and genotype-phenotype correlation.

Charcot-Marie-Tooth disease (CMT) is a genetically heterogeneous disorder that has been associated with alterations of several proteins: peripheral myelin protein 22, myelin protein zero, connexin 32, early growth response factor 2, periaxin, myotubularin related protein 2, N-myc downstream regulated gene 1 product, neurofilament light chain, and kinesin 1B. To determine the frequency of mutations in these genes among patients with CMT or a related peripheral neuropathy, we identified 153 unrelated patients who enrolled prior to the availability of clinical testing, 79 had a 17p12 duplication (CMT1A duplication), 11 a connexin 32 mutation, 5 a myelin protein zero mutation, 5 a peripheral myelin protein 22 mutation, 1 an early growth response factor 2 mutation, 1 a periaxin mutation, 0 a myotubularin related protein 2 mutation, 1 a neurofilament light chain mutation, and 50 had no identifiable mutation; the N-myc downstream regulated gene 1 and the kinesin 1B gene were not screened for mutations. In the process of screening the above cohort of patients as well as other patients for CMT-causative mutations, we identified several previously unreported mutant alleles: two for connexin 32, three for myelin protein zero, and two for peripheral myelin protein 22. The peripheral myelin protein 22 mutation W28R was associated with CMT1 and profound deafness. One patient with a CMT2 clinical phenotype had three myelin protein zero mutations (I89N+V92M+I162M). Because one-third of the mutations we report arose de novo and thereby caused chronic sporadic neuropathy, we conclude that molecular diagnosis is a necessary adjunct for clinical diagnosis and management of inherited and sporadic neuropathy.