• DHPLC, denaturing high performance liquid chromatography

Opitz syndrome (G/BBB syndrome, MIM145410 and MIM300000) is a midline congenital malformation characterised by hypertelorism, hypospadias and oesophagolaryngotracheal defects leading to swallowing difficulties and a hoarse cry.¹ Additional defects include cleft lip with or without cleft palate, imperforate anus, anomalies of the central nervous system (including corpus callosum agenesis or vermis agenesis and hypoplasia),² congenital heart defects (atrial and ventricular septal defects, patent ductus arteriosus and coarctation of the aorta),³ and developmental delay in two thirds of patients. This condition is genetically heterogeneous with an X-linked recessive form mapped to Xp22.3 and at least one autosomal dominant form mapped to chromosome 22q11.2.⁴ Also, several patients with an autosomal Opitz syndrome have been reported with a 22q11 deletion.^5,6 Recently, mutations in MID1, a member of the B-box protein family have been identified in the X-linked form of the disease⁷ but the gene for the autosomal dominant form on 22q11 remains unknown.

MID1 encodes a protein belonging to a novel subclass of RING, B-box, Coiled-Coil proteins characterised by a fibronectin type III motif and a C-terminal domain. Although the function of MID1 remains unknown, recent experiments have demonstrated that MID1 is a microtubule associated protein, belonging to a large multiprotein complex^8,9 involved in ubiquitination through microtubules.¹⁰ MID1 association with microtubules is regulated by dynamic phosphorylation involving MAP kinase and protein phosphatase 2A that is targeted specifically to MID1 by a regulatory α4 subunit.

Here, we report on six MID1 mutations in a cohort of 14 patients with Opitz syndrome and on heart and hindbrain expression of MID1 during early human development using mRNA in situ hybridisation. In addition, we investigate the contribution of chromosome X-inactivation studies to identify the X-linked form of the disease.