Embryonic expression of the human MID1 gene and its mutations in Opitz syndrome

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 hoarse voice. 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. Recently, mutations in MID1 have been identified in the X-linked form of the disease but the gene for the autosomal dominant form on 22q11 remains unknown. Here we report on MID1 mutations screening in a series of 14 patients with Opitz syndrome and the MID1 expression pattern in human embryos using hybridisation in situ. Finally, we investigated the contribution of chromosome X-inactivation studies to identify the X-linked form of the disease. Six MID1 mutations were identified in our series. All mutations were novel except the R495X mutation previously reported in three unrelated patients. We report heart and hindbrain expression of MID1 during early human development. Obligate carrier mothers showed a random pattern of X-inactivation. Vermis hypoplasia or agenesis was frequently present (4/9) in patients with MID1 mutation. The heart and hindbrain expression of MID1 during early human development further supports the view that heart defects and vermis hypoplasia or agenesis are features to be included in the malformative spectrum of the syndrome. Finally, the study of X-inactivation pattern in women does not help discrimination between X-linked and autosomal forms of the disease.