The Cellular Function Of MID2 And Its Potential Role In The Pathogenesis Of Opitz Syndrome

MID2 is a homologue of MID1, the causative gene of Opitz syndrome(OS). MID1 and MID2 are similar at both nucleic acid level (70%) and protein level (77%). MIDl and MID2 displayed overlapping cellular functions, eg, both interacted with alpha4, the regulatory subunit of phosphotase 2A. Also, experimental data supported that the homo-/hetero-dimerization of these two proteins was necessary for their cellular functions such as microtubule-association. According to these data, we proposed that MID2 could be a potential OS-related gene or a modifying gene in some Opitz patients. In this study, we found a MID2 mutation (a C＞Atransition at the 1073 nucleotide), which resulted in a missense mutation (from 358Alanine to 358Aspartie acid), in three unrelated OS patients that had no detectable mutation in MID1. The same mutation was not found in a screen of 200 normal controls using site-specific PCR. It suggested that the MID2A358D is OS-related.To further address the function of MID2A358D, a yeast-two hybrid system and immunofluorescence were used to study whether MID2A358D demonstrate any functional abnormalies as to its cellular localization and protein-protein interactions, comparing to wildtype MID2. Given that the 358 alanine locates at the C-teminal to the coiled-coil domain, which is responsible for MID1/MID2 homo/hetero-dimerization and microtubule association, the Coiled-Coil domain (205aa-365aa) was cloned and its ability ofdimerization was tested in the same yeast two-hybrid system. PEPP2, a PH domain containing protein with unknown function that were identified as a interactor of MID2, was shown to also interact MIDl. It provided further evidence for a related function for MID1 and MID2. Although no diseernable differences on protein-protein interactions were found for MID2 and MID2A358D in our yeast two-hybrid system, the cellular function of MID2A358D and its role in OS pathogenesis awaited for further studies. Due to the structural similarities and the closely related functions, we propesed that MID2 is a potential modifying factors for MID1 and in some cases may even contribute to the pathogenesis of OS.