N-terminal MID1 variants affect brain development
The X-linked form of Opitz BBB/G syndrome (OS) is a monogenic disorder in which symptoms are established early during embryonic development. OS is caused by pathogenic variants in the X-linked gene MID1. Disease-associated variants are distributed across the entire gene locus, with the exception of the N-terminal RING domain that encompasses the E3 ubiquitin ligase activity. By employing genome-edited human induced pluripotent stem cell (hiPSC) lines, we here show that absence of the N-terminal RING domain of MID1 cause severe patterning defects in human brain organoids. We observed a prominent neurogenic deficit with a reduction of neural tissue and a concomitant increase in choroid plexus-like structures. Transcriptome analyses revealed a deregulation of patterning pathways very early on, even preceding neural induction. Notably, the observed phenotypes starkly contrast with  those observed in MID1 full-knockout organoids, indicating the presence of a distinct mechanism that underlies the patterning defects. The severity and early onset of these phenotypes could potentially account for the absence of patients carrying pathogenic variants in exon 1 of the MID1 gene coding for the N-terminal RING domain.