Pathogenic variants in TMEM184B cause a neurodevelopmental syndrome associated with alteration of metabolic signaling
Transmembrane protein 184B (TMEM184B) is an endosomal 7-pass transmembrane protein with evolutionarily conserved roles in synaptic structure and axon degeneration. We report six pediatric cases who have de novo heterozygous variants in TMEM184B. All individuals harbor rare missense or mRNA splicing changes and have neurodevelopmental deficits including developmental delay, corpus callosum hypoplasia, seizures, and/or microcephaly. TMEM184B is predicted to contain a pore domain, wherein many human disease-associated variants cluster. Structural modeling suggests that all missense variants alter TMEM184B protein stability. To understand the contribution of TMEM184B to neural development in vivo, we suppressed the TMEM184B ortholog in zebrafish and observed microcephaly and reduced anterior commissural neurons, aligning with symptoms of affected individuals. Ectopic TMEM184B expression resulted in dominant effects for Lys184Glu and Gly162Arg. However, in vivo complementation studies demonstrate that all other variants tested result in diminished protein function and indicate a haploinsufficiency basis for disease. Human induced pluripotent stem cells (iPSC) with monoallelic expression of Lys184Glu show mRNA disruptions in key metabolic pathways and molecules including those controlling mechanistic target of rapamycin (mTOR) activity. Expression of Lys184Glu and other variants increased apoptosis in cell lines and altered nuclear localization of transcription factor EB (TFEB), a master regulator of lysosomal biogenesis, further supporting disruptions to nutrient signaling pathways. Together, our data indicate that TMEM184B variants cause cellular metabolic disruption likely through divergent molecular effects that all result in abnormal neural development. Overall design: RNAseq profiling of induced pluripotent stem cells (iPSCs) to determine mRNA levels of TMEM184b and expression of transcripts related to metabolism and cellular pH. The assessed cell lines belonged to one of three conditions, each containing four biological replicates: wild type (WT), Lys184Glu/+, or Lys184Glu/Lys184Glu. Non-WT samples exhibited either a heterozygous or homozygous mutation to TMEM184b that mimic mutations observed in human patients.