👤 Gaia Novarino

Autism spectrum disorders are a genetically heterogeneous constellation of syndromes characterized by impairments in reciprocal social interaction. Available somatic treatments have limited efficacy. We have identified inactivating mutations in the gene BCKDK (Branched Chain Ketoacid Dehydrogenase Kinase) in consanguineous families with autism, epilepsy, and intellectual disability. The encoded protein is responsible for phosphorylation-mediated inactivation of the E1α subunit of branched-chain ketoacid dehydrogenase (BCKDH). Patients with homozygous BCKDK mutations display reductions in BCKDK messenger RNA and protein, E1α phosphorylation, and plasma branched-chain amino acids. Bckdk knockout mice show abnormal brain amino acid profiles and neurobehavioral deficits that respond to dietary supplementation. Thus, autism presenting with intellectual disability and epilepsy caused by BCKDK mutations represents a potentially treatable syndrome. Show less

Inactivation of the mainly endosomal 2Cl(-)/H(+)-exchanger ClC-5 severely impairs endocytosis in renal proximal tubules and underlies the human kidney stone disorder Dent's disease. In heterologous expression systems, interaction of the E3 ubiquitin ligases WWP2 and Nedd4-2 with a "PY-motif" in the cytoplasmic C terminus of ClC-5 stimulates its internalization from the plasma membrane and may influence receptor-mediated endocytosis. We asked whether this interaction is relevant in vivo and generated mice in which the PY-motif was destroyed by a point mutation. Unlike ClC-5 knock-out mice, these knock-in mice displayed neither low molecular weight proteinuria nor hyperphosphaturia, and both receptor-mediated and fluid-phase endocytosis were normal. The abundances and localizations of the endocytic receptor megalin and of the Na(+)-coupled phosphate transporter NaPi-2a (Npt2) were not changed, either. To explore whether the discrepancy in results from heterologous expression studies might be due to heteromerization of ClC-5 with ClC-3 or ClC-4 in vivo, we studied knock-in mice additionally deleted for those related transporters. Disruption of neither ClC-3 nor ClC-4 led to proteinuria or impaired proximal tubular endocytosis by itself, nor in combination with the PY-mutant of ClC-5. Endocytosis of cells lacking ClC-5 was not impaired further when ClC-3 or ClC-4 was additionally deleted. We conclude that ClC-5 is unique among CLC proteins in being crucial for proximal tubular endocytosis and that PY-motif-dependent ubiquitylation of ClC-5 is dispensable for this role. Show less