ABSTRACT: P/Q-type channels are the principal presynaptic calcium channels in brain functioning in neurotransmitter release. They are composed of the pore-forming CaV2.1 ?1 subunit and the auxiliary ?2?-2 and ?4 subunits. ?4 is encoded by CACNB4, and its multiple splice variants serve isoform-specific functions as channel subunits and transcriptional regulators in the nucleus. In two siblings with intellectual disability, psychomotor retardation, blindness, epilepsy, movement disorder and cerebellar atrophy we identified rare homozygous variants in the genes LTBP1, EMILIN1, CACNB4, MINAR1, DHX38 and MYO15 by whole-exome sequencing. In silico tools, animal model, clinical, and genetic data suggest the p.(Leu126Pro) CACNB4 variant to be likely pathogenic. To investigate the functional consequences of the CACNB4 variant, we introduced the corresponding mutation L125P into rat ?4b cDNA. Heterologously expressed wild-type ?4b associated with GFP-CaV1.2 and accumulated in presynaptic boutons of cultured hippocampal neurons. In contrast, the ?4b-L125P mutant failed to incorporate into calcium channel complexes and to cluster presynaptically. When co-expressed with CaV2.1 in tsA201 cells, ?4b and ?4b-L125P augmented the calcium current amplitudes, however, ?4b-L125P failed to stably complex with ?1 subunits. These results indicate that p.Leu125Pro disrupts the stable association of ?4b with native calcium channel complexes, whereas membrane incorporation, modulation of current density and activation properties of heterologously expressed channels remained intact. Wildtype ?4b was specifically targeted to the nuclei of quiescent excitatory cells. Importantly, the p.Leu125Pro mutation abolished nuclear targeting of ?4b in cultured myotubes and hippocampal neurons. While binding of ?4b to the known interaction partner PPP2R5D (B56?) was not affected by the mutation, complex formation between ?4b-L125P and the neuronal TRAF2 and NCK interacting kinase (TNIK) seemed to be disturbed. In summary, our data suggest that the homozygous CACNB4 p.(Leu126Pro) variant underlies the severe neurological phenotype in the two siblings, most likely by impairing both channel and non-channel functions of ?4b.
