Objective: To report the identification of bi-allelic mutations in BZRAP1, encoding the active zone (AZ) protein RIM-binding protein 1 (RBP1), as a novel cause of dystonia.

Background: Dystonia is characterized by excessive muscle contractions leading to abnormal involuntary movements and postures. While the precise functional neuroanatomy and pathophysiological molecular events of dystonia are unclear, abnormal synaptic homeostasis has been observed in several models of dystonia.

Method: We studied 3 pedigrees using homozygosity mapping and whole-exome sequencing and independently identified homozygous BZRAP1 mutations as the likely genetic cause of dystonia in our patients. We characterized the motor phenotype and brain pathology of BZRAP1-KO mice and Purkinje cells (PC)-only BZRAP1-KO mice. To directly test how the mutation p.Gly1808Ser impacts synaptic function, we measured its effect on neurotransmission in neuronal cultures.

Results: 2 homozygous truncating BZRAP1 (p.Ala180Profs*8 and p.Gln817*) were found in 4 subjects from 2 families. These cases all presented in early teenage years with progressive generalized dystonia with prominent involvement of the cranial and laryngeal muscles and striking progressive cerebellar atrophy. A homozygous missense variant (p.Gly1808Ser) was identified in three members of a family with a significantly milder phenotype, consisting of adult-onset cervical and upper-limb dystonia. BZRAP1-KO mice showed several motor abnormalities, including increased spontaneous locomotion, abnormal beam-walking test, dystonic clasping of hindlimbs and a dramatic pathological motor response to cholinergic stimulation. This motor phenotype was only partially recapitulated by selective BZRAP1 KO in PC. BZRAP1-KO did not affect cerebellar gross volume, but impacted significantly gross dendritic morphology of PC and number of synaptic inputs onto these cells.  Analysis of the p.Gly1808Ser variant showed a gain-of-function effect on neurotransmitter release through enhanced presynaptic calcium entry.

Conclusion: We describe the identification of bi-allelic BZRAP1 mutations, both loss- and gain-of-function, in patients with different forms of dystonia. RBPs mediate tethering of voltage-gated Ca2+ channels to the presynaptic AZ, ensuring fidelity of synaptic vesicle release in response to action potentials. Our results demonstrate a direct link between the dysfunction of the presynaptic machinery and dystonia pathogenesis.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/homozygous-bzrap1-mutations-cause-autosomal-recessive-dystonia/