The SYT9 gene encodes a member of the synaptotagmin family. Synaptotagmins are integral membrane proteins of synaptic vesicles. They contain a transmembrane domain at their N-terminal, followed by C2A and C2B domains. These proteins carry out calcium-dependent phospholipid binding through their C2A domain and are suggested to be involved in the calcium-dependent exocytosis of secretory vesicles. By carrying out its functions, the SYT9 protein mediates the release of neurotransmitters such as norepinephrine and maintains fast synaptic transmission. It is also believed to function as a calcium-ion sensor during vesicular trafficking.
The SYT9 gene is located on the short arm of chromosome 11. It spans a length of 230.2 kb of DNA and its coding sequence is spread across 12 exons. The protein product encoded by this gene has a molecular mass of 56.1 kDa and consists of 491 amino acids. SYT9 is mainly expressed in the brain and spinal cord.
Monies et al. (2017) analyzed the findings of 1000 diagnostic panels and exomes carried out at a next generation sequencing lab in Saudi Arabia. One patient, a 16-year-old male, suffered from seizures, retinitis pigmentosa and obesity. His parents were consanguineous and he had a family history of this phenotype. Whole exome sequencing helped identify a dual molecular diagnosis in this patient. A homozygous mutation (c.773T>G, p.L258W) was found in exon 8 of the patient’s RLBP1 gene, associated with Bothnia retinal dystrophy. The patient also had a heterozygous variant (c.1471C>T, p.R491X) in exon 7 of the SYT9 gene and this was suggested to be associated with epilepsy. The SYT9 mutation was considered a candidate for pathogenicity as it was a novel variant predicted to be deleterious; and its deficiency in mice led to severely impaired synaptic transmission. The authors noted that dual molecular diagnoses were rare and only occurred in 1.5% of the cohort.
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