Biochemical analysis of a TrkB receptor mutation that causes a developmental epileptic encephalopathy

TrkB, a receptor tyrosine kinase encoded by gene NTRK2, is essential for diverse biological processes in both the developing and mature mammalian nervous systems. Whole exome sequencing of children with developmental epileptic encephalopathy revealed an intriguing syndrome caused by a rare de novo recurrent variant of TrkB, namely Y434C. Investigating the biochemical properties of the Y434C mutant protein is an important initial step toward understanding how this mutation causes this devastating disease. This led us to establish and characterize multiple cell lines stably expressing mouse WT or Y434C TrkB. In comparison to WT, Y434C mutant cell lines expressed low to undetectable levels of mature form (145 kDa) of TrkB protein and varying levels of mutant forms migrating at sizes ranging from 40 to 110 kDa. Y434C mutant cell lines exhibited striking impairments of brain-derived neurotrophic factor-mediated activation of TrkB signaling. Reducing agents reduced high molecular weight forms of Y434C protein multimers detected in protein gel electrophoresis, consistent with disulfide bond formation between the Y434C mutant proteins. We propose that conversion of tyrosine to cysteine at amino acid 434 results in a novel intermolecular disulfide bond between Y434C mutant proteins, thereby modifying their structure and enhancing their proteolytic digestion. The ensuing reductions of the mature form of TrkB likely underlie impaired TrkB signaling. The proteolytic fragments of TrkB may themselves have deleterious consequences, which contribute to the phenotypic manifestations of the Y434C TrkB mutation.