pigk Mutation underlies macho behavior and affects Rohon-Beard cell excitability

The study of touch-evoked behavior allows investigation of both the cells and circuits that generate a response to tactile stimulation. We investigate a touch-insensitive zebrafish mutant, macho (maco), previously shown to have reduced sodium current amplitude and lack of action potential firing in sensory neurons. In the genomes of mutant but not wild-type embryos, we identify a mutation in the pigk gene. The encoded protein, PigK, functions in attachment of glycophosphatidylinositol anchors to precursor proteins. In wild-type embryos, pigk mRNA is present at times when mutant embryos display behavioral phenotypes. Consistent with the predicted loss of function induced by the mutation, knock-down of PigK phenocopies maco touch insensitivity and leads to reduced sodium current (INa) amplitudes in sensory neurons. We further test whether the genetic defect in pigk underlies the maco phenotype by overexpressing wild-type pigk in mutant embryos. We find that ubiquitous expression of wild-type pigk rescues the touch response in maco mutants. In addition, for maco mutants, expression of wild-type pigk restricted to sensory neurons rescues sodium current amplitudes and action potential firing in sensory neurons. However, expression of wild-type pigk limited to sensory cells of mutant embryos does not allow rescue of the behavioral touch response. Our results demonstrate an essential role for pigk in generation of the touch response beyond that required for maintenance of proper INa density and action potential firing in sensory neurons.

Whole exome sequencing and functional studies identify an intronic mutation in TRAPPC2 that causes SEDT

Skeletal dysplasias are challenging to diagnose because of their phenotypic variability, genetic heterogeneity, and diverse inheritance patterns. We conducted whole exome sequencing of a Turkish male with a suspected X-linked skeletal dysplasia of unknown etiology as well as his unaffected mother and maternal uncle. Bioinformatic filtering of variants implicated in skeletal system development revealed a novel hemizygous mutation, c.341-(11_9)delAAT, in an intron of TRAPPC2, the causative locus of spondyloepiphyseal dysplasia tarda (SEDT). We show that this deletion leads to the loss of wild-type TRAPPC2 and the generation of two functionally impaired mRNAs in patient cells. These consequences are predicted to disrupt function of SEDLIN/TRAPPC2. The clinical and research data were returned, with appropriate caveats, to the patient and informed his disease status and reproductive choices. Our findings expand the allelic repertoire of SEDT and show how prior filtering of the morbid human genome informed by inheritance pattern and phenotype, when combined with appropriate functional tests in patient-derived cells, can expedite discovery, overcome issues of missing data and help interpret variants of unknown significance. Finally, this example shows how the return of a clinically confirmed mutational finding, supported by research allele pathogenicity data, can assist individuals with inherited disorders with life choices.