Background: Congenital diarrheal disorders (CDDs) are a large group of life-threatening genetic disorders that are frequently difficult to diagnose. We report four siblings from consanguineous kindred with persistent generalized malabsorptive diarrhea hypothyroidism, GH deficiency, intermittent diabetes insipidus, and monogenic obesity.
Objective and hypotheses: To find the genetic etiology for the CDD in four cases from consanguineous family using homozygosity mapping and whole exom sequencing.
Method: Following clinical presentation with CDD, homozygosity mapping and whole exom next generation sequencing were followed by Sanger sequencing. The mutated PC1/3 protein enzymatic activity was assayed in HEK293 and Neuro2A cells.
Results: Homozygosity mapping identified five regions, comprising 337 protein-coding genes that were shared by the three affected siblings. Exome sequencing identified a novel homozygous N309K mutation in the PCSK1 gene, encoding the neuroendocrine convertase 1 precursor (PC1/3) which was recently reported to cause CDD. The N309K mutation located in an evolutionarily conserved locus was not observed in the ~7500 individuals sequenced by the 1000 genomes and NHLBI exome projects, and was predicted to be deleterious by in silico analysis. The PCSK1 mutation affected the oxyanion hole transition state-stabilizing amino acid within the active site, which is critical to proprotein maturation and enzyme activity. Unexpectedly, the N309K mutant protein exhibited normal prodomain removal and was efficiently secreted from both HEK293 and Neuro2A cells. However, the secreted enzyme showed no catalytic activity, and was not processed into the 66 kDa form in either cell line.
Conclusion: The novel N309K mutation in the very recently characterized PCSK1 gene causes a severe decrease in PC1/3 catalytic activity against in trans substrates. This results in a dysfunction of enteroendocrine and glandular endocrine cells, causing severe infantile diarrhea and endocrinopathies. Further studies are underway to elucidate the mechanism for the late onset presentations of different endocrinopathies including monogenic (PC1/3) obesity.
18 Sep 2014 - 20 Sep 2014