Molecular characterization of a nonneuronal human UNC18 homolog

Munc18b is an essential gene in mice whose expression is limiting for secretion by airway epithelial and mast cells

Airway mucin secretion and MC (mast cell) degranulation must be tightly controlled for homoeostasis of the lungs and immune system respectively. We found the exocytic protein Munc18b to be highly expressed in mouse airway epithelial cells and MCs, and localized to the apical pole of airway secretory cells. To address its functions, we created a mouse with a severely hypomorphic Munc18b allele such that protein expression in heterozygotes was reduced by ~50%. Homozygous mutant mice were not viable, but heterozygotes showed a ~50% reduction in stimulated release of mucin from epithelial cells and granule contents from MCs. The defect in MCs affected only regulated secretion and not constitutive or transporter-mediated secretion. The severity of passive cutaneous anaphylaxis was also reduced by ~50%, showing that reduction of Munc18b expression results in an attenuation of physiological responses dependent on MC degranulation. The Munc18b promoter is controlled by INR (initiator), Sp1 (specificity protein 1), Ets, CRE (cAMP-response element), GRE (glucocorticoid-response element), GATA and E-box elements in airway epithelial cells; however, protein levels did not change during mucous metaplasia induced by allergic inflammation. Taken together, the results of the present study identify Munc18b as an essential gene that is a limiting component of the exocytic machinery of epithelial cells and MCs.

Distinct mutations in STXBP2 are associated with variable clinical presentations in patients with familial hemophagocytic lymphohistiocytosis type 5 (FHL5)

Familial hemophagocytic lymphohistiocytosis (FHL) is a genetically determined hyperinflammatory syndrome caused by uncontrolled immune response mediated by T-lymphocytes, natural killer (NK) cells, and macrophages. STXBP2 mutations have recently been associated with FHL5. To better characterize the genetic and clinical spectrum of FHL5, we analyzed a cohort of 185 patients with suspected FHL for mutations in STXBP2. We detected biallelic mutations in 37 patients from 28 families of various ethnic origins. Missense mutations and mutations affecting 1 of the exon 15 splice sites were the predominant changes detectable in this cohort. Patients with exon 15 splice-site mutations (n = 13) developed clinical manifestations significantly later than patients with other mutations (median age, 4.1 year vs 2 months) and showed less severe impairment of degranulation and cytotoxic function of NK cells and CTLs. Patients with FHL5 showed several atypical features, including sensorineural hearing deficit, abnormal bleeding, and, most frequently, severe diarrhea that was only present in early-onset disease. In conclusion, we report the largest cohort of patients with FHL5 so far, describe an extended disease spectrum, and demonstrate for the first time a clear genotype-phenotype correlation.

Gene structure and promoter function of murine Munc18-2, a nonneuronal exocytic Sec1 homolog

Sec1 family proteins are regulators of diverse exocytic processes, from yeast to man. Three mammalian homologues, Munc18-1, -2, and -3 have been described. We have studied the structure and expression of the mouse Munc18-2 gene. The Munc18-2 gene comprises 19 exons whose sizes range from 50 to 158 bp, with a total gene size of approximately 11 kb. A single transcript of 2.1 kb is expressed in multiple non-neuronal murine tissues. Munc18-2 has a striking resemblance to Munc18-1 in structure despite only 60% sequence identity, suggesting a recent gene duplication event. Analysis of the region upstream of the transcription start site shows that Munc18-2 has a TATA-less promoter, with a consensus initiator (Inr) sequence at the start of transcription, several Sp1 binding sites, and strong promoter activity in RBL-2H3 mast cells. The region from +5 to -430 is more active than +5 to -800, suggesting upstream repressor elements.