CHRM3 codes for the M3 muscarinic acetylcholine receptor that is located on the surface of smooth muscle cells of the detrusor, the muscle that effects urinary voiding. Previously, we reported brothers in a family affected by a congenital prune belly‐like syndrome with mydriasis due to homozygous CHRM3 frameshift variants. In this study, we describe two sisters with bladders that failed to empty completely and pupils that failed to constrict fully in response to light, who are homozygous for the missense CHRM3 variant c.352G > A; p.(Gly118Arg). Samples were not available for genotyping from their brother, who had a history of multiple urinary tract infections and underwent surgical bladder draining in the first year of life. He died at the age of 6 years. This is the first independent report of biallelic variants in CHRM3 in a family with a rare serious bladder disorder associated with mydriasis and provides important evidence of this association.

Mastocytosis is a rare myeloid neoplasm characterized by uncontrolled expansion of mast cells, driven in >80% of affected individuals by acquisition of the KIT D816V mutation. To explore the hypothesis that inherited variation predisposes to mastocytosis, we performed a two-stage genome-wide association study, analyzing 1,035 individuals with KIT D816V positive disease and 17,960 healthy control individuals from five European populations. After quality control, we tested 592,007 SNPs at stage 1 and 75 SNPs at stage 2 for association by using logistic regression and performed a fixed effects meta-analysis to combine evidence across the two stages. From the meta-analysis, we identified three intergenic SNPs associated with mastocytosis that achieved genome-wide significance without heterogeneity between cohorts: rs4616402 (p meta ¼ 1.37 3 10 À15 , OR ¼ 1.52), rs4662380 (p meta ¼ 2.11 3 10 À12 , OR ¼ 1.46), and rs13077541 (p meta ¼ 2.10 3 10 À9 , OR ¼ 1.33). Expression quantitative trait analyses demonstrated that rs4616402 is associated with the expression of CEBPA (p eQTL ¼ 2.3 3 10 À14 ), a gene encoding a transcription factor known to play a critical role in myelopoiesis. The role of the other two SNPs is less clear: rs4662380 is associated with expression of the long non-coding RNA gene TEX41 (p eQTL ¼ 2.55 3 10 À11 ), whereas rs13077541 is associated with the expression of TBL1XR1, which encodes transducin (b)-like 1 X-linked receptor 1 (p eQTL ¼ 5.70 3 10 À8 ). In individuals with available data and non-advanced disease, rs4616402 was associated with age at presentation (p ¼ 0.009; beta ¼ 4.41; n ¼ 422). Additional focused analysis identified suggestive associations between mastocytosis and genetic variation at TERT, TPSAB1/TPSB2, and IL13. These findings demonstrate that multiple germline variants predispose to KIT D816V positive mastocytosis and provide novel avenues for functional investigation.

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