Myh11 haploinsufficiency recapitulates megacystis and voiding dysfunction in a mouse model of MMIHS

Megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS) is a fatal rare disorder characterized by bladder and colonic smooth muscle dysfunction. Mutations in the MYH11 gene have been identified as one of the causative factors of MMIHS.

This study aimed to generate a Myh11 knockout mouse model, evaluate the phenotypes and function of the bladder and intestines, and investigate the association between Myh11 deficiency and the core clinical features of MMIHS.

Gene expression was assessed by qRT-PCR and Western blot. Protein localization and expression were determined using immunohistochemistry. Histomorphological changes were evaluated through hematoxylin-eosin (H&E) and Masson staining. Bladder function was analyzed via void spot assay and post-void residual volume measurements, while smooth muscle contractility was measured using muscle tension assays. Intestinal motility was assessed using established gastrointestinal transit tests, and cell proliferation was examined by BrdU incorporation assay.

Compared with wildtype mice, Myh11^+/- heterozygote mice showed significantly reduced Myh11 expression at both mRNA and protein levels in bladder tissues (P < 0.001). Immunohistochemical staining revealed that Myh11 was predominantly expressed in the bladder smooth muscle, and a significant reduction in its expression level was observed in the bladders of Myh11^+/- mice (P < 0.01). Phenotypically, Myh11^+/- mice exhibited a markedly enlarged bladder volume (P < 0.05), an increased bladder-to-body weight ratio (P < 0.01), a thickened smooth muscle layer (P < 0.01), and enhanced collagen deposition (P < 0.01). Functionally, Myh11^+/- mice displayed reduced urinary frequency and total urine output (P < 0.05), an increased residual urine volume (P < 0.001), a slowed bladder contraction (P < 0.01), along with mild intestinal dysmotility (prolonged time to first black stool, P < 0.05) and an elevated fecal water content (P < 0.05). At the molecular level, the up-regulation of PCNA protein expression and the increased incorporation of BrdU suggests active cell proliferation (P < 0.05).

Myh11^+/- mice successfully recapitulate the core bladder phenotypes of MMIHS, including megacystis and voiding dysfunction. This confirms the critical role of Myh11 in both the development and contractile function of the bladder, thereby providing a reliable in vivo model for in-depth investigation into the pathological mechanisms underlying bladder dysfunction in MMIHS.

Myh11 deficiency leads to significant bladder enlargement, smooth muscle thickening, collagen accumulation, and voiding dysfunction, establishing a good disease model for MMIHS.