Table 1.

Summary of known cytoskeletal mutations associated with visceral myopathy

Gene	Function in SMCs	Myopathy-Associated Mutations	Inheritance Pattern	Clinical Phenotype	Disease Mechanisms Determined by basic science studies	Ref.
ACTG2	Predominant actin isoform in visceral SMCs. Exact role in cells is unknown. May contract SMCs or control compliance.	R38H, P39L, P39R, R40H, R40C, H41Q, V44A, M45T, R63G, R63Q, P113S, Y134N, L143F, Y144F, G147C, R148S, R148L, T149R, R178H, R178L, R178C, R178S, D185Y, T195I, E196D, G198D, A205T, R211Q, D245G, R257C, R257H, G269E, *c. 502C>T; p. R168Ter, **c.187C>T; p.R63Ter, ***c. 1006C>T; p.R336W	Mostly autosomal dominant	MMIHS, PIPO, or familial CIPO; highly variable presentation	-Impaired actin polymerization -Decreased cell contraction in collagen gel contraction assay with some mutations	(2, 11–13, 15–22, 24, 25, 27, 44–47)
ACTA2	Predominant actin isoform in vascular SMCs. Also present in visceral SMCs. Required for contractility.	R179C, R179H All other ACTA2 mutations cause vascular disease, but not visceral myopathy.	Autosomal dominant	Multisystemic smooth muscle dysfunction: MMIHS, vascular disease, heart defects, congenital mydriasis	-Impaired actin polymerization with increased severing of R179H F-actin by cofilin, increased binding of actin to profilin and formin -Decreased movement of R179H F-actin along myosin may explain decrease in contractility - ACTA2 knockout mouse has hypocontractile bladder in vitro	(28, 48,49)
MYH11	Smooth muscle-specific myosin heavy chain (MHC). Four isoforms (SM1A, SM1B, SM2A, SM2B) by alternative splicing. Required for SMC contraction. Different isoforms may play specific roles in SMC contraction in various tissues and across developmental time points.	Compound heterozygous mutations: -2 bp deletion in exon 22 (c.2809_2810del, p.R937Gfs*7, paternal) and 49 bp deletion in exon 26 (c.3422_3470del, p.K1141Tfs*20, maternal) -c.2051 G > A, p.R684H and c.3540_3541delinsTT, p.E1180D, Q1181Ter - Heterozygous missense variant c.379C>T, P127S and a heterozygous 1.3 Mb deletion in 16q13.11 Homozygous mutations:-c.3598 A > T, p.K1200Ter -c.1591C>T, p.R531TerHeterozygous:-c.5819delC, p.P1940HfsTer91 heterozygous frameshift variant -c.5819_5820insCA, p.Q1941Nfs*91 2 bp insertion with addition of 90 unique amino acids before the stop codon	Compound heterozygous and homozygous mutations are recessive. Heterozygous mutations are autosomal dominant.	Homozygous and compound heterozygous mutations: MMIHS, multisystemic smooth muscle dysfunctionHeterozygous autosomal dominant: CIPO and severe esophageal dysmotility	-No basic science studies on the specific mutations identified in humans. -Several animal studies on different Myh11 isoform-specific knockout mice have been used to understand the roles of individual isoforms as well as Myh11 knockouts that affect all isoforms (50–56). SM2 knockout mice have bowel and bladder distension, urinary retention, hydronephrosis, and early death (51). -Bladder muscle strips in SM2 knockouts are hypercontractile, suggesting that SM2 is a negative regulator of SMC contraction.	(29–34)
MYL9	Regulatory myosin light chain (rMLC) of class II myosins in smooth muscle. Critical for actin-myosin interaction by increasing ATPase activity of MHC.	Homozygous deletion including the last exons Two mutations: 9 bp deletion removes canonical splice donor site at exon 2 (c.184 + 2_184 + 10del), and loss of exon 4	Autosomal recessive Compound heterozygous	MMIHS with mild mydriasis MMHIS, mydriasis, broncho-pulmonary dysplasia, diffuse microvascular disease, progressive white matter loss	-Homozygous deletion of MYL9 is hypothesized to reduce actin-myosin binding.	(12, 36, 57)
MYLK	Myosin light chain kinase (MLCK) phosphorylates regulatory MLC needed for actin-myosin interaction.	-7 bp duplication (c.3838_3844dupGAAAGCG, p.E1282Gfs*51) -Putative splice-site variant (c.3985 + 5C>A) causes a frameshift and creates a premature termination codon (PTC)	Autosomal recessive	MMIHS	-Unproven hypothesis that lack of phosphorylation of the rMLC prevents myosin activation and interaction with actin, impairing contractility. -Mylk smooth-muscle specific knockout mice have severe gut dysmotility, bladder dysfunction, and hypotension due to inhibition of RLC phosphorylation (58). -Less severe vascular phenotypes could be due to compensation by other kinases.	(35)
LMOD1	Actin-binding protein that nucleates new actin filaments.	Premature termination codon (PTC) in Exon 2 of LMOD1 c.1108C>T, p.R370Ter).	Autosomal recessive	MMIHS	-Lmod1 knockout mouse with PTC in exon 1. Stomach and bladder dilation, but no microcolon. Jejunal rings show lower passive tensile strength and weaker contractility in response to agonists. Reduction in F-actin by phalloidin staining and fractionation of monomeric and filamentous actin. No filament changes at ultrastructural level, but elongation of dense bodies in knockout mice. -siRNA knockdown of LMOD1 in human intestinal SMCs led to decrease in collagen gel contraction.	(59)
FLNA	Actin-binding protein that crosslinks F-actin networks. May be important for cell shape and motility.	-Exon skipping due to 4-bp deletion in exon 40 with translation of mutant FLNA missing 41 amino acids. -Partial duplication including first 28 exons -Hemizygous nonsense mutation c.7021C>T; Q2341Ter -2-bp deletion in exon 2 with frameshift leading to PTC - no-stop FLNA mutation (c.7941_7942delCT, p.('2648Sext' 100))	X-linked recessive	CIPO, cardiovascular defects, brain malformation, periventricular nodular heterotopia (PVNH)	-Previously believed to be only neuropathic in origin, but detailed immunohisto-chemical analysis demonstrated diffuse abnormal layering (DAL) of intestinal smooth muscle with no enteric neuron involvement (37). -FLNA has two isoforms (60). The longer isoform is the predominant isoform in intestinal smooth muscle, and mutations in this isoform cause CIPO with PVNH in the brain.	(37, 61–63)

ACTA2, actin alpha 2; ACTG2, actin gamma 2; CIPO, chronic intestinal pseudo-obstruction; ECM, extracellular matrix; ER, endoplasmic reticulum; F-actin, filamentous actin; FLNA, Filamin A; G-actin, globular actin; LMOD1, Leiomodin 1; MHC, myosin heavy chain; MLC, myosin light chain; MMIHS, megacystis-microcolon-intestinal hypoperistalsis syndrome; MRTFs, myocardin-related transcription factors; MYH11, myosin heavy chain 11; MYL9, myosin light chain 9; MYLK, myosin light chain kinase; PIPO, pediatric intestinal pseudo-obstruction; SMC, smooth muscle cell; YAP, Yes-associated protein.

*One reported autosomal recessive null mutation.

**One reported heterozygous null mutation.

***One reported autosomal recessive missense mutation.