Unregulated smooth-muscle myosin in human intestinal neoplasia

Abstract

A recent study described a recessive ATPase activating germ-line mutation in smooth-muscle myosin (smmhc/myh11) underlying the zebrafish meltdown (mlt) phenotype. The mlt zebrafish develops intestinal abnormalities reminiscent of human Peutz-Jeghers syndrome (PJS) and juvenile polyposis (JP). To examine the role of MYH11 in human intestinal neoplasia, we searched for MYH11 mutations in patients with colorectal cancer (CRC), PJS and JP. We found somatic protein-elongating frameshift mutations in 55% of CRCs displaying microsatellite instability and in the germ-line of one individual with PJS. Additionally, two somatic missense mutations were found in one microsatellite stable CRC. These two missense mutations, R501L and K1044N, and the frameshift mutations were functionally evaluated. All mutations resulted in unregulated molecules displaying constitutive motor activity, similar to the mutant myosin underlying mlt. Thus, MYH11 mutations appear to contribute also to human intestinal neoplasia. Unregulated MYH11 may affect the cellular energy balance or disturb cell lineage decisions in tumor progenitor cells. These data challenge our view on MYH11 as a passive differentiation marker functioning in muscle contraction and add to our understanding of intestinal neoplasia.

Fig. 1.

Examples of mutations observed in the study. Mutations found in the C8 mononucleotide repeat of MYH11. (a) WT sequence. (b) Deletion of one cytosine in a MSI CRC sample. (c) A homozygous deletion of one cytosine in HCA7 MSI CRC cell line. (d) Germ-line insertion of one cytosine in a patient with PJS. (e) WT amino acid sequence of the last exon of MYH11 SM2. (f and g) Consequences of insertion (f) and deletion (g) of one cytosine to the amino acid sequence. Both mutations disrupt the C-terminal amino acids and elongate the predicted peptide. (h and i) Homology comparison between species shows that MYH11 amino acids R501 (1) and K1044 (3) are highly conserved. (2) indicates the amino acid mutated in zebrafish meltdown phenotype. Peptide sequences: Mm, ENSMUSP00000087756; Rn, ENSRNOP00000047484; Hs, ENSP00000300036; Gg, ENSGALP00000032964; Dr, ENSDARP00000041141.

Fig. 2.

ATPase activity measurements. ATPase activity of WT, R501L, and K1044N mutant smooth-muscle heavy meromyosins.