MYO1A (myosin IA)

	Figure 1. Diagram of DNA/RNA.
Description	There are several transcripts described for MYO1A. The two transcripts better characterized contain 28 and 29 exons spanning over 21 kb and both code for an identical protein of 1043 amino acids.

	Figure 2. Protein structure of MYO1A.
Description	The protein encoded by the MYOSIN-IA gene belongs to the myosin superfamily. Like all myosin-1 isoforms, MYO1A contain these three core domains (figure 2): an N-terminal motor domain that coordinates ATP hydrolysis with actin binding and force generation; a central neck region made up of varying numbers of IQ motifs, which bind calmodulin or calmodulin-like proteins; and a tail region, which includes a highly basic C-terminal tail homology 1 (TH1) domain that is responsible for membrane binding (Coluccio and Bretscher, 1990; Krendel and Mooseker, 2005; McConnell and Tyska, 2010; Nambiar et al., 2010).
	Figure 3. Relative MYO1A mRNA levels in human normal tissues. MYO1A mRNA levels in human normal and tumor samples were obtained from a collection of 667 normal human samples from different tissues (Gene Expression Omnibus: GSE7307) and 10000 normal and tumor samples from GeneSapiens.org (Kilpinen et al., 2008).
Expression	Myo1a is highly expressed in the enterocytes that line the mucosa of the small intestine (Matsudaira and Burgess, 1979; Skowron and Mooseker, 1999). Expression of MYO1A has also been observed at relatively high levels in gastric epithelium when compared to other organs such as endometrium, myometrium, ovary and prostate (figure 3). In tumor samples, MYO1A mRNA expression in human gastric adenocarcinomas is comparable to intestinal adenocarcinomas, and significantly higher than in other tumor types (Mazzolini et al., 2013) (figure 4). Myo1a transcripts are also present in rodent inner ear at low level (Dumont et al., 2002).
	Figure 4. Relative MYO1A mRNA levels in human tumors of different origin. Box-whisker plot of the gene's expression in cancer tissues. The bottom of the box is the 25th percentile of the data, the top of the box is the 75th percentile, and the vertical red line is the median. The whiskers extend to 1.5 times the interquartile range from the edges of the box, and any data points beyond this are considered outliers, marked by hollow circles. Filled grey bars are gastrointestinal carcinomas.
Localisation	Myo1a localizes to the cellular membrane through to the C-terminal tail domain. In the enterocytes that line the mucosa of the small intestine, MYO1A localizes to the apical brush border membrane (Matsudaira and Burgess, 1979; Collins and Borysenko, 1984; Skowron and Mooseker, 1999) (figure 5).
	Figure 5. MYO1A localizes to the apical membrane of intestinal epithelial cells. MYO1A-GFP was transfected into the colon cancer cell line Caco-2. The image was taken by confocal microscopy and represents an orthogonal stuck of a monolayer of cells. (A) Actin staining with rhodamine-phalloidin shows the apical and baso-lateral membtanes of the cells. (B) EGFP-MYO1A localizing in the apical membrane. (C) Overlay (modified from Mazzolini et al., 2012).
Function	Myosin Ia (MYO1A) is a major component of the cytoskeleton that underlies and supports the apical brush border of the enterocytes. MYO1A forms a spiral array of bridges that links the microvillar actin core to the membrane (Chantret et al., 1988; West et al., 1988; Beaulieu et al., 1990). Here, Myosin-1a plays a critical role in maintaining the brush border composition, structure, and regulating the microvillar membrane tension (Tyska et al., 2005; Nambiar et al., 2009), Myo1a also plays a role in powering the release of vesicles from the tips of the microvilli (McConnell et al., 2009).

	Table 1. MYO1A mutations related to sensorineural bilateral hearing loss (Donaudy et al., 2003). Figure 6. Somatic mutations of MYO1AA7MUT. (A) Frameshift mutations in the A8 track in Exon 28 of MYO1A. (B) Co-transfection of wild type EGFP-MYO1Awt and mutant ERFP-MYO1AA7MUT demonstrated that the mutant protein mislocalized to the cytoplasm of undifferentiated Caco2 cells (modified from Mazzolini et al., 2012). (C) Localization of additional mutations found in colorectal tumors without microsatellite instability.
Germinal	The following germinal mutations have been reported in eight unrelated patients coming from central and southern Italy and affected by sensorineural bilateral hearing loss of variable degree: one nonsense mutation, one trinucleotide insertion leading to an additional amino acid, and six missense mutations (Donaudy et al., 2003) (table 1).
Somatic	Mazzolini et al. reported frequent frame-shift somatic mutations in colorectal and gastric cancer. These mutations were found with the following frequencies: 44,4% (16 of 36) in microsatellite-instable colorectal cancer cell lines; 31,3% (42 of 134) in primary colon tumors; 46,8% (22/47) in gastric microsatellite-instable primary tumors. No mutations were observed in the matching healthy intestinal mucosa. All the mutations observed were insertions or deletions in an A8 microsatellite tract located in exon 28. The most frequent mutation is the deletion of one A (MYO1AA7MUT). All the mutations found appeared to be heterozygous as the wild type allele was also visible in all cases (figure 6, panel A). The MYO1AA7MUT mutation causes sub-cellular mislocalization (figure 6, panel B) and decreased stability of Myosin-1a (Mazzolini et al., 2012; Mazzolini et al., 2013). Additional mutations have been found in colorectal tumors without microsatellite instability (TCGA; figure 6, panel C).

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