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MYOSIN, HEAVY CHAIN 16, SKELETAL MUSCLE, PSEUDOGENE; MYH16

MYOSIN, HEAVY CHAIN 16, SKELETAL MUSCLE, PSEUDOGENE; MYH16

HGNC Approved Gene Symbol: MYH16Cytogenetic location: 7q22 Genomic coordinates (GRCh38): 7:104,900,000-107,800,000▼ DescriptionThe MYH16 gene, encoding a sar...

HGNC Approved Gene Symbol: MYH16

Cytogenetic location: 7q22 Genomic coordinates (GRCh38): 7:104,900,000-107,800,000

▼ Description
The MYH16 gene, encoding a sarcomeric myosin heavy chain expressed in nonhuman primate masticatory muscles, is inactivated in humans. Stedman et al. (2004) hypothesized that the decrement in masticatory muscle size caused by the inactivation of MYH16 removed an evolutionary constraint on encephalization in early man.

▼ Cloning and Expression
By screening genomic cosmid libraries for myosin heavy chain genes, followed by cDNA hybridization and PCR amplification, Desjardins et al. (2002) cloned MYH16.

Using degenerate PCR, Stedman et al. (2004) obtained a DNA sequence for a human sarcomeric myosin gene, MYH16. They annotated the locus using homology to other sarcomeric myosins and generated the complete sequence.

▼ Gene Structure
Stedman et al. (2004) found that the human MYH16 gene comprises 42 exons and spans 67,983 basepairs.

▼ Gene Function
Stedman et al. (2004) detected transcription of human MYH16 and its M. fascicularis homolog only in muscles derived from the first pharyngeal arch, denoted as masticatory muscles, including temporalis and tensor veli palatini. No MYH16 protein is expressed in human. The relative size of individual masticatory muscle homologs contrasts remarkably in gross anatomic comparisons between humans and great apes and monkeys. At the histologic level, the difference between human and nonhuman primate temporalis muscle is highlighted by staining for type II (all fast-twitch) sarcomeric myosin and interstitial laminin. Human type II fibers are about one-eighth the size of those of M. fascicularis, while unstained type I fibers are the same size in both species. Protein gel electrophoresis and peptide sequencing using mass spectrometry showed that the dominant heavy chain isoform in the temporalis of M. fascicularis, specific to the masticatory muscles and undetectable in human temporalis, is the product of the M. fascicularis MYH16 ortholog. That MYH16 is still appropriately transcribed indicates that the coding sequence deletion was not preceded by a silencing mutation in a transcriptional control domain. Stedman et al. (2004) postulated that the volume of the skeletal muscle fibers expressing the MYH16 gene transcript is proportional to the total amount of myosin heavy chain accumulating in the cell, and that reliance on the translation of the less abundant MYH1 (160730) and MYH2 (160740) transcripts in the face of a frameshift mutation in MYH16 resulted in the 8-fold reduction in the size of the type II fibers in the human masticatory muscles as compared with macaque. Using the coding sequence for the myosin rod domains as a molecular clock, Stedman et al. (2004) estimated that this mutation appeared approximately 2.4 million years ago, predating the appearance of modern human body size and emigration of Homo from Africa. Stedman et al. (2004) hypothesized that this smaller muscle size led to gracilization of facial bones and accelerated encephalization in early Homo.

▼ Molecular Genetics
Stedman et al. (2004) identified a frameshift deletion at codon 660 in all modern human samples, including natives of Africa, South America, western Europe, Iceland, Japan, and Russia; thus, the inactivating mutation seems to be fixed in Homo sapiens. In contrast, all of the nonhuman primates for which sequence was obtained, including woolly monkey, pigtail macaque, rhesus, orangutan, gorilla, bonobo, and chimpanzee, have an ACC codon that encodes a highly conserved threonine. The frameshift in the human coding sequence truncates the predicted 224-kD myosin heavy chain to a 76-kD fragment containing an unstable portion of the myosin head domain.

▼ Mapping
By genomic sequence analysis, Desjardins et al. (2002) mapped the MYH16 gene to chromosome 7.

By analysis of genomic sequence, Stedman et al. (2004) determined that the human MYH16 gene resides in the region of human chromosome 7q22 flanked 5-prime by SMURF1 (605568) and 3-prime by ARPC1A (604220).

Tags: 7q22

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