HUMAN MYOSIN HEAVY CHAIN GENE ORGANIZATION AND EXPRESSION
SAEZ, LINO J.
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To define MHC gene organization and expression in human skeletal muscle, we have isolated and characterized MHC genomic and cDNA sequences. Genomic clone analysis indicates that the human sarcomeric MHC gene family is composed of a minimum of 10-12 genes that share regions of sequence homology. We have also determined that MHC genes are highly conserved throughout vertebrate evolution. Whether all the human MHC genomic fragments that we have isolated are part of functional genes or pseudogenes is not yet known.;The diversity of expression of this multigene family within one muscle type was examined by cDNA clone analysis. There are as many as five different forms of MHC expressed simultaneously in adult human skeletal muscle. Two different skeletal muscle MHC cDNA clones were sequenced, one of which encodes the entire light meromyosin segment of MHC. The protein coding sequences of the two different isoforms show a high degree of homology with MHC sequences from other organisms. Northern analysis and sequence comparisons with physiologically defined MHC types allow us to identify the two human forms as a skeletal fast fiber form while the other clone represents a cardiac gene that is coexpressed in skeletal muscle.;The 3' untranslated sequences of skeletal MHC cDNAs from widely separated species are homologous to each other as are cardiac MHC cDNAs. Since the 3' untranslated region of cardiac and skeletal genes show no homology to each other, these regions can thus define isotype specific markers. The isotype-specific preservation of the 3' untranslated sequences during evolution suggests a functional role for these regions.;An oligonucleotide complementary to the 3' untranslated region of the human cardiac MHC mRNA was used to isolate its corresponding gene. This gene was found to be closely linked to another MHC gene. From DNA sequence analysis we determined that the two genes correspond to cardiac (alpha)- and (beta)-MHC genes that are organized in tandem.;Chromosomal mapping of human MHC genes, using somatic cell hybrids between primary human cells and a permanent mouse cell line shows that human skeletal and cardiac MHC genes are located on different chromosomes. All human skeletal MHC genes map to chromosome 17 and the (alpha) and (beta) cardiac MHC genes map to chromosome 14. These results indicate that there is no preferential chromosome arrangement for coexpression of members of this multigene family. However, sequential expression during development may require linkage. (Abstract shortened with permission of author.).