MHCs gene cluster organization and their mRNA expression in dog skeletal muscles

Myosin heavy chains (MHCs) are actin-based motor proteins that play a key role in determining muscle contraction speed. MHC isoforms expressed in mammalian muscles are encoded by a multigene family. This MHC gene superfamily presents a highly conserved cluster organisation in the human and mouse genomes (Weiss et al., 1999). In this work we used computer–based programs to identify and characterise dog MHC sequences in dog genome from NCBI Dog Genome Resources web site. Moreover, we analysed the expression pattern of the identified dog MHC isoforms by RT-PCR in the trunk, limb and specialised (masticatory, intrinsic laryngeal and extraocular) muscles. BLAST searches suggest that the genes coding for the fast dog isoforms (embryonic, 2A, 2X, 2B, neonatal and extraocular) cluster on chromosome 5; the two “cardiac” genes, coding for MHC-β (slow) and MHC-α (cardiac), are arranged tandemly on chromosome 8; the gene for masticatory isoform (2M MHC, characteristic of masticatory muscles of dog) has been located on chromosome 6. The phylogenetic analysis confirmed the position of the dog orthologous isoforms within each specific MHC group with high similarity values (94-98%). The limb and trunk muscles express 1, 2A and 2X MHC, although a fourth cDNA, corresponding to 2B MHC, has been found in semimembranosus but not in longissimus dorsi. In contrast, in specialised muscles (retractor bulbi, rectus lateralis and in some laryngeal muscles) the 2B MHC mRNA was constantly present. Our results confirm that the fourth band identified by Wu et al. (2000) in thyroarytenoideus muscle correspond to the 2B MHC isoform.