Expression analysis of ten myosin heavy chain isoforms in human extraocular muscles

cHuman extraocular muscles (EO) express, together with skeletal MyHC isoforms (type 1, 2A, 2X), the following: developmental isoforms (Embryonic and Perinatal) all life long;  cardiac isoform, present in other particular muscles as masseter and laryngeal; EO isoform that is muscle specific; 2B isoform found in a recent research only in these muscles. The type M isoform, typical of masticatory muscles of some species such as carnivores and non human primates, is not express. Moreover, in EO muscles another isoform was identified, the slow tonic, but not characterized yet at genomic level. Recently Schiaffino and collaborators (Rossi, J Physiol 588.2, 2010) studied two new genes concluding that MYH14/7b correspond to slow tonic and MYH15 gene to ventricular isoform of chicken. Another author found, on the contrary, that the slow tonic is codified by MYH15 gene (Rahnert, Cell Tissues Organs 191, 2010). Our results, assessed by Realtime PCR and electrophoresis, comparing skeletal, laryngeal, EO, masseter and atrium muscles, tend to link MYH15 gene to slow tonic (express only in EO), while MYH14/7b to a chicken embryonic isoform, express in atrium, laryngeal and EO. MyHC expression is muscle dependent and its regulation is under the control of several factors. Recently Harrison (Skeletal Muscle 1, 2011) correlated the lack of 2B expression in human skeletal muscles to the promoter region that differ from the mouse promoter (that express the 2B). We aligned promoter sequence of other species and our results do not confirm his assumptions. EO muscles are, therefore, extremely complicated, able to express, also at protein level, 10 of 11 isoforms identified at genomic level and the problem of this complexity remain open yet.