Expression of an embryonic fibronectin splicing variant in human masseter muscle

Fibronectin is an extracellular matrix protein that is a key constituent of all skeletal muscles. Its deposition increases in a number of pathological conditions, including some muscular dystrophies in which a progressive increase in lower-face height is often noted. It has been shown in other organ systems that increased deposition of fibronectin is associated with changes in the expression of differentially spliced isoforms of the molecule. This investigation documents the expression of mRNA coding for fibronectin and its splicing variants, EIIIA and EIIIB, in biopsies of masseter muscle from normal and long-face patients, using reverse transcriptase-polymerase chain reaction. Expression was compared with that occurring in anatomically and embryologically differing somatic skeletal muscle. Masseter expressed fibronectin mRNA containing the EIIIA but not the EIIIB splicing variant. Conversely, somatic skeletal muscle expressed neither the EIIIA nor EIIIB variants. There were no differences between expression of fibronectin containing the EIIIA splicing variants in masseter from normal and long faces. These results suggest that variations in fibronectin expression reflect the differing functional demands of muscles from different anatomical locations and indicate that jaw and somatic muscle belong to different allotypes.