Monoclonal antibodies to titin in conjunction with antibodies to desmin separate rhabdomyosarcomas from other tumor types.

Titin is a major constituent protein of sarcomeric muscles and is thought to give rise to an elastic filament component underlying the myofibrillar organization. Monoclonal antibodies to titin have been characterized on normal and pathological human material and on human cell lines in culture. A positive immunocytochemical reaction was restricted to sarcomeric muscles and did not occur on visceral or vascular smooth muscles or on various nonmuscle tissues. When different tumor types were examined titin antibodies reacted solely with rhabdomyosarcomas and did not react with leiomyosarcoma or leiomyoma, or with the nonmuscle tumor types tested. In rhabdomyosarcomas a noticeably smaller population of cells were positive with antibodies to titin than with antibodies to desmin showing that individual cells within a rhabdomyosarcoma achieve different degrees of myogenic differentiation. The results reinforce the use of desmin as a marker for muscle sarcomas and show that a positive identification of rhabdomyosarcoma can be achieved by immunocytochemistry with the parallel use of desmin and titin antibodies.