The pathological process in myofibrillar myopathy (MFM) (previously also referred to as ''desmin storage'' or ''intermediate filament myopathy'') results in dissolution of myofibrils, accumulation of products of the degradative process, and abnormal ectopic expression of desmin, dystrophin, gelsolin, NCAM, and N-terminal components of beta-amyloid precursor protein. We now demonstrate that the abnormal fiber regions in MFM immunoreact strongly for (a) CDC2 kinase, the mitotic kinase that phosphorylates and disassembles intermediate filaments; (b) cyclin-dependent kinases CDK2, CDK4, and CDK7, which are involved in regulation of the cell cycle; (c) lamin B, which normally supports the inner nuclear membrane; and (d) the nuclear matrix associated protein. The normal muscle fiber lies in a terminally differentiated state and is refractory to reentry into the cell cycle. The abnormal expression of multiple cyclin-dependent kinases in the terminally differentiated muscle fiber implies inappropriate activation of positive regulators bf mitosis and may signal a mitotic catastrophe. The dissolution of myofibrils may be due to hyperphosphorylation occurring during this event.
