[Effect of the serum of a patient affected by myeloma with diffuse muscular hypertrophy on the growth of human muscle cells in vitro].

Diffuse skeletal muscle enlargement appeared in a 62 years old male without any detectable endocrine disturbance. The patient's weakness contrasted with his athletic appearance. There was no myotonia nor fasciculations. The laboratory investigations were unrevealing until 18 months after the apparent beginning of the disease: free kappa light chains in serum and urine and plasmocytosis in the bone marrow established the diagnosis of multiple myeloma. No amyloid infiltration was observed on the biopsied tissues: skin, muscle and tongue. The diameters of the muscle fibers were at the upper limit of the normal range. There was no fiber type grouping. Kappa light chain deposits surrounding the muscle fibers and numerous coated vesicles beneath the sarcolemmal membrane were found by electron microscopy. The patient's serum was tested on human muscle cell cultures and displayed trophic properties for these cells. This effect was not due to an acceleration of the myoblast proliferation, but to an enhancement of their differentiation. The fusion was more rapid than in presence of control serum and lead to larger and highly branched myotubes. Protein synthesis was increased as demonstrated by the significantly higher 3H-leucine incorporation into the trichloroacetic-acid precipitable material. An identical trophic effect was also found with the kappa light chains purified from the patient's serum but only in the presence of normal human serum.