Myofibrillar myopathy (MFM) is a pathologically defined group of hereditary human muscle diseases, characterized by focal myofibrillar destruction and cytoplasmic aggregates that contain several Z-disc-related proteins. The previously reported MFM-associated mutation (8130G → W2710X) in the filamin C gene (FLNC) leads to a partial disturbance of the secondary structure of the dimerization domain of filamin C, resulting in massive protein aggregation in skeletal muscle fibers of the patients. Here, we provide a thorough characterization of the biochemical, biophysical and cellular properties of the mutated filamin C polypeptide. Our experiments revealed that the mutant dimerization domain is less stable and more susceptible to proteolysis. As a consequence, it does not dimerize properly and forms aggregates in vitro. Furthermore, the expression of mutant filamin in cultured cells results in the formation of protein aggregates. The mutant filamin does not associate with wild type filamin. These findings are of great importance to explain the pathomechanism of this disease.