How much mutant protein is needed to cause a protein aggregate myopathy in vivo? Lessons from an exceptional desminopathy.
Myofibrillar myopathies are caused by mutations in desmin, alphaB-crystallin, myotilin, ZASP, and filamin C genes. Since the vast majority of myofibrillar myopathy causing mutations are heterozygous single amino acid substitutions or small in-frame deletions, the pathogenic role of mutant versus wild-type protein cannot be assessed in human skeletal muscle by standard immunodetection techniques. We report on an exceptional desminopathy due to a heterozygous c.735G>C mutation. Immunoblotting detected full-length 53 kDa desmin and a truncated 50 kDa variant in skeletal muscle from three affected patients of two different families. RT-PCR identified three desmin mRNA species encoding for wild-type and two mutant proteins, p.Glu245Asp and p.Asp214_Glu245del. Since previous functional studies on the p.Glu245Asp mutant showed biological properties identical to wild-type desmin, the truncated p.Asp214_Glu245del desmin is the disease-causing mutant. Semiquantitative RT-PCR established a fraction of the truncated desmin mRNA species in a range from 24% to 37%. Initial quantification of corresponding desmin proteins in the muscle biopsy of the index patient of one family indicated a fraction of only 10% of the truncated species. However, serial analyses of different sections from each muscle biopsy revealed a high intra- and interindividual variability of the truncated desmin protein level within a range from 5% to 43%. Desmin assembly studies in vitro have established clear-cut pathogenic ratios of mutant versus wild-type proteins. However, our findings point out a far more complex situation in human skeletal muscle. The heterogeneously distributed mutation load within and between individual specimens, which reflects local differences in the expression and/or turnover of the mutant protein in different areas containing multiple myonuclear domains, renders it impossible to define an exact pathogenic threshold of a specific mutant in vivo.
Clemen
Christoph S
CS
Institute of Biochemistry I, Medical Faculty, University of Cologne, Germany.
Fischer
Dirk
D
Reimann
Jens
J
Eichinger
Ludwig
L
Müller
Clemens R
CR
Müller
Harald D
HD
Goebel
Hans H
HH
Schröder
Rolf
R
D001483:Base Sequence; D015153:Blotting, Western; D004252:DNA Mutational Analysis; D003893:Desmin / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D005192:Family Health; D005260:Female; D015870:Gene Expression; D006801:Humans; D008297:Male; D008854:Microscopy, Electron; D018482:Muscle, Skeletal / Q000378:metabolism / Q000473:pathology / Q000648:ultrastructure; D009135:Muscular Diseases / Q000235:genetics* / Q000378:metabolism / Q000473:pathology; D009154:Mutation; D010375:Pedigree; D020133:Reverse Transcriptase Polymerase Chain Reaction; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization
D003893:Desmin