BACKGROUND: Extensive genetic screening results in the identification of thousands of rare variants that are difficult to interpret. Because of its sheer size, rare variants in the titin gene (TTN) are detected frequently in any individual. Unambiguous interpretation of molecular findings is almost impossible in many patients with myopathies or cardiomyopathies. OBJECTIVE: To refine the current classification framework for TTN-associated skeletal muscle disorders and standardize the interpretation of TTN variants. METHODS: We used the guidelines issued by the American College of Medical Genetics and Genomics (ACMG) and the Association for Molecular Pathology (AMP) to re-analyze TTN genetic findings from our patient cohort. RESULTS: We identified in the classification guidelines three rules that are not applicable to titin-related skeletal muscle disorders; six rules that require disease-/gene-specific adjustments and four rules requiring quantitative thresholds for a proper use. In three cases, the rule strength need to be modified. CONCLUSIONS: We suggest adjustments are made to the guidelines. We provide frequency thresholds to facilitate filtering of candidate causative variants and guidance for the use and interpretation of functional data and co-segregation evidence. We expect that the variant classification framework for TTN-related skeletal muscle disorders will be further improved along with a better understanding of these diseases.

• MeSH
• Cardiomyopathies * classification   congenital   genetics   MeSH
• Connectin genetics   MeSH
• Humans MeSH
• Muscular Diseases * classification   congenital   genetics   MeSH
• Practice Guidelines as Topic standards   MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH

Myasthenia gravis (MG) is an autoimmune disease caused by antibodies targeting the neuromuscular junction of skeletal muscles. Triple-seronegative MG (tSN-MG, without detectable AChR, MuSK and LRP4 antibodies), which accounts for ~10% of MG patients, presents a serious gap in MG diagnosis and complicates differential diagnosis of similar disorders. Several AChR antibody positive patients (AChR-MG) also have antibodies against titin, usually detected by ELISA. We have developed a very sensitive radioimmunoprecipitation assay (RIPA) for titin antibodies, by which many previously negative samples were found positive, including several from tSN-MG patients. The validity of the RIPA results was confirmed by western blots. Using this RIPA we screened 667 MG sera from 13 countries; as expected, AChR-MG patients had the highest frequency of titin antibodies (40.9%), while MuSK-MG and LRP4-MG patients were positive in 14.6% and 16.4% respectively. Most importantly, 13.4% (50/372) of the tSN-MG patients were also titin antibody positive. None of the 121 healthy controls or the 90 myopathy patients, and only 3.6% (7/193) of other neurological disease patients were positive. We thus propose that the present titin antibody RIPA is a useful tool for serological MG diagnosis of tSN patients.

• MeSH
• Autoantibodies blood   MeSH
• Enzyme-Linked Immunosorbent Assay MeSH
• Connectin immunology   MeSH
• Humans MeSH
• International Cooperation MeSH
• Myasthenia Gravis blood   diagnosis   epidemiology   MeSH
• LDL-Receptor Related Proteins immunology   MeSH
• Radioimmunoprecipitation Assay MeSH
• Receptors, Cholinergic immunology   MeSH
• Receptor Protein-Tyrosine Kinases immunology   MeSH
• Check Tag
• Humans MeSH
• Male MeSH
• Female MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH

• MeSH
• Connectin chemistry   adverse effects   MeSH
• Skin Diseases etiology   classification   MeSH
• Humans MeSH
• Paraproteinemias * drug therapy   classification   pathology   MeSH
• Rheumatic Diseases diagnosis   pathology   MeSH
• Sjogren's Syndrome diagnosis   pathology   MeSH
• Lupus Erythematosus, Systemic diagnosis   pathology   MeSH
• Check Tag
• Humans MeSH
• Publication type
• Review MeSH