J Hum Genet. 2021 Apr;66(4):419-429. doi: 10.1038/s10038-020-00855-0. Epub 2020 Oct 10.
DNA analysis of benign adult familial myoclonic epilepsy reveals associations between the pathogenic TTTCA repeat insertion in SAMD12 and the nonpathogenic TTTTA repeat expansion in TNRC6A.
Benign adult familial myoclonic epilepsy (BAFME) is an autosomal dominant disease characterized by adult-onset tremulous hand movement, myoclonus, and infrequent epileptic seizures. Recently, intronic expansion of unstable TTTCA/TTTTA pentanucleotide repeats in SAMD12, TNRC6A, or RAPGEF2 was identified as pathological mutations in Japanese BAFME pedigrees. To confirm these mutations, we performed a genetic analysis on 12 Japanese BAFME pedigrees. A total of 143 participants, including 43 familial patients, 5 suspected patients, 3 sporadic nonfamilial patients, 22 unaffected familial members, and 70 unrelated controls, were screened for expanded abnormal pentanucleotide repeats in SAMD12, TNRC6A, RAPGEF2, YEAT2, MARCH6, and STARD7. DNA samples were analyzed using Southern blotting, long-range polymerase chain reaction (PCR), repeat-primed PCR, and long-range PCR followed by Southern blotting. Of the 51 individuals with clinically diagnosed or suspected BAFME, 49 carried a SAMD12 allele with an expanded TTTCA/TTTTA pentanucleotide repeat. Genetic and clinical anticipation was observed. As in previous reports, the one patient with homozygous mutant alleles showed more severe symptoms than the heterozygous carriers. In addition, screening for expanded pentanucleotide repeats in TNRC6A revealed that the frequency of expanded TTTTA repeat alleles in the BAFME group was significantly higher than in the control group. All patients who were clinically diagnosed with BAFME, including those in the original family reported by Yasuda, carried abnormally expanded TTTCA/TTTTA repeat alleles of SAMD12.
Patients with BAFME also frequently carried a TTTTA repeat expansion in TNRC6A, suggesting that there may be unknown factors in the ancestry of patients with BAFME that make pentanucleotide repeats unstable.
常染色体顕性遺伝形式を示す良性家族性成人ミオクローヌスてんかん（BAFME)は、日本人の多い疾患でてんかん学会で多数の症例の登録を行ってきた。8q23.3–q24.13との連鎖がしめされていたが、Long-read NGSでSAMD12遺伝子内にTTTCA/TTTTA リピートを認め、家系内の発症年齢の低年齢化とリP-ト数の増加の関係を認めた。