Objective: To identify the genetic cause in a familial cortical myoclonic tremor with epilepsy (FCMTE) pedigree without reported pathogenic(TTTCA)n insertions.

Background: FCMTE is an autosomal dominant neurodegenerative disease, characterized by cortical tremor and epileptic seizures1. Intronic (TTTCA)n expansions inserted between polymorphic (TTTTA)n expansion and poly(A) tails of Alu elements in the SAMD12, TNRC6A and RAPGEF2 genes have been identified as causes in 78 pedigrees from Japan and China2, 3. However, these (TTTCA)n insertions can only explain 18 out of 21 pedigrees in our FCMTE cohort, indicating the existence of other genetic causes3.

Method: The FCMTE pedigree in this study is one of the two pedigrees with no (TTTCA)n insertion identified in our previous report [detection of (TTTCA)n insertion or (TTTTA)n expansion in TNRC6A and RAPGEF2 was also negative]3. Repeat-primed PCR, long-range PCR and Sanger sequencing were used to find a candidate genetic cause in this pedigree. Targeted long-read sequencing in PacBio RS II platform was performed to identify the detailed structure of this novel intronic pentanucleotide expanded repeat insertion.

Results: A novel expanded intronic (TTTGA)n insertion at the same site of the previously reported (TTTCA)n insertion in SAMD12 co-segregating with the disease in the FCMTE pedigree was identified (figure 1). Targeted long-read sequencing in PacBio RS II platform was performed in two patients (II:6 and III:4) and one asymptomatic carrier (IV:2) in this pedigree, with one previously reported patient (P-I-III2 from an another pedigree, Pedigree I) carrying (TTTCA)n insertion as positive control (figure 2) 3, 4. A total of 302, 159, 207 and 50 on-target subreads (accuracy ≥90%) spanning the target repeat expansion region were generated for these individuals respectively, which uncovered the accurate structures of the repeat expansions as (TTTTA)114-124(TTTGA)108-116 in the pedigree and (TTTTA)38(TTTCA)479 in P-I-III2 (figure 3 and table 1).

Conclusion: The targeted long-read sequencing helps us elucidate the accurate structures of the pathogenic (TTTGA)n and (TTTCA)n insertions. Our finding further expands the mutational spectrum of SAMD12-associated FCMTE and sheds light on the identification of the genetic causes of FCMTE pedigrees with when no (TTTCA)n insertion could be detected in the reported causative genes.

References: 1. van den Ende T, Sharifi S, van der Salm SMA, van Rootselaar AF. Familial Cortical Myoclonic Tremor and Epilepsy, an Enigmatic Disorder: From Phenotypes to Pathophysiology and Genetics. A Systematic Review. Tremor Other Hyperkinet Mov (N Y) 2018;8:503. 2. Ishiura H, Doi K, Mitsui J, Yoshimura J, Matsukawa MK, Fujiyama A, Toyoshima Y, Kakita A, Takahashi H, Suzuki Y, Sugano S, Qu W, Ichikawa K, Yurino H, Higasa K, Shibata S, Mitsue A, Tanaka M, Ichikawa Y, Takahashi Y, Date H, Matsukawa T, Kanda J, Nakamoto FK, Higashihara M, Abe K, Koike R, Sasagawa M, Kuroha Y, Hasegawa N, Kanesawa N, Kondo T, Hitomi T, Tada M, Takano H, Saito Y, Sanpei K, Onodera O, Nishizawa M, Nakamura M, Yasuda T, Sakiyama Y, Otsuka M, Ueki A, Kaida KI, Shimizu J, Hanajima R, Hayashi T, Terao Y, Inomata-Terada S, Hamada M, Shirota Y, Kubota A, Ugawa Y, Koh K, Takiyama Y, Ohsawa-Yoshida N, Ishiura S, Yamasaki R, Tamaoka A, Akiyama H, Otsuki T, Sano A, Ikeda A, Goto J, Morishita S, Tsuji S. Expansions of intronic TTTCA and TTTTA repeats in benign adult familial myoclonic epilepsy. Nat Genet 2018. 3. Cen Z, Jiang Z, Chen Y, Zheng X, Xie F, Yang X, Lu X, Ouyang Z, Wu H, Chen S, Yin H, Qiu X, Wang S, Ding M, Tang Y, Yu F, Li C, Wang T, Ishiura H, Tsuji S, Jiao C, Liu C, Xiao J, Luo W. Intronic pentanucleotide TTTCA repeat insertion in the SAMD12 gene causes familial cortical myoclonic tremor with epilepsy type 1. Brain 2018. 4. Cen Z, Huang C, Yin H, Ding X, Xie F, Lu X, Ouyang Z, Lou Y, Qiu X, Wang Z, Xiao J, Ding M, Luo W. Clinical and neurophysiological features of familial cortical myoclonic tremor with epilepsy. Mov Disord 2016;31:1704-1710.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/a-novel-intronic-pentanucleotide-tttga-repeat-insertion-in-samd12-causes-familial-cortical-myoclonic-tremor-with-epilepsy-type-1/