RT Journal Article SR Electronic T1 Intronic SMCHD1 variants in FSHD: testing the potential for CRISPR-Cas9 genome editing JF Journal of Medical Genetics JO J Med Genet FD BMJ Publishing Group Ltd SP jmedgenet-2019-106402 DO 10.1136/jmedgenet-2019-106402 A1 Remko Goossens A1 Marlinde L van den Boogaard A1 Richard J L F Lemmers A1 Judit Balog A1 Patrick J van der Vliet A1 Iris M Willemsen A1 Julie Schouten A1 Ignazio Maggio A1 Nienke van der Stoep A1 Rob C Hoeben A1 Stephen J Tapscott A1 Niels Geijsen A1 Manuel A F V Gonçalves A1 Sabrina Sacconi A1 Rabi Tawil A1 Silvère M van der Maarel YR 2019 UL http://jmg.bmj.com/content/early/2019/11/18/jmedgenet-2019-106402.abstract AB Background Facioscapulohumeral dystrophy (FSHD) is associated with partial chromatin relaxation of the DUX4 retrogene containing D4Z4 macrosatellite repeats on chromosome 4, and transcriptional de-repression of DUX4 in skeletal muscle. The common form of FSHD, FSHD1, is caused by a D4Z4 repeat array contraction. The less common form, FSHD2, is generally caused by heterozygous variants in SMCHD1.Methods We employed whole exome sequencing combined with Sanger sequencing to screen uncharacterised FSHD2 patients for extra-exonic SMCHD1 mutations. We also used CRISPR-Cas9 genome editing to repair a pathogenic intronic SMCHD1 variant from patient myoblasts.Results We identified intronic SMCHD1 variants in two FSHD families. In the first family, an intronic variant resulted in partial intron retention and inclusion of the distal 14 nucleotides of intron 13 into the transcript. In the second family, a deep intronic variant in intron 34 resulted in exonisation of 53 nucleotides of intron 34. In both families, the aberrant transcripts are predicted to be non-functional. Deleting the pseudo-exon by CRISPR-Cas9 mediated genome editing in primary and immortalised myoblasts from the index case of the second family restored wild-type SMCHD1 expression to a level that resulted in efficient suppression of DUX4.Conclusions The estimated intronic mutation frequency of almost 2% in FSHD2, as exemplified by the two novel intronic SMCHD1 variants identified here, emphasises the importance of screening for intronic variants in SMCHD1. Furthermore, the efficient suppression of DUX4 after restoring SMCHD1 levels by genome editing of the mutant allele provides further guidance for therapeutic strategies.