Inspired by a colleague's patient
Machol K, Rousseau J, Ehresmann S, Garcia T, Nguyen TTM, Spillmann RC, Sullivan JA, Shashi V, Jiang YH, Stong N, Fiala E, Willing M, Pfundt R, Kleefstra T, Cho MT, McLaughlin H, Rosello Piera M, Orellana C, Martínez F, Caro-Llopis A, Monfort S, Roscioli T, Nixon CY, Buckley MF, Turner A, Jones WD, van Hasselt PM, Hofstede FC, van Gassen KLI, Brooks AS, van Slegtenhorst MA, Lachlan K, Sebastian J, Madan-Khetarpal S, Sonal D, Sakkubai N, Thevenon J, Faivre L, Maurel A, Petrovski S, Krantz ID, Tarpinian JM, Rosenfeld JA, Lee BH; Undiagnosed Diseases Network, Campeau PM. Expanding the Spectrum of BAF-Related Disorders: De Novo Variants in SMARCC2 Cause a Syndrome with Intellectual Disability and Developmental Delay. Am J Hum Genet. 2019 Jan 3;104(1):164-178.
SMARCC2 (BAF170) is one of the invariable core subunits of the ATP-dependent chromatin remodeling BAF (BRG1-associated factor) complex and plays a crucial role in embryogenesis and corticogenesis. Pathogenic variants in genes encoding other components of the BAF complex have been associated with intellectual disability syndromes. Despite its significant biological role, variants in SMARCC2 have not been directly associated with human disease previously. Using whole-exome sequencing and a web-based gene-matching program, we identified 15 individuals with variable degrees of neurodevelopmental delay and growth retardation harboring one of 13 heterozygous variants in SMARCC2, most of them novel and proven de novo. The clinical presentation overlaps with intellectual disability syndromes associated with other BAF subunits, such as Coffin-Siris and Nicolaides-Baraitser syndromes and includes prominent speech impairment, hypotonia, feeding difficulties, behavioral abnormalities, and dysmorphic features such as hypertrichosis, thick eyebrows, thin upper lip vermilion, and upturned nose. Nine out of the fifteen individuals harbor variants in the highly conserved SMARCC2 DNA-interacting domains (SANT and SWIRM) and present with a more severe phenotype. Two of these individuals present cardiac abnormalities. Transcriptomic analysis of fibroblasts from affected individuals highlights a group of differentially expressed genes with possible roles in regulation of neuronal development and function, namely H19, SCRG1, RELN, and CACNB4. Our findings suggest a novel SMARCC2-related syndrome that overlaps with neurodevelopmental disorders associated with variants in BAF-complex subunits.
Martínez F, Caro-Llopis A, Roselló M, Oltra S, Mayo S, Monfort S, Orellana C. High diagnostic yield of syndromic intellectual disability by targeted next-generation sequencing. J Med Genet. 2017 Feb;54(2):87-92.
Intellectual disability is a very complex condition where more than 600 genes have been reported. Due to this extraordinary heterogeneity, a large proportion of patients remain without a specific diagnosis and genetic counselling. The need for new methodological strategies in order to detect a greater number of mutations in multiple genes is therefore crucial.
In this work, we screened a large panel of 1256 genes (646 pathogenic, 610 candidate) by next-generation sequencing to determine the molecular aetiology of syndromic intellectual disability. A total of 92 patients, negative for previous genetic analyses, were studied together with their parents. Clinically relevant variants were validated by conventional sequencing.
A definitive diagnosis was achieved in 29 families by testing the 646 known pathogenic genes. Mutations were found in 25 different genes, where only the genes KMT2D, KMT2A and MED13L were found mutated in more than one patient. A preponderance of de novo mutations was noted even among the X linked conditions. Additionally, seven de novo probably pathogenic mutations were found in the candidate genes AGO1, JARID2, SIN3B, FBXO11, MAP3K7, HDAC2 and SMARCC2. Altogether, this means a diagnostic yield of 39% of the cases (95% CI 30% to 49%).
The developed panel proved to be efficient and suitable for the genetic diagnosis of syndromic intellectual disability in a clinical setting. Next-generation sequencing has the potential for high-throughput identification of genetic variations, although the challenges of an adequate clinical interpretation of these variants and the knowledge on further unknown genes causing intellectual disability remain to be solved.