Seizures-scoliosis-macrocephaly syndrome (OMIM 616682)

Inspired by a patient

...seizures, scoliosis, and macrocephaly/microcephaly syndrome (SSMS) is caused by homozygous or compound heterozygous mutation in the EXT2 gene (608210) on chromosome 11p12.

Seizures, scoliosis, and macrocephaly/microcephaly syndrome (SSMS) is an autosomal recessive neurodevelopmental disorder characterized by global developmental delay apparent from early infancy, impaired intellectual development, behavioral problems, poor or absent speech, seizures, dysmorphic facial features with macro- or microcephaly, and skeletal abnormalities, including scoliosis and delayed bone age. Other features may include hypotonia, gastrointestinal problems, and exostoses 

https://www.omim.org/entry/616682

Gentile M, Agolini E, Cocciadiferro D, Ficarella R, Ponzi E, Bellacchio E, Antonucci MF, Novelli A. Novel exostosin-2 missense variants in a family with autosomal recessive exostosin-2-related syndrome: further evidences on the phenotype. Clin Genet. 2019 Jan;95(1):165-171. doi: 10.1111/cge.13458. Epub 2018 Oct 24. PMID: 30288735.

Abstract

Biallelic exostosin-2 (EXT2) pathogenic variants have been described as the cause of the Seizures-Scoliosis-Macrocephaly syndrome (OMIM 616682) characterized by intellectual disability, facial dysmorphisms and seizures. More recently, it has been proposed to rename this disorder with the acronym AREXT2 (autosomal recessive EXT2-related syndrome). Here, we report the third family affected by AREXT2 syndrome, harboring compound missense variants in EXT2, p.Asp227Asn, and p.Tyr608Cys. In addition, our patients developed multiple exostoses, which were not observed in the previously described families. AREXT2 syndrome can be considered as a multiorgan Congenital Disorder of Glycosylation caused by a significant, but non-lethal, decrease in EXT2 expression, thereby affecting the synthesis of the heparan sulfate proteoglycans, which is relevant in many physiological processes. Our finding expands the clinical and molecular spectrum of the AREXT2 syndrome and suggests a possible genotype/phenotype correlation in the development of the exostoses.

Gupta A, Ewing SA, Renaud DL, Hasadsri L, Raymond KM, Klee EW, Gavrilova RH. Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants. Clin Case Rep. 2019 Feb 19;7(4):632-637. doi: 10.1002/ccr3.2010. PMID: 30997052; PMCID: PMC6452521.

Abstract

We report a patient with developmental delay, autism, epilepsy, macrocephaly, facial dysmorphism, gastrointestinal, and behavioral issues due to EXT2 compound heterozygous likely pathogenic variants. This case report expands the EXT2 gene mutation database and the clinical spectrum of patients with deficiencies in the heparan sulfate pathway.

El-Bazzal L, Atkinson A, Gillart AC, Obeid M, Delague V, Mégarbané A. A novel EXT2 mutation in a consanguineous family with severe developmental delay, microcephaly, seizures, feeding difficulties, and osteopenia extends the phenotypic spectrum of autosomal recessive EXT2-related syndrome (AREXT2). Eur J Med Genet. 2019 Apr;62(4):259-264. doi: 10.1016/j.ejmg.2018.07.025. Epub 2018 Jul 31. PMID: 30075207.

Abstract

We report a consanguineous family where 2 boys presented with developmental delay, hypotonia, microcephaly, seizures, gastro-intestinal abnormalities, osteopenia, and neurological regression. Whole exome sequencing performed in one of the boys revealed the presence of a novel homozygous missense variant in the EXT2 gene: c.11C > T (p.Ser4Leu). Segregation analysis by Sanger sequencing confirmed homozygous by descent autosomal recessive transmission of this mutation. Another family was previously reported with homozygous mutations in this gene in four siblings affected with a nearly similar clinical condition (Farhan et al., 2015). We discuss the similarities and differences between the two syndromes and propose AREXT2 as a new acronym for EXT2-related diseases.

Farhan SM, Wang J, Robinson JF, Prasad AN, Rupar CA, Siu VM; FORGE Canada Consortium, Hegele RA. Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses. J Med Genet. 2015 Oct;52(10):666-75. doi: 10.1136/jmedgenet-2015-103279. Epub 2015 Aug 5. PMID: 26246518.

Abstract

Background: Heparan sulfate proteoglycans are vital components of the extracellular matrix and are essential for cellular homeostasis. Many genes are involved in modulating heparan sulfate synthesis, and when these genes are mutated, they can give rise to early-onset developmental disorders affecting multiple body systems. Herein, we describe a consanguineous family of four sibs with a novel disorder, which we designate as seizures-scoliosis-macrocephaly syndrome, characterised by seizures, intellectual disability, hypotonia, scoliosis, macrocephaly, hypertelorism and renal dysfunction. 

Methods: Our application of autozygosity mapping and whole-exome sequencing allowed us to identify mutations in the patients. To confirm the autosomal-recessive mode of inheritance, all available family members were genotyped. We also studied the effect of these mutations on protein expression and function in patient cells and using an in vitro system.

Results: We identified two homozygous mutations p.Met87Arg and p.Arg95 Cys in exostosin 2, EXT2, a ubiquitously expressed gene that encodes a glycosyltransferase required for heparan sulfate synthesis. In patient cells, we observed diminished EXT2 expression and function. We also performed an in vitro assay to determine which mutation has a larger effect on protein expression and observed reduced EXT2 expression in constructs expressing either one of the mutations but a greater reduction when both residues were mutated. 

Conclusions: In short, we have unravelled the genetic basis of a new recessive disorder, seizures-scoliosis-macrocephaly syndrome. Our results have implicated a well-characterised gene in a new developmental disorder and have further illustrated the spectrum of phenotypes that can arise due to errors in glycosylation.