Clinical and molecular studies of EXT1/EXT2 in Bulgaria

Novel deletion and 2397 G>T mutations of the EXT1 gene identified in two Chinese pedigrees with hereditary multiple exostoses using exon sequencing

BACKGROUND

Hereditary multiple exostoses (HME), a rare genetic pediatric disorder, has a peculiar pathogenic mechanism. The results of previous studies have shown that HME is associated with mutations of the EXT1 and EXT2 genes at a molecular genetics level. In our study, two families who received therapy in the Department of Orthopedics of Shanghai Children's Hospital between June, 2017 and November, 2018 were recruited, and a mutational analysis of the EXT1 genes was conducted to further elucidating the relationship between HME and EXT1.

METHODS

Venous blood samples were collected from individuals with HME and their families. Exon sequencing and RT-PCR were performed to comprehensively analyze 11 exons of the EXT1 gene.

RESULTS

The deletion of exon 7 and the 2397 G>T mutation in exon 7 caused deletion mutation and nonsense mutation only in the HME patients. The mutations in exon 7 were tested and verified by Sanger sequencing. RT-PCR showed that the mRNA expression of EXT1 was significantly decreased in the mutation samples compared with the normal samples, which exerted a great influence on the function of EXT1.

CONCLUSIONS

This study identified new mutation sites for the pathogenesis of HME and further clarified the relationship between HME and EXT1.

Multiple Hereditary Exostoses: Report of an EXT2 Gene Mutation in a Colombian Family

Multiple hereditary exostoses (MHE) is a rare disease with autosomal dominant inheritance, caused by heterozygous germline mutations in the EXT1 or EXT2 genes. This disorder is characterized by the growth of prominences surrounded by cartilage in the growth plates and the long bones. Here, we report a family affected by MHE. In this family, a pathogenic variant c.544C > T (p. Arg182Ter) was identified in the EXT2 gene. This variant has been previously described in the literature, and here we are reporting the relationship with clinical findings. MHE is suspected according to the clinical manifestations; molecular research should be performed to establish the most frequent mutations. A support, diagnosis, and follow-up group should be created, and genetic counseling should be available for patients and families.

Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses

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.

A broad spectrum of genomic changes in latinamerican patients with EXT1/EXT2-CDG

Multiple osteochondromatosis (MO), or EXT1/EXT2-CDG, is an autosomal dominant O-linked glycosylation disorder characterized by the formation of multiple cartilage-capped tumors (osteochondromas). In contrast, solitary osteochondroma (SO) is a non-hereditary condition. EXT1 and EXT2, are tumor suppressor genes that encode glycosyltransferases involved in heparan sulfate elongation. We present the clinical and molecular analysis of 33 unrelated Latin American patients (27 MO and 6 SO). Sixty-three percent of all MO cases presented severe phenotype and two malignant transformations to chondrosarcoma (7%). We found the mutant allele in 78% of MO patients. Ten mutations were novel. The disease-causing mutations remained unknown in 22% of the MO patients and in all SO patients. No second mutational hit was detected in the DNA of the secondary chondrosarcoma from a patient who carried a nonsense EXT1 mutation. Neither EXT1 nor EXT2 protein could be detected in this sample. This is the first Latin American research program on EXT1/EXT2-CDG.

Insights into complexity of congenital disorders of glycosylation

Biochemical and biological properties of glycoconjugates are strongly determined by the specific structure of its glycan parts. Glycosylation, the covalent attachment of sugars to proteins and lipids, is very complex and highly-coordinated process involving > 250 gene products. Deficiency of glycosylation enzymes or transporters results in impaired glycosylation, and consequently pathological modulation of many physiological processes. Inborn defects of glycosylation enzymes, caused by the specific mutations, lead to the development of rare, but severe diseases – congenital disorders of glycosylation (CDGs). Up today, there are more than 45 known CDGs. Their clinical manifestations range from very mild to extremely severe (even lethal) and unfortunately, only three of them can be eff ectively treated nowadays. CDG symptoms highly vary, though some are common for several CDG types but also for other unrelated diseases, especially neurological ones, leaving the possibility that many CDGs cases are under- or mis-diagnosed. Glycan analysis of serum transferrin (by isoelectric focusing or more sophisticated methods, such as HPLC (high-performance liquid chromatography) or MALDI (matrix-assisted laser desorption/ionization)) or serum N-glycans (by MS), enzyme activity assays and DNA sequence analysis are the most frequently used methods for CDG screening and identification, since no specific tests are available yet. In this review we summarize the current knowledge on the clinical, biochemical and genetic characteristic of distinct CDGs, as well as existing diagnostic and therapeutic procedures, aiming to contribute to the awareness on the existence of these rare diseases and encourage the eff orts to elucidate its genetic background, improve diagnostics and develop new strategies for their treatment.