Mutation spectrum of EXT1 and EXT2 in the Saudi patients with hereditary multiple exostoses

Abstract

Background: Hereditary Multiple Exostoses (HME), also known as Multiple Osteochondromas (MO) is a rare genetic disorder characterized by multiple benign cartilaginous bone tumors, which are caused by mutations in the genes for exostosin glycosyltransferase 1 (EXT1) and exostosin glycosyltransferase 2 (EXT2). The genetic defects have not been studied in the Saudi patients.

Aim of study: We investigated mutation spectrum of EXT1 and EXT2 in 22 patients from 17 unrelated families.

Methods: Genomic DNA was extracted from peripheral leucocytes. The coding regions and intron-exon boundaries of both EXT1 and EXT2 genes were screened for mutations by PCR-sequencing analysis. Gross deletions were analyzed by MLPA analysis.

Results: EXT1 mutations were detected in 6 families (35%) and 3 were novel mutations: c.739G > T (p. E247*), c.1319delG (p.R440Lfs*4), and c.1786delA (p.S596Afs*25). EXT2 mutations were detected in 7 families (41%) and 3 were novel mutations: c.541delG (p.D181Ifs*89), c.583delG (p.G195Vfs*75), and a gross deletion of approximately 10 kb including promoter and exon 1. Five patients from different families had no family history and carried de novo mutations (29%, 5/17). No EXT1 and EXT2 mutations were found in the remaining four families. In total, EXT1 and EXT2 mutations were found in 77% (13/17) of Saudi HME patients.

Conclusion: EXT1 and EXT2 mutations contribute significantly to the pathogenesis of HME in the Saudi population. In contrast to high mutation rate in EXT 1 (65%) and low mutation rate in EXT2 (25%) in other populations, the frequency of EXT2 mutations are much higher (41%) and comparable to that of EXT1 among Saudi patients. De novo mutations are also common and the six novel EXT1/EXT2 mutations further expands the mutation spectrum of HME.

Keywords: EXT1; EXT12; Exostoses; Mutation; Osteochondromas.

Fig. 1

Radiology of patients with osteochondromas. Patient#1 has an osteochondroma at left hip joint; Patient #15 has an osteochondroma at right proximal humerus; Patient#18 has an osteochondroma at left distal radius; and Patient # 21 has a right pelvic osteochondroma with malignant transformation. Osteochondroma is indicated by an arrow

Fig. 2

Sequence analysis of EXT1 and EXT2 in the patients with hereditary multiple exostoses. Representative electropherograms of previously reported EXT1and EXT2 mutations are shown. Heterozygous mutations are present in the patients and affected family members except for the affected mother (patient#12) in Family 9 who carries a homozygous mutation whereas her daughter (patient#13) has a heterozygous mutation. The mutation is indicated by an arrow

Fig. 3

Detection of novel EXT1 and EXT2 mutations. a Sequence analysis of EXT1 and EXT2 in the patients with hereditary multiple exostoses. Representative electropherograms of 5 novel EXT1 and EXT2 mutations are shown. They are also de novo mutations except for c.541delG (p.D181Ifs*89) in Family 6. Heterozygous mutations are present only in the patients. The mutation is indicated by an arrow. b Agarose gel analysis of a homozygous EXT2 exon 1 deletion. PCR products were run in a 1.3% agarose gel. Exon 1 was not amplified from patient #11 whereas the remaining exons 2–14 were amplified (only exon 2 amplification was shown)