Multiple osteochondromas (also called
hereditary multiple exostoses) is an autosomal dominant disorder characterized by multiple cartilaginous
tumors, which are caused by mutations in the genes for
exostosin-1 (EXT1) and
exostosin-2 (EXT2). The goal of this study was to elucidate the genetic alterations in a family with three affected members. Isolation of
RNA from the patients' blood followed by reverse transcription and PCR amplification of selected fragments showed that the three patients lack a specific region of 90 bp from their EXT1
mRNA. This region corresponds to the sequence of exon 8 from the EXT1 gene. No splice site mutation was found around exon 8. However, long-range PCR amplification of the region from intron 7 to intron 8 indicated that the three patients contain a deletion of 4318 bp, which includes exon 8 and part of the flanking introns. There is evidence that the deletion was caused by non-homologous end joining because the breakpoints are not located within a repetitive
element, but contain multiple copies of the deletion hotspot sequence TGRRKM. Exon 8 encodes part of the active site of the EXT1
enzyme, including the DXD signature of all
UDP-
sugar glycosyltransferases. It is conceivable that the
mutant protein exerts a dominant negative effect on the activity of the EXT
glycosyltransferase since it might interact with normal copies of the
enzyme to form an inactive hetero-oligomeric complex. We suggest that sequencing of
RNA might be superior to exome sequencing to detect short deletions of a single exon.