[93] Somatic Mosaicism Is Causative in EXT1/2 Negative Multiple Osteochondroma Detected by Tiling Resolution Array-CGH

K Szuhai, J de Jong, JVMG Bovee, I Jennes, W Wuyts, M Wiweger, HJ Tanke, PCW Hogendoorn. Leiden University Medical Center, Leiden, Netherlands; Leiden Unviersity Medical Center, Leiden, Netherlands; University of Antwerp, Antwerp, Belgium

Background: Multiple osteochondroma (MO) (previously known as multiple exostosis) is a skeletal disorder characterized by the presence of bony outgrowths at the epiphyseal region of long bone. Mutations in EXT1 or EXT2 genes have been observed, both, in sporadic and hereditary cases. Approximately in 70-75% of MO cases point mutation or small deletions of single exons cause the development of the disease. However, in about 15-20 % MO, genomic alteration can not be detected implying the potential role of other alteration such as inversion, translocation or mosaicism. The involvement of other genes or the identification of the putative EXT3 gene still awaits investigation.
Design: We have designed a custom-made Agilent oligonucleotide based chip containing 44000 probes with a tiling coverage of EXT1/2 genes and additional 30 genes involved in heparan sulfate biosynthesis. 16 patient samples with confirmed clinico-pathological appearance of MO and negative for mutation in EXT1 and EXT2 when sequenced and MLPA tested were used (in this study). Array hybridizations, scanning, image and data processing were performed according to standard protocols. As control, DNA samples of 5 MO patients with known deletions in EXT1 or EXT2 genes were selected.
Results: In the control samples all deletions detected by MLPA were confirmed by targeted array-CGH. From the 16 samples with previously undetected mutation, a low degree of deletion of the EXT1 gene was detected in two patients. These results indicated the presence of a mosaic deletion in about 10-15% of the blood cell and were confirmed by interphase FISH.
Conclusions: By using a highly sensitive, custom made chip we identified mosaic deletion of the EXT1 gene in 2 out of 16 analyzed MO patients with otherwise negative test results. Here we showed that oligo-array-CGH approach with tiling resolution have better sensitivity compared to either MLPA based detection or sequencing. For the first time somatic mosaicism with large genomic deletions as the underlying mechanism in MO formation was identified. Furthermore, the existence of mosaic mutations, not alterations of other heparin sulfate biosynthesis related genes, has a significant role in the development of MO in patients who are negative for mutation in Exostosins when analysed by MLPA.
Category: Bone & Soft Tissue

Tuesday, March 10, 2009 9:30 AM

Poster Session III # 11, Tuesday Morning