Combined genome-wide allelotyping and copy number analysis identify frequent genetic losses without copy number reduction in medulloblastoma.

Detailed analysis of mechanisms of genetic loss for specific tumor suppressor genes (TSGs; e.g., RB1, APC and NF1) indicates that TSG inactivation can occur by allelic loss of heterozygosity (LOH), without any alteration in DNA copy number. However, the role and prevalence of such events in the pathogenesis of specific malignancies remains to be established on a genome-wide basis. We undertook a detailed molecular assessment of chromosomal defects in a panel of nine cell lines derived from primary medulloblastomas, the most common malignant brain tumors of childhood, by parallel genome-wide assessment of LOH (allelotyping) and copy number aberrations (comparative genomic hybridization and fluorescence in situ hybridization). The majority of genetic losses observed were detected by both copy number and LOH methods, indicating they arise through the physical deletion of chromosomal material. However, a significant proportion of losses (17/42, 40%) represented regions of allelic LOH without any associated copy number reduction; these events involved both whole chromosomes (10/17) and sub-chromosomal regions (7/17). Using this approach, we identified medulloblastoma-characteristic alterations, e.g., isochromosome for 17q, MYC amplification and losses on chromosomes 10, 11, and 16, alongside novel regions of genetic loss (e.g., 10q21.1-26.3, 11q24.1-qter). This detailed genetic characterization of the majority of medulloblastoma cell lines provides important precedent for the widespread involvement of copy number-neutral genetic losses in medulloblastoma and demonstrates that combined assessment of copy number aberrations and LOH will be necessary to accurately determine the contribution of chromosomal defects to tumor development.