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Microalterations of Inherently Unstable Genomic Regions in Rat Mammary Carcinomas as Revealed by Long Oligonucleotide Array-Based Comparative Genomic Hybridization

¹ Human and Molecular Genetics Center and Departments of ² Surgery and ³ Physiology and Paediatrics, Medical College of Wisconsin, Milwaukee, Wisconsin; ⁴ Hubrecht Institute and University Medical Center, Cancer Genomics Center, Utrecht, The Netherlands; ⁵ Department of Physics and Comprehensive Diabetes Center, University of Alabama at Birmingham, Birmingham, Alabama; and ⁶ Department of Surgery, University of Iowa, Iowa City, Iowa

Requests for reprints: Tatjana Adamovic, Lars Kaggsgatan 35E, #25, 415 03 Göteborg, Sweden. Phone: 0046-0705-10-56-90; E-mail: tatjana.adamovic{at}hotmail.com.

Key Words: rat mammary cancer • SS and BN rat • aCGH • reference DNA • CNV

The presence of copy number variants in normal genomes poses a challenge to identify small genuine somatic copy number changes in high-resolution cancer genome profiling studies due to the use of unpaired reference DNA. Another problem is the well-known rearrangements of immunoglobulin and T-cell receptor genes in lymphocytes (a commonly used reference), which may misdirect the researcher to a locus with no relevance in tumorigenesis. We here show real gains of the IgG heavy chain V gene region in carcinogen-induced rat mammary tumor samples after normalization to paired mammary gland, a tissue without lymphocyte infiltration. We further show that the segmental duplication region encompassing the IgG heavy chain V genes is a copy number variant between the susceptible (SS) and the resistant (BN) to mammary tumor development inbred rat strains. Our data suggest that the already inherently unstable genomic region is a convenient target for additional structural rearrangements (gains) at the somatic level when exposed to a carcinogen (7,12-dimethylbenz[a]anthracene), which subsequently seem to benefit tumor development in the mammary gland of the susceptible strain. Thus, the selection of an appropriate reference DNA enabled us to identify immunoglobulin genes as novel cancer targets playing a role in mammary tumor development. We conclude that control DNA in array-based comparative genomic hybridization experiments should be selected with care, and DNA from pooled spleen (contains immature lymphocytes and is used as reference in animal studies) or blood may not be the ideal control in the study of primary tumors. [Cancer Res 2009;69(12):5159–67]