Dominant genetic alterations in immortalization: Role for 20q gain

Abstract

Gain of 20q has been observed in many cancer types, including bladder cancers. However, the biological significance of low-copy-number 20q gain in human cancer pathogenesis has not yet been defined. We reported that immortalization of human uroepithelial cells (HUC) transformed with human papillomavirus 16 (HPV 16) E7 is associated with single-copy 20q gain (P = 2 x 10(-7)). We also observed 20q13.2 amplification in some cell lines, but only after 20 passages. Thus, we hypothesized that low-copy gain of 20q gene(s) contributes in a dominant way to bypassing HUC senescence. To test this hypothesis, we fused precrisis E7-transformed HUCs (pcE7s) with three independent immortal E7-HUCs that acquired a single-copy 20q gain at immortalization. In one of these lines, a single-copy gain of 20q and a 10p12.1-pter loss were the only cytogenetic alterations. Immortal cell hybrids were obtained with all three crosses. Southern analysis for unique HPV16 insertion sites, as well as fluorescence in situ hybridization (FISH) with whole chromosome 20 painting probes (WCP20) for marker chromosomes in the immortal clones, confirmed the hybrid and independent nature of representative immortal clones. In contrast, when we used the same protocol, no immortal somatic cell hybrids were obtained when HPV16 E6 immortal HUC (E6-HUC) that showed 3p and 9p losses, but no 20q gain, were fused with precrisis E6-transformed HUC (pcE6s). This latter observation is consistent with many results demonstrating that recessive changes are required for cell immortalization. Therefore, the new results reported herein for the first time demonstrate that dominant changes can contribute to bypassing senescence, and that such genes may be located on 20q. Genes Chromosomes Cancer 26:304-311, 1999.

Expression of hTERT immortalises normal human urothelial cells without inactivation of the p16/Rb pathway

“…The lack of immortalisation by these modulations alone is consistent with the observation that HPV E7-transduced NHUC immortalise only after a crisis period in which additional aberrations such as gains on chromosome 20q are acquired (Cuthill et al, 1999). A requirement for alterations of the Rb pathway has also been suggested when NHUC are immortalised by HPV E6, as in these cases p16 expression is commonly lost due to hemizygous deletion of 9p21 (Reznikoff et al, 1996).…”

BORIS, a paralogue of the transcription factor, CTCF, is aberrantly expressed in breast tumours

“…This chromosome region is often amplified in many cancers and is believed to contain a dominant immortalising or transforming gene(s) (Tanner et al, 1994;Cuthill et al, 1999). Recent reports show that BORIS is a downstream regulator of cancer -testis genes: expression of BORIS in normal cells leads to derepression of cancer -testis genes MAGE-A1, NY-ESO-1 and others Vatolin et al, 2005).…”

Amplification of chromosome 5p correlates with increased expression of Skp2 in HPV-immortalized keratinocytes

“…Aberrations of each chromosome were observed; however, the most striking were structural rearrangements and gain of chromosomes 5 a b c (Cuthill et al, 1999;Reznikoff et al, 1994;Savelieva et al, 1997) and trisomy of chromosome 20 is observed in human prostate epithelial cells immortalized with HPV16 E6 and E7. M-FISH data revealed amplification of chromosome 5 material to be a consistent event, as a result of small metacentric chromosome formation or translocation.…”