cyclin D1, cyclin E 在乳腺良、恶性病变中的表达及其与p16,p21waf1及Rb 蛋白的关系

为探讨cyclinD1,cyclinE在乳腺癌发生发展中的作用及其与细胞周期调控相关基因蛋白的关系,采用免疫组化检测17例非增生乳腺导管上皮、19例不同程度增生的导管上皮及59例乳腺癌中cyclinD1,cyclinE,p16,p21waf1及Rb基因蛋白的表达.结果显示1.非增生乳腺导管上皮除1例cyclinE过表达外均无cyclinD1和cyclinE的过表达.乳腺癌的cyclinD1和cyclinE的过表达率均明显高于良性乳腺组织(P<0.05).2.乳腺癌cyclinD1过表达与淋巴结转移呈正相关(P<0.05),瘤体直径大于5cm者cyclinE过表达呈增加趋势,但差异无显著性意义.3.CyclinD1和cyclinE的过表达呈正相关(P<0.05).4.从非增生乳腺导管上皮到增生直至乳腺癌,p16,p21与cyclinD1,cyclinE含量的比值逐渐递减,而p21含量高于cyclinD1的乳腺癌体积小、淋巴结转移率低(P<0.05).p21阳性率与cyclinD1过表达呈正相关(P<0.01),也随cyclinE的过表达呈上升趋势.Rb基因蛋白的强表达与cyclinD1过表达呈正相关(P<0.01).结果表明CyclinD1和cyclinE蛋白过表达频发于乳腺癌早期,它们可能与p16、p21waf1、pRb共同参与了乳腺癌的发生发展.     

Cyclin E and c-Myc promote cell proliferation in the presence of p16INK4a and of hypophosphorylated retinoblastoma family proteins.

Retroviral expression of the cyclin-dependent kinase (CDK) inhibitor p16(INK4a) in rodent fibroblasts induces dephosphorylation of pRb, p107 and p130 and leads to G1 arrest. Prior expression of cyclin E allows S-phase entry and long-term proliferation in the presence of p16. Cyclin E prevents neither the dephosphorylation of pRb family proteins, nor their association with E2F proteins in response to p16. Thus, cyclin E can bypass the p16/pRb growth-inhibitory pathway downstream of pRb activation. Retroviruses expressing E2F-1, -2 or -3 also prevent p16-induced growth arrest but are ineffective against the cyclin E-CDK2 inhibitor p27(Kip1), suggesting that E2F cannot substitute for cyclin E activity. Thus, cyclin E possesses an E2F-independent function required to enter S-phase. However, cyclin E may not simply bypass E2F function in the presence of p16, since it restores expression of E2F-regulated genes such as cyclin A or CDC2. Finally, c-Myc bypasses the p16/pRb pathway with effects indistinguishable from those of cyclin E. We suggest that this effect of Myc is mediated by its action upstream of cyclin E-CDK2, and occurs via the neutralization of p27(Kip1) family proteins, rather than induction of Cdc25A. Our data imply that oncogenic activation of c-Myc, and possibly also of cyclin E, mimics loss of the p16/pRb pathway during oncogenesis.     

Expression of retinoblastoma gene product in respiratory epithelium and sinonasal neoplasms: relationship with p16 and cyclin D1 expression

Transition from G1 to S phase of the cell cycle is mediated by interactions between the Retinoblastoma gene product (pRb), p16, and cyclin D1. To determine the expression of these proteins in the sinonasal mucosa immunohistochemistry was carried out on archived tissue sections from 46 patients (37 men, 9 women, age range 17 to 82 years, median 55 years). Nuclear immunostaining for these proteins was assessed and the expression rates (percentages of immunoreactive nuclei) in normal respiratory epithelium, inverted sinonasal papillomas, cylindrical (oncocytic) sinonasal papillomas, and squamous cell carcinomas were compared. Normal respiratory epithelium showed significantly higher pRb expression in surface cells compared to basal cells (p < 0.05). In contrast, abundant pRb expression in surface and basal cells was detected in columnar differentiation in sinonasal papillomas and adjacent mucosa. Cuboidal and squamous metaplasia in inverted papillomas showed significantly reduced pRb expression in surface cells compared to columnar epithelium in inverted papillomas (p < 0.05, respectively). Expression of p16 was detected in all epithelial cell layers of normal respiratory epithelium, sinonasal papillomas, and adjacent mucosa. Cuboidal and squamous metaplasia in inverted papillomas showed increased p16 expression in surface cells compared to columnar epithelium in inverted papillomas (p < 0.05 between squamous metaplasia and columnar epithelium). Sinonasal squamous cell carcinomas showed the coexpression of pRb and p16. Expression rates of cyclin D1 higher than 10% were detected only in invasive carcinomas but not in carcinoma in situ, sinonasal papillomas or respiratory epithelium. Conclusively, pRb expression accompanies terminal differentiation in columnar surface cells. Expression of pRb in proliferating basal cells is present in sinonasal papillomas and adjacent mucosa but not in normal respiratory epithelium. Cuboidal and squamous metaplasia in inverted papillomas involves downregulation of pRb expression along with increased p16 expression in surface cells. Sinonasal squamous cell carcinomas coexpress pRb and p16. Overexpression of cyclin D1 in sinonasal lesions is confined to invasive squamous cell carcinomas.     

Characterization of novel inhibitors of cyclin-dependent kinases.

In epithelial cells progression through the G1 phase of the cell cycle and preparing the cell for the S phase is regulated by cyclin D1-cdk4. Cells that express the retinoblastoma protein (pRb) are dependent on cyclin D1-cdk4 activity for their proliferation while cells that do not express pRb are not. Overexpression of cyclin D1 and/or cdk4, and loss of expression of p16 (the natural inhibitor of cyclin D1-cdk4 activity), have been implicated in several cancers. These data suggest that the aberrant activity of cyclin D1-cdk4 correlates with the tumor phenotype. Hence, blocking cyclin D1-cdk4 activity may prove to be an effective anticancer therapy for pRb(+) tumors. In this paper, we report the identification of four novel compounds that selectively inhibit cyclin D1-cdk4 activity to various degrees. We further demonstrate that two of these compounds also selectively inhibit the target, pRb(+) tumor cells. The implications of these discoveries and their utility as anticancer agents are discussed.     

Id2 specifically alters regulation of the cell cycle by tumor suppressor proteins.

Cells which are highly proliferative typically lack expression of differentiated, lineage-specific characteristics. Id2, a member of the helix-loop-helix (HLH) protein family known to inhibit cell differentiation, binds to the retinoblastoma protein (pRb) and abolishes its growth-suppressing activity. We found that Id2 but not Id1 or Id3 was able to bind in vitro not only pRb but also the related proteins p107 and p130. Also, an association between Id2 and p107 or p130 was observed in vivo in transiently transfected Saos-2 cells. In agreement with these results, expression of Id1 or Id3 did not affect the block of cell cycle progression mediated by pRb. Conversely, expression of Id2 specifically reversed the cell cycle arrest induced by each of the three members of the pRb family. Furthermore, the growth-suppressive activities of cyclin-dependent kinase inhibitors p16 and p21 were efficiently antagonized by high levels of Id2 but not by Id1 Id3. Consistent with the role of p16 as a selective inhibitor of pRb and pRb-related protein kinase activity, p16-imposed cell cycle arrest was completely abolished by Id2. Only a partial reversal of p21-induced growth suppression was observed, which correlated with the presence of a functional pRb. We also documented decreased levels of cyclin D1 protein and mRNA and the loss of cyclin D1-cdk4 complexes in cells constitutively expressing Id2. These data provide evidence for important Id2-mediated alterations in cell cycle components normally involved in the regulatory events of cell cycle progression, and they highlight a specific role for Id2 as an antagonist of multiple tumor suppressor proteins.     

TSG101 expression in gynecological tumors: relationship to cyclin D1, cyclin E, p53 and p16 proteins.

Recent studies have shown that in vitro steady-state expression of the tumor susceptibility gene TSG101 is important for maintenance of genomic stability and cell cycle regulation. To determine the contribution of TSG101 expression in neoplastic formation, expression of TSG101 protein levels were evaluated in primary ovarian and endometrial adenocarcinoma tumors. Expression of TSG101 was also examined in various tumor cell lines (PA-1, AN3CA, HeLa, HS578T, HCT116). Full-length TSG101 protein was detected in these tumors and cell lines indicating that intragenic deletions were not characteristic of TSG101. In addition, TSG101 protein levels were compared with aberrations of prominent cell cycle regulatory molecules such as cyclin D1, cyclin E, p16 and p53. Reduced TSG101 protein was observed in 36% (8/22) of ovarian and 17% (1/6) of endometrial adenocarcinoma. Aberrant levels of p53, p16, cyclin D or E were comparable to published studies indicating that the clinicopathological distribution of these cases did not favor advanced stage tumors. Altogether, these findings suggest that a down-regulation of TSG101 is associated with tumorigenesis in a subgroup of gynecological tumors.     

Differential regulation of retinoblastoma protein by hormonal and antihormonal agents in T47D breast cancer cells

Phosphorylation of the tumor suppressor protein, retinoblastoma (pRb), regulates the progression of the cell cycle. Previous work from this laboratory had shown that estradiol (E(2)) regulates tumor suppressor proteins, p53 and retinoblastoma in breast cancer cells. In the present study, we have examined the phosphorylation of pRB in T47D breast cancer cells following treatments with R5020 and antiprogestins. In growth medium containing serum depleted of endogenous steroids by charcoal treatment, pRb appeared mainly in its hypophosphorylated form. Addition of 10 nM R5020 to the culture medium caused hyperphosphorylation of pRb within 24 h, but the hypophosphorylated form of pRb began to accumulate after 72 h. Upon prolonged R5020 treatment (72-96 h), pRb was detected exclusively in its hypophosphorylated form. While treatment of cells with R5020 caused a transient increase in the level of cyclin D1, E(2) addition caused a sustained increase in the level of cyclin D1 consistent with its role in stimulating pRb phosphorylation. Antagonists of both estrogen receptor (ER) and progesterone receptor (PR) blocked the E(2) and R5020-induced pRb phosphorylation, respectively. These results suggest that R5020 induces pRb phosphorylation via a transient increased expression of cyclin D1, whereas E(2) treatment results in sustained expression of cyclin D1 and increased pRb phosphorylation. Furthermore, R5020 effects on pRb phosphorylation appear PR-mediated as no cross-antagonism of pRb phosphorylation was observed: the R5020 effects were blocked by RU486 and ZK98299, but not by the pure ER antagonist, ICI 182, 780 (ICI).     

Cell cycle and biochemical effects of PD 0183812. A potent inhibitor of the cyclin D-dependent kinases CDK4 and CDK6

Progression through the G1 phase of the cell cycle requires phosphorylation of the retinoblastoma gene product (pRb) by the cyclin D-dependent kinases CDK4 and CDK6, whose activity can specifically be blocked by the CDK inhibitor p16(INK4A). Misregulation of the pRb/cyclin D/p16(INK4A) pathway is one of the most common events in human cancer and has lead to the suggestion that inhibition of cyclin D-dependent kinase activity may have therapeutic value as an anticancer treatment. Through screening of a chemical library, we initially identified the [2,3-d]pyridopyrimidines as inhibitors of CDK4. Chemical modification resulted in the identification of PD 0183812 as a potent and highly selective inhibitor of both CDK4 and CDK6 kinase activity, which is competitive with ATP. Flow cytometry experiments showed that of the cell lines tested, only those expressing pRb demonstrated a G1 arrest when treated with PD 0183812. This arrest correlated in terms of incubation time and potency with a loss of pRb phosphorylation and a block in proliferation, which was reversible. These results suggest a potential use of this chemical class of compounds as therapeutic agents in the treatment of tumors with functional pRb, possessing cell cycle aberrations in other members of the pRb/cyclin D/p16(INK4A) pathway.     

Immunohistochemical expression of retinoblastoma gene product (Rb), p53 protein, MDM2, c-erbB-2, HLA-DR and proliferation indices in human urinary bladder carcinoma

Archival biopsy specimens from transitional cell bladder cancers (n=88) were analysed immunohistochemically for the expression of the retinoblastoma (Rb) gene protein, p53, mdm2, c-erbB-2, HLA-DR antigen and proliferation indices. An altered nuclear expression of Rb, p53 and mdm2 was observed in 55.2%, 33.3% and 18.2% of tumors respectively. Cytoplasmic membrane immunoreactivity (>25% tumor cells) for c-erbB-2 was detected in 14.1% of tumors and aberrant HLA-DR antigen cytoplasmic staining (>5% of tumor cells) in 22.2% of the cases. P53 overexpression was associated with higher tumor grade and stage. Aberrant HLA-DR antigen expression and PCNA were also correlated with the grade of differentiation and tumor stage. MIB1 was statistically correlated with stage. pRb scores and HLA-DR antigen expression were correlated with proliferation activity as determined by PCNA and MIB1 immunostaining. p53 protein was also strongly correlated with the proliferation index PCNA. A strong correlation between PCNA and MIB1 (p<0.0001) was also found. In addition a statistically positive correlation between p53 and HLA-DR antigen expression was observed. Our data show that, although pRb and p53 protein expressions are not associated between them, they may contribute to the growth fraction of the bladder cancer. In addition, p53 and HLA-DR antigen expression could be indicators of aggressive behavior of bladder cancer.     

Cyclin D1 determines mitochondrial function in vivo

The cyclin D1 gene encodes a regulatory subunit of the holoenzyme that phosphorylates and inactivates the pRb tumor suppressor to promote nuclear DNA synthesis. cyclin D1 is overexpressed in human breast cancers and is sufficient for the development of murine mammary tumors. Herein, cyclin D1 is shown to perform a novel function, inhibiting mitochondrial function and size. Mitochondrial activity was enhanced by genetic deletion or antisense or small interfering RNA to cyclin D1. Global gene expression profiling and functional analysis of mammary epithelial cell-targeted cyclin D1 antisense transgenics demonstrated that cyclin D1 inhibits mitochondrial activity and aerobic glycolysis in vivo. Reciprocal regulation of these genes was observed in cyclin D1-induced mammary tumors. Cyclin D1 thus integrates nuclear DNA synthesis and mitochondrial function.     

p16、survivin、cyclin D1 在膀胱尿路上皮癌中的表达及意义

目的:探讨抑癌基因p16、细胞周期蛋白cyclin D1和凋亡抑制基因survivin在膀胱移行细胞癌中的表达及意义。方法:膀胱移行细胞癌组67例,10例正常正常膀胱粘膜作为对照,采用免疫组织化学方法检测p16和cyclin D1、survivin蛋白表达,然后分析上述三种蛋白在膀胱癌组织中的表达情况,以及随着不同临床分期和病理分级表达的变化。结果:所有膀胱癌患者平均年龄58.16岁,其中男性患者38例。免疫组织化学分析表明,p16和cyclin D1、survivin蛋白均表达在细胞的细胞核。膀胱癌组织中P16表达明显低于正常对照组,而cyclin D1和survivin表达明显高于正常对照组。随着临床分期的进展,p16表达明显下降,cyclinD1表达明显上升;而随着膀胱癌病理分级升高,p16表达明显下降,survivin表达上升。此外,膀胱癌组织中,p16与cyclin D1p16之间存在着明确的负相关。结论:p16、cyclin D1、survivin在膀胱移行细胞癌的生物学行为中起重要作用,p16,cyclin D1和survivin与膀胱移行细胞癌的恶性进展有关。     

p27kip1、Cyclin D1在卵巢癌中的表达及临床意义

目的探讨p27kip1、Cyclin D1在卵巢癌发生方面的意义.方法应用免疫组织化学方法及半定量分析方法,检测50例卵巢癌、19例卵巢良性上皮肿瘤、 13例正常卵巢组织中的p27kip1和Cyclin D1表达,并分析它们与良恶性肿瘤、病理学分级、临床分期的相关性. 结果正常卵巢和卵巢良性肿瘤间,p27和Cyclin D1的各自表达无明显差异;p27在正常卵巢组织和卵巢良性上皮肿瘤中高表达,在卵巢癌中表达降低(P<0.05),且随着肿瘤分级、分期增高(恶性程度增高),阳性表达率逐渐下降;而Cyclin D1的表达则相反;两者在肿瘤中的表达呈负相关.结论 p27kip表达下降、Cyclin D1过表达可能在卵巢癌的发生中起重要作用,检测p27kip1、Cyclin D1在卵巢癌中的表达可预测肿瘤生物学行为特征,可以作为判断预后的指标.     

Correlation of STAT1 with Apoptosis and Cell-Cycle Markers in Esophageal Squamous Cell Carcinoma

We recently found evidence that STAT1 in esophageal squamous carcinoma (ESCC) cells exerts tumor suppressor function, and it regulates five key regulators of apoptosis or cell-cycle progression, including Bcl-2, Bcl-xL, survivin, cyclin D1 and p21. In this study, we confirmed these findings in four ESCC cell lines. Using immunohistochemistry, we also assessed the expression of these proteins in 62 primary tumors. The expression of these markers was heterogeneous, ranging 39 to 69% of the cohort. Significant correlation was found between STAT1 and three proteins (p21, Bcl-xL and survivin), whereas only a trend was identified for cyclin D1 and Bcl-2. We then correlated the expression of these proteins with several clinicopathologic parameters including lymph node metastasis, depth of invasion, clinical stage and overall survival. Significant correlations were found between Bcl-2 and deep invasion (p = 0.033), survivin and lymph node metastasis (p = 0.006), as well as cyclin D1 and clinical stage (p = 0.014). Patients with p21-positive tumors had a significantly longer survival compared to those with p21-negative tumors (p = 0.031). To conclude, our findings support the concept that STAT1 exerts its tumor suppressor effects in ESCC via modulating the expression of key regulators of apoptosis and cell-cycle progression.     

Cell cycle regulators in bladder cancer: relationship to schistosomiasis

Dysregulation of cell cycle control may lead to genomic instability, neoplastic transformation and tumor progression. In terms of the particular roles in regulation of the cell-cycle, p21(WAF1) causes growth arrest through inhibition of cyclin-dependant kinases required for G1/S transition. P16 (INK4A) and p15 (INK4B) are thought to act as tumor suppressors, since their inactivation and/or deletion are observable in various types of malignancies. Cyclin D1 is hypothesized to control cell cycle progression through the G1-S check point. The present study evaluated p21 expression, p16 and p15 gene deletion and cylin D1 expression in bladder carcinoma among Egyptian patients, in relation to different clinicopathological features of the tumors and presence or absence of bilharziasis. Tissue specimens were obtained from 132 patients with bladder carcinoma and 50 normal tissue samples from the same patients served as control. P21 was determined by Western blot (WB) and enzyme immunoassay (EIA), p16 and p15 gene deletions were examined by polymerase chain reaction (PCR) and Cyclin D1 was detected by WB. Levels of p21 were lower in malignant tumors than in normal tissues. Lower expression of p21 was evident in lymph node positive, well differentiated tumors and squamous cell carcinoma (SCC) than in lymph node negative, poorly differentiated tumors and transitional cell carcinoma (TCC). In all normal samples, p15 and p16 genes were detected while cyclin D1 was not detected. P16 and p15 genes were deleted in 38.7% (41/106) and 30.2% (32/106) of bladder tumors respectively. The deletion of both genes was associated with poor differentiation grade and presence of bilharziasis. P16 deletion was also correlated to advancing tumor stage. Cyclin D1 was expressed in 57.5% of bladder tumors (69/120), where its expression was correlated to early stage, well differentiation grade, schistomiasis, and low levels of p21. Cell cycle is dysregulated in bladder carcinoma. This was evident from the increased expression of cyclin D1, the decreased levels of p21 and the deletion of p15 and p16 genes. Moreover, p16 and p15 gene deletion was related to tumor progression and might have a role in bilharzial bladder carcinogenesis. Cyclin D1 over-expression appears to be an early event in bladder cancer and might explain bilharzial associated bladder carcinogenesis.     

Cooperation between p53 and p130(Rb2) in induction of cellular senescence

To determine pathways cooperating with p53 in cellular senescence when the retinoblastoma protein (pRb)/p16INK4a pathway is defunct, we stably transfected the p16INK4a-negative C6 rat glioma cell line with a temperature-sensitive mutant p53. Activation of p53(Val-135) induces a switch in pocket protein expression from pRb and p107 to p130(Rb2) and stalls the cells in late G1, early S-phase at high levels of cyclin E. Maintenance of the arrest depends on the functions of p130(Rb2) repressing cyclin A. Inactivation of p53 in senescent cultures restores the pocket proteins to initial levels and initiates progression into S-phase, but the cells fail to resume proliferation, likely due to DNA damage becoming apparent in the arrest and activating apoptosis subsequent to the release from p53-dependent growth suppression. The data indicate that p53 can cooperate selectively with p130(Rb2) to induce cellular senescence, a pathway that may be relevant when the pRb/p16INK4a pathway is defunct.     

Differential roles for cyclin-dependent kinase inhibitors p21 and p16 in the mechanisms of senescence and differentiation in human fibroblasts

The irreversible G1 arrest in senescent human diploid fibroblasts is probably caused by inactivation of the G1 cyclin-cyclin-dependent kinase (Cdk) complexes responsible for phosphorylation of the retinoblastoma protein (pRb). We show that the Cdk inhibitor p21(Sdi1,Cip1,Waf1), which accumulates progressively in aging cells, binds to and inactivates all cyclin E-Cdk2 complexes in senescent cells, whereas in young cells only p21-free Cdk2 complexes are active. Furthermore, the senescent-cell-cycle arrest occurs prior to the accumulation of the Cdk4-Cdk6 inhibitor p16(Ink4a), suggesting that p21 may be sufficient for this event. Accordingly, cyclin D1-associated phosphorylation of pRb at Ser-780 is lacking even in newly senescent fibroblasts that have a low amount of p16. Instead, the cyclin D1-Cdk4 and cyclin D1-Cdk6 complexes in these cells are associated with an increased amount of p21, suggesting that p21 may be responsible for inactivation of both cyclin E- and cyclin D1-associated kinase activity at the early stage of senescence. Moreover, even in the late stage of senescence when p16 is high, cyclin D1-Cdk4 complexes are persistent, albeit reduced by 相似文献   

A role for cell cycle proteins in the serum-starvation resistance of Epstein-Barr virus immortalized B lymphocytes.

Epstein-Barr virus (EBV) is a B-lymphotropic human herpes virus that infects B lymphocytes and is associated with a broad spectrum of benign and malignant diseases. B cell infection by EBV causes indefinite cell proliferation that results in the development of immortalized lymphoblastoid cell lines (LCLs). We found that SNU-1103, a latency type III EBV-transformed LCL developed from a Korean cancer patient, resisted the G1 arrest that was normally caused by serum starvation. Western blot analyses revealed several alterations in the expression of key regulatory cell cycle proteins involved in the G1 phase. High expression of cyclin D2 and time-dependent increases in cyclin-dependent kinase 6 (CDK6) and cyclin D3 were observed in SNU-1103 during serum starvation. Very unexpectedly, in SNU-1103, the key G1 phase CDK inhibitor p21CiP1 was expressed at a consistently high level, while p27KiP1 expression was increased. Of three pRb family proteins, pRb expression was reduced and it became hypophosphorylated in SNU-1103 during serum starvation. Instead, p107 and p130 were expressed at consistently high levels in SNU-1103 during serum starvation. In conclusion, compared with an EBV-negative BJAB cell line, multiple cell cycle regulatory proteins were abnormally or inversely expressed in SNU-1103 during serum starvation.     

Functional ablation of pRb activates Cdk2 and causes antiestrogen resistance in human breast cancer cells

Estrogens are required for the proliferation of hormone dependent breast cancer cells, making estrogen receptor (ER) positive tumors amenable to endocrine therapies such as antiestrogens. However, resistance to these agents remains a significant cause of treatment failure. We previously demonstrated that inactivation of the retinoblastoma protein (pRb) family tumor suppressors causes antiestrogen resistance in MCF-7 cells, a widely studied model of estrogen responsive human breast cancers. In this study, we investigate the mechanism by which pRb inactivation leads to antiestrogen resistance. Cdk4 and cdk2 are two key cell cycle regulators that can phosphorylate and inactivate pRb, therefore we tested whether these kinases are required in cells lacking pRb function. pRb family members were inactivated in MCF-7 cells by expressing polyomavirus large tumor antigen (PyLT), and cdk activity was inhibited using the cdk inhibitors p16(INK4A) and p21(Waf1/Cip1). Cdk4 activity was no longer required in cells lacking functional pRb, while cdk2 activity was required for proliferation in both the presence and absence of pRb function. Using inducible PyLT cell lines, we further demonstrated that pRb inactivation leads to increased cyclin A expression, cdk2 activation and proliferation in antiestrogen arrested cells. These results demonstrate that antiestrogens do not inhibit cdk2 activity or proliferation of MCF-7 cells in the absence of pRb family function, and suggest that antiestrogen resistant breast cancer cells resulting from pRb pathway inactivation would be susceptible to therapies that target cdk2.