Structural rearrangement of the retinoblastoma gene in human breast carcinoma

Murine models of neoplasia: functional analysis of the tumour suppressor genes Rb-1 and p53

Loss of function of one or both of the two tumour suppressor genes p53 and RB-1 has been recognised as an important step in the development of a variety of human neoplasias for some time. By virtue of the ability to manipulate the genome of murine embryonic stem cells in culture, it has become possible to generate strains of mice which bear inactivations of the murine counterparts of these genes. This article attempts to bring together some of the many results obtained from these murine strains which are shedding light both on the normal role played by both of these genes and the consequences of their dysfunction. Surprisingly neither gene product is revealed to have an indispensable role at the level of the single cell. Hence, even though the Rb-1 gene product clearly has an important role in cell cycle regulation animals constitutively deficient in this gene develop relatively normally for the first 10 days of embryogenesis. It is only at and beyond this stage of development that a requirement for Rb-1 becomes clear, in the regulation of certain cell populations through control of both proliferation and apoptosis. That loss of function of Rb-1 is associated with tumorigenesis is confirmed by the development of tumours of the pituitary gland within heterozygotes. The retinas of these animals, the target organ for tumorigenesis in human RB-1 heterozygotes, remain unaffected. The majority of mice homozygous for an inactivating p53 mutation survive to birth, but then rapidly succumb to tumorigenesis. Heterozygotes also develop tumours, but with a delayed time course and altered spectrum. Analysis of several tissue types from the mutant animals has shown p53 to be crucial for the normal induction of apoptosis following DNA damage, and it is thought that failure of this process is a key predisposing step towards tumorigenesis within the mutant animals. Finally, studies on these and other transgenic strains have revealed interactions between pathways governed by these two genes. For example, the fate of Rb-1 deficient cells has been shown, in some tissues at least, to be dependent upon the functional status of p53.

Abnormalities of retinoblastoma gene expression in hematological malignancies

The human retinoblastoma gene product which is involved in cell cycle control and also acts as a transcriptional repressor of genes involved in growth control, is constitutively expressed as a phosphoprotein in normal hemopoietic cells. Abnormalities of the retinoblastoma gene expression leading to loss of protein expression either due to structural changes, mutations or transcriptional abnormalities have been found in a variety of hematological malignancies. There is evidence that loss of Rb protein expression is particularly associated with tumour progression and an adverse response to therapy which may be linked to the biological effect of Rb protein loss on the growth characteristics of tumour cells.

Human cyclin E, a nuclear protein essential for the G1-to-S phase transition

Cyclin E was first identified by screening human cDNA libraries for genes that would complement G1 cyclin mutations in Saccharomyces cerevisiae and has subsequently been found to have specific biochemical and physiological properties that are consistent with it performing a G1 function in mammalian cells. Most significantly, the cyclin E-Cdk2 complex is maximally active at the G1/S transition, and overexpression of cyclin E decreases the time it takes the cell to complete G1 and enter S phase. We have now found that mammalian cells express two forms of cyclin E protein which differ from each other by the presence or absence of a 15-amino-acid amino-terminal domain. These proteins are encoded by alternatively spliced mRNAs and are localized to the nucleus during late G1 and early S phase. Fibroblasts engineered to constitutively overexpress either form of cyclin E showed elevated cyclin E-dependent kinase activity and a shortened G1 phase of the cell cycle. The overexpressed cyclin E protein was detected in the nucleus during all cell cycle phases, including G0. Although the cyclin E protein could be overexpressed in quiescent cells, the cyclin E-Cdk2 complex was inactive. It was not activated until 6 to 8 h after readdition of serum, 4 h earlier than the endogenous cyclin E-Cdk2. This premature activation of cyclin E-Cdk2 was consistent with the extent of G1 shortening caused by cyclin E overexpression. Microinjection of affinity-purified anti-cyclin E antibodies during G1 inhibited entry into S phase, whereas microinjection performed near the G1/S transition was ineffective. These results demonstrate that cyclin E is necessary for entry into S phase. Moreover, we found that cyclin E, in contrast to cyclin D1, was required for the G1/S transition even in cells lacking retinoblastoma protein function. Therefore, cyclins E and D1 control two different transitions within the human cell cycle.

Familial breast cancer and genes involved in breast carcinogenesis

Breast cancer has often been reported to run in families, and the most important risk factor for the disease is a family history of breast cancer. Numerous pedigrees and segregation analyses have suggested an autosomal dominant transmitted susceptibility to breast cancer. Familial breast cancer occurs alone or associated with other cancers in clinically distinguishable syndromes. Such cases may be characterized by early onset, bilateral disease, prolonged survival, and anticipation, mainly seen as a higher penetrance or earlier onset in subsequent generations. Studies of patients and tumors from these families as well as sporadic cases have led to localization and/or identification of a number of genes implicated in breast carcinogenesis of familial and sporadic breast cancer.

Functional interactions between the retinoblastoma (Rb) protein and Sp-family members: superactivation by Rb requires amino acids necessary for growth suppression

The transient expression of the retinoblastoma protein (Rb) regulates the transcription of a variety of growth-control genes, including c-fos, c-myc, and the gene for transforming growth factor beta 1 via discrete promoter sequences termed retinoblastoma control elements (RCE). Previous analyses have shown that Sp1 is one of three RCE-binding proteins identified in nuclear extracts and that Rb functionally interacts with Sp1 in vivo, resulting in the "superactivation" of Sp1-mediated transcription. By immunochemical and biochemical criteria, we report that an Sp1-related transcription factor, Sp3, is a second RCE-binding protein. Furthermore, in transient cotransfection assays, we report that Rb "superactivates" Sp3-mediated RCE-dependent transcription in vivo and that levels of superactivation are dependent on the trans-activator (Sp1 or Sp3) studied. Using expression vectors carrying mutated Rb cDNAs, we have identified two portions of Rb required for superactivation: (i) a portion of the Rb "pocket" (amino acids 614-839) previously determined to be required for physical interactions between Rb and transcription factors such as E2F-1 and (ii) a novel amino-terminal region (amino acids 140-202). Since both of these regions of Rb are targets of mutation in human tumors, our data suggest that superactivation of Sp1/Sp3 may play a role in Rb-mediated growth suppression and/or the induction of differentiation.

Detection of a novel cell cycle-regulated kinase activity that associates with the amino terminus of the retinoblastoma protein in G2/M phases

Recent genetic and functional evidence suggests that the amino terminus of the retinoblastoma (Rb) protein plays an important role in Rb-mediated growth suppression. To explore the mechanism(s) by which this portion of Rb may regulate cell growth, we have sought to characterize cellular proteins that associate with the Rb amino terminus using an in vitro protein-binding assay. Here we report that at least one such protein is a cell cycle-regulated Rb/histone H1 kinase (RbK) whose enzymatic and/or Rb association activity is most prevalent in G2/M phases of cells. In contrast to previously characterized cyclin-dependent and Rb-associated kinases, such as cdk1 (cdc2) and cdk2, G2/M RbK 1) is not depleted by incubation with p13suc-beads, 2) is not detected with antisera against several Rb-associated cyclins-cdks, and 3) associated with Rb via the Rb amino terminus, a region that is dispensable for interaction with other Rb-associated kinases. RbK is clearly distinct from previously characterized mitotic cdks since cyclin A-cdc2, cyclin A-cdk2, cyclin B-cdc2, and cyclin B-cdk2 did not associate with the Rb amino terminus. Coprecipitation experiments with Rb antisera confirmed the association of Rb with a RbK-like kinase in metaphase-arrested cells in vivo. Interestingly, G2/M RbK did not appreciably associate with an analogous portion of p107, a Rb-related protein. Taken together, these data indicate that the Rb amino terminus specifically associates with a novel cell cycle-regulated kinase in late cell cycle stages.

Retinoblastoma gene mutations in primary human bladder cancer

Inactivation of the retinoblastoma (RB) gene is known to be implicated in the pathogenesis of several types of human cancers. Since structural alterations of the RB gene have not been well examined in human bladder cancer, we looked for mutations in the entire coding region of this gene using polymerase chain reaction (PCR) and single-strand conformational polymorphism analysis of RNA. We also examined allelic loss of the RB gene using PCR-based restriction fragment length polymorphism analysis. Of 30 samples obtained from patients with bladder cancer, eight (27%) were found to have RB gene mutations. DNA sequencing of the PCR products revealed five cases with single point mutations and three cases with small deletions. These mutations included one (10%) of ten low-grade (grade 1) tumours, four (50%) of eight intermediate-grade (grade 2) tumours and three (25%) of 12 high-grade (grade 3) tumours. Likewise, mutations were found in four (21%) of 19 superficial (pTa and pT1) tumours and four (36%) of 11 invasive (pT2 or greater) tumours. In 15 informative cases, loss of heterozygosity at the RB locus was shown in five cases (33%), three cases with RB mutations and two without them. These results suggest that RB gene mutations are involved in low-grade and superficial bladder cancers as well as in high-grade and invasive cancers.

Loss of heterozygosity in ductal carcinoma in situ of the breast

Loss of heterozygosity (LOH) at loci reported to show allele loss in invasive breast cancers was examined in ductal in situ carcinomas of the breast using polymorphic short tandem repeats and the polymerase chain reaction (PCR). LOH was detected at all loci examined in at least 11 per cent of the samples examined. The proportion of cases of in situ carcinoma showing LOH at these loci was similar to that previously reported in invasive cancers. Cases of pure in situ cancer without an invasive component exhibited an overall lower frequency of allele loss. LOH at more than one locus was observed in some intraductal cancers. In a small number of cases, LOH was present in the invasive but not in the intraductal component of the tumour, suggesting that mutation at the locus concerned was associated with development of invasive behaviour.

p53, Rb and bcl-2 expression during the cell cycle: a study in phytohaemagglutinin stimulated lymphocytes and microwave irradiated lymphoid tissue sections

AIMS

To determine the expression of p53, Rb, and bcl-2 during the cell cycle in stimulated peripheral blood lymphocytes (PBLs) and microwave heated reactive lymphoid tissue sections.

METHODS

The expression of p53, Rb and bcl-2 proteins in paraffin wax embedded tonsil tissue sections was detected by immunohistochemistry using an (APAAP) technique following microwave irradiation. Flow cytometric analysis as performed on phytohaemagglutinin (PHA) stimulated PBLs, with simultaneous S fraction determination.

RESULTS

Expression of p53 protein was detected in reactive tonsil germinal centre cells, in some suprabasal cells in the surface and cryptic epithelium, and in some endothelial cells. Analysis of p53 in PHA stimulated PBLs revealed expression of p53 by non-tumoral activated lymphocytes. Rb protein expression was increased in PHA stimulated PBLs and was usually detected in most germinal centre B cells, in isolated paracortical cells, in a fraction of endothelial cells, and in most epithelial suprabasal cells. Expression of bcl-2 in stimulated lymphocytes was inversely correlated with proliferation. This confirms findings in reactive tonsil tissue samples, where proliferating cells located in the germinal centres and paracortical area are mostly bcl-2 negative.

CONCLUSIONS

Expression of these three oncogenic and tumour suppressor proteins varies during the cell cycle in non-tumoral cells. Consequently, tumoral growth fraction must be taken into account when analysing dysregulation of these three genes in lymphomas and other tumours. The p53 protein may be detected in benign conditions, as its expression is not synonymous with malignancy or mutation of the p53 gene.

Detection of Heterozygous Mutation in the Retinoblastoma Gene in a Human Pituitary Adenoma Using PCR-SSCP Analysis and Direct Sequencing

Genomic deoxyribonucleic acid from surgical specimens of 25 pituitary adenomas was screened for the presence of mutations in the tumor suppressor gene retinoblastoma gene, using polymerase chain reaction and single-strand conformation polymorphism analysis, followed by direct deoxyribonucleic acid sequencing. Mutation causing an amino acid change was found in one of the 25 pituitary adenomas. The mutation site was in exon 19 (codon 621) of the retinoblastoma gene. In addition, there were three types of silent mutations in introns of the gene. The patient in whom the retinoblastoma mutation was identified had a tumor with high clinical malignancy, a high percentage of c-myc protein-labeled cells, and a diagnosis of plurihormonal pituitary adenoma based on the presence of cells immunoreactive for five pituitary hormones. This article suggests that point mutation of retinoblastoma gene is rare in human pituitary adenomas but may provide a marker for aggressive pituitary adenoma.

Growth-factor stimulation reveals two mechanisms of retinoblastoma gene inactivation in human myelogenous leukemia cells

Mutation or deletion of the retinoblastoma tumor suppressor gene (Rb) or abnormal Rb protein expression is found in many types of human solid tumors. Low or absent levels of Rb protein are usually found in the leukemic cells of patients with acute myelogenous leukemia (AML) who have an extremely poor prognosis. The absence of Rb protein in these AML cells could result from defects in the Rb gene or from abnormal cell cycle regulation that affects Rb expression. To test these possibilities and to examine whether a low level of Rb protein in AML cells could be up-regulated, we studied the effect that growth factors interleukin 3 (IL3) and granulocyte-macrophage colony stimulating factor (GM-CSF) had on the levels of Rb protein and Rb phosphorylation in AML cells from patients with low Rb or no Rb protein expression. We observed three responses to growth factor-stimulation in leukemic cells taken from patients with AML: (1) some AML cell samples entered a proliferative phase, and Rb protein levels increased with the appearance of normally phosphorylated forms of Rb protein and positive nuclear staining for Rb protein; (2) some AML cell samples became more proliferative, but the levels of Rb protein remained low or absent; and (3) some AML cell samples showed no response. These results indicate that at least two different mechanisms may be responsible for the lack of Rb protein in the leukemic cells of some patients with AML.

Radiation-induced lymphomas in MSM, (BALB/cHeA x MSM) F1 and (BALB/cHeA x STS/A) F1 hybrid mice

M.MOL-MSM (MSM) mice derived from Mus musculus molossinus progenitors showed extreme resistance to the induction of lymphomas following whole-body X-irradiation with four doses of 1.7 Gy. (BALB/cHeA x MSM) F1 mice between a high lymphoma strain, BALB/cHeA and the MSM showed a high incidence of radiation-induced lymphomas which was quite similar to that in BALB/cHeA mice, but the latent period was prolonged in the hybrids. Susceptibility in incidence was dominant over resistance in these crosses. Incidences of (BALB/cHeA x MSM)F1 hybrids irradiated with four doses of 2.5 Gy X-rays were 77% in females and 88% in males. F1 hybrids between BALB/cHeA and another resistant strain STS/A, (BALB/cHeA x STS/A) F1, also showed a high level of susceptibility, that is, lymphoma incidence was 64% in females and 63% in males. The mean latent period in the (BALB/cHeA x STS/A) F1 hybrids was similar to that in (BALB/cHeA x MSM) F1 hybrids. As all cases of tumors developed in F1 hybrids are informative concerning the detection of the loss of heterozygosity in the loci depending on the combination of two parental strains, the radiation-induced lymphomas obtained from (BALB/cHeA x MSM) F1 and (BALB/cHeA x STS/A) F1 hybrids could be useful for fine analysis of the genetic alterations involved in lymphomagenesis.

Loss of the retinoblastoma susceptibility gene (RB1) is a frequent and early event in prostatic tumorigenesis

Loss of the RB1 gene is an important event in the initiation and progression of many tumours. Prostate tissue from 43 patients with prostate cancers and ten with benign prostatic hypertrophy (BPH) were studied for loss of heterozygosity of the RB1 gene. Four intragenic polymorphic loci were studied with two techniques. These were restriction fragment length polymorphism (RFLP). Southern blotting and hybridisation with the p123m1.8 and p68RS2.0 probes (to introns 1 and 17 respectively) and also the polymerase chain reaction (PCR) to amplify loci within introns 17 and 20. Protein product (pRB) expression was determined by immunohistochemistry using the NCL-RB antibody in nine patients with cancer and four patients with BPH. Loss of heterozygosity was found in 24 out of 40 (60%) informative patients with cancer. Loss of RB1 occurred with a similar frequency in early-stage and low-grade cancers as in more advanced cancers. Loss of RB1 was also found in one patient with BPH. Expression of pRB was completely absent from seven cancers and markedly reduced in the other two, while nuclear pRB staining was always present in areas of BPH, whether alongside cancer-containing tissue or with BPH alone. We conclude that loss of RB1 is an early event in prostatic tumorigenesis.

Dephosphorylation of the retinoblastoma gene product induced by differentiation and its relevancy to growth inhibition in normal human keratinocytes

The retinoblastoma gene product (pRB) is a tumor suppressor gene product. Recently, it has been shown that the dephosphorylation of pRB leads a variety of cells into G1/G0 arrest. Keratinocytes were grown in serum-free MCDB 153 medium containing 0.1 mM Ca2+, followed by addition of Ca2+ to 1.8 mM (calcium switch). Under-phosphorylated pRB band appeared at 48 h and increased up to 72 h after calcium switch. S phase cells decreased at 48 and 72 h in a time-dependent manner. In contrast, the number of involucrin positive cells, differentiated cells, increased even at 24 h, preceding the dephosphorylation of pRB and continued to increase up to 72 h. These results indicate that the dephosphorylation of pRB is closely related to growth inhibition of human keratinocytes associated with differentiation.

Localization of a breast cancer susceptibility gene, BRCA2, to chromosome 13q12-13

A small proportion of breast cancer, in particular those cases arising at a young age, is due to the inheritance of dominant susceptibility genes conferring a high risk of the disease. A genomic linkage search was performed with 15 high-risk breast cancer families that were unlinked to the BRCA1 locus on chromosome 17q21. This analysis localized a second breast cancer susceptibility locus, BRCA2, to a 6-centimorgan interval on chromosome 13q12-13. Preliminary evidence suggests that BRCA2 confers a high risk of breast cancer but, unlike BRCA1, does not confer a substantially elevated risk of ovarian cancer.

RB tumor suppressor gene expression in hepatocellular carcinomas from patients infected with the hepatitis B virus

Hepatitis B virus (HBV) infection is closely associated with the development of hepatocellular carcinoma (HCC), but definite mechanisms by which it could play an etiologic role have not yet been identified. Modifications of the function of the RB tumor suppressor gene, which regulates the cell cycle, could provide such a mechanism. In the present study, the expression of the protein product of RB, pRB, was evaluated by immunohistochemical staining in HCC tissues from 25 patients from China and the United States, adjacent nontumorous liver from 19 of those patients, five human HCC cell lines, three human hepatoblastoma cell lines, and five specimens of normal human liver. Representative samples were also evaluated by western blot. Altered expression of RB was detected in eight HCC tissues (pRB undetectable in five HCCs and detected in < 1% of nuclei of HCC cells in three others); all eight had detectable hepatitis B surface or core antigen in the adjacent nontumorous liver, indicating active HBV infection. pRB was detected in 10-95% of nuclei (normal expression) in the remaining 17 HCCs, and in many nuclei in all 19 nontumorous livers, and in the 5 normal livers. No pRB staining was detected in the nuclei of three HCC cell lines, but pRB was detected in > 90% of nuclei of the other HCC and hepatoblastoma cell lines. The relationship of pRB expression to mutations of the p53 tumor suppressor gene was also examined. The absence of detectable nuclear pRB by immunohistochemical staining was associated with the presence of presumed mutant p53 detected by immunohistochemical staining in four out of five HCC cases.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a second tumor suppressor gene on 17p linked to high S-phase index in primary human breast carcinomas

The short area of chromosome 17 is a frequent target for deletions in human tumors, including breast cancer. We have investigated by restriction fragment polymorphism analysis the pattern of loss of heterozygosity (LOH) at four loci on 17p13.1-17pter in a panel of 110 primary human breast carcinomas. A copy of the p53 gene was lost in 23% of the informative cases. Point mutations in the p53 gene were statistically associated with LOH at the same locus (p = 0.003) but not at other loci on 17p13.3-17pter. A second region bordered by the loci D17S5/D17S28 (17p13.3) and D17S34 (17pter) is also affected by LOH, independent of point mutations in the p53 gene. We propose the presence of a second tumor suppressor gene within this region. In support of this hypothesis is the significant association (p = 0.005) between LOH at the D17S5/D17S28, but not at the TP53 or D17S34 loci, and tumors having a high S-phase index.

Cyclin D1 overexpression vs. retinoblastoma inactivation: implications for growth control evasion in non-small cell and small cell lung cancer

The cyclin-dependent kinases and their associated regulatory cyclins control cell cycle progression and cell growth. Antibodies against these proteins were used to determine their levels in several lung tumor-derived cell lines and a "normal" immortalized bronchoepithelial cell line in order to investigate their potential roles in the etiology of lung cancer. All the cell lines expressed roughly equal levels of cdk-1; cdk-2; PSTAIRE-sequence containing kinases; proliferating cell nuclear antigen; and cyclins A, B1, and E. Cyclin D1, however, was present at 4- to 100-fold higher levels in 11 of 12 non-small cell lung cancer cell lines than in the bronchoepithelial line and all but one of the small cell lung cancer lines. Furthermore, immunoblots of the retinoblastoma gene product, pRB, revealed a perfect correlation between pRB levels and tumor type with normal levels of phosphorylation-competent pRB in all of the non-small cell lung cancer lines and undetectable levels of pRB in all of the small cell lung cancer lines. These data suggest the possibility that small cell and non-small cell lung cancer may evade normal growth controls by different mechanisms: loss of the proliferation inhibitor pRB in small cell lung cancer and overexpression of the growth promoting cyclin D1 in non-small cell lung cancer.

The retinoblastoma gene product RB stimulates Sp1-mediated transcription by liberating Sp1 from a negative regulator

Studies have demonstrated that the retinoblastoma susceptibility gene product, RB, can either positively or negatively regulate expression of several genes through cis-acting elements in a cell-type-dependent manner. The nucleotide sequence of the retinoblastoma control element (RCE) motif, GCCACC or CCACCC, and the Sp1 consensus binding sequence, CCGCCC, can confer equal responsiveness to RB. Here, we report that RB activates transcription of the c-jun gene through the Sp1-binding site within the c-jun promoter. Preincubation of crude nuclear extracts with monoclonal antibodies to RB results in reduction of Sp1 complexes in a mobility shift assay, while addition of recombinant RB in mobility shift assay mixtures with CCL64 cell extracts leads to an enhancement of DNA-binding activity of SP1. These results suggest that RB is directly or indirectly involved in Sp1-DNA binding activity. A mechanism by which RB regulates transactivation is indicated by our detection of a heat-labile and protease-sensitive Sp1 negative regulator(s) (Sp1-I) that specifically inhibits Sp1 binding to a c-jun Sp1 site. This inhibition is reversed by addition of recombinant RB proteins, suggesting that RB stimulates Sp1-mediated transactivation by liberating Sp1 from Sp1-I. Additional evidence for Sp1-I involvement in Sp1-mediated transactivation was demonstrated by cotransfection of RB, GAL4-Sp1, and a GAL4-responsive template into CV-1 cells. Finally, we have identified Sp1-I, a approximately 20-kDa protein(s) that inhibits the Sp1 complexes from binding to DNA and that is also an RB-associated protein. These findings provide evidence for a functional link between two distinct classes of oncoproteins, RB and c-Jun, that are involved in the control of cell growth, and also define a novel mechanism for the regulation of c-jun expression.

The retinoblastoma gene product RB stimulates Sp1-mediated transcription by liberating Sp1 from a negative regulator

Studies have demonstrated that the retinoblastoma susceptibility gene product, RB, can either positively or negatively regulate expression of several genes through cis-acting elements in a cell-type-dependent manner. The nucleotide sequence of the retinoblastoma control element (RCE) motif, GCCACC or CCACCC, and the Sp1 consensus binding sequence, CCGCCC, can confer equal responsiveness to RB. Here, we report that RB activates transcription of the c-jun gene through the Sp1-binding site within the c-jun promoter. Preincubation of crude nuclear extracts with monoclonal antibodies to RB results in reduction of Sp1 complexes in a mobility shift assay, while addition of recombinant RB in mobility shift assay mixtures with CCL64 cell extracts leads to an enhancement of DNA-binding activity of SP1. These results suggest that RB is directly or indirectly involved in Sp1-DNA binding activity. A mechanism by which RB regulates transactivation is indicated by our detection of a heat-labile and protease-sensitive Sp1 negative regulator(s) (Sp1-I) that specifically inhibits Sp1 binding to a c-jun Sp1 site. This inhibition is reversed by addition of recombinant RB proteins, suggesting that RB stimulates Sp1-mediated transactivation by liberating Sp1 from Sp1-I. Additional evidence for Sp1-I involvement in Sp1-mediated transactivation was demonstrated by cotransfection of RB, GAL4-Sp1, and a GAL4-responsive template into CV-1 cells. Finally, we have identified Sp1-I, a approximately 20-kDa protein(s) that inhibits the Sp1 complexes from binding to DNA and that is also an RB-associated protein. These findings provide evidence for a functional link between two distinct classes of oncoproteins, RB and c-Jun, that are involved in the control of cell growth, and also define a novel mechanism for the regulation of c-jun expression.

The retinoblastoma gene and its significance

The first human tumour suppressor gene, the Retinoblastoma Susceptibility gene (RB1) was first demonstrated in retinoblastoma, a rare paediatric eye tumour which has been studied extensively over the last century. Genetic studies of retinoblastoma have yielded unique insights into familial cancer syndromes and the mechanisms of oncogenesis by tumour suppressor genes such as the RB1 gene. In this view, we will summarize past research into the genetics of retinoblastoma that led to the discovery of the RB1 gene and discuss the influence these results have had on the field of cancer research. In addition, we will discuss current research into RB1 as it relates to cancer and its potential for new therapies.

Cyclin D1 protein expression and function in human breast cancer

Cyclin D1 is a cell-cycle regulator essential for G1 phase progression and a candidate proto-oncogene implicated in pathogenesis of several human tumour types, including breast carcinomas. In spite of the accumulating genetic evidence, however, there are no data regarding abundance and properties of the cyclin D1 protein in breast cancer. We now report aberrant nuclear overexpression/accumulation of the cyclin D1 protein in about half of the 170 primary breast carcinoma specimens analyzed by monoclonal antibody immunohistochemistry, indicating that the frequency of cyclin D1 abnormalities may be considerably higher than previously deduced from DNA amplification studies. A comparison of the expression patterns in matched lesions at different stages of tumour progression revealed that the cyclin D1 protein aberration appears to reflect a relatively early event and that, when acquired by a tumour, it is maintained throughout breast cancer progression including metastatic spread. In both tumour tissues and breast cancer cell lines, the abundance of this protein shows characteristic variations consistent with a cell-cycle oscillation and the peak levels expressed in G1. In all 7 cell lines whose retinoblastoma (Rb) protein is mutant or complexed to SV40 T antigen, exceptionally low levels of cyclin D1 protein and mRNA were found. Antibody-mediated and anti-sense oligonucleotide knockout experiments demonstrate the requirement for the cell-cycle regulatory function of cyclin D1 in breast cancer lines with single or multiple copies of the gene and reveal the absence of such a requirement in the cell lines with Rb defects. Our data are consistent with the notion that the emerging "Rb-cyclin D1 pathway" represents a frequent target of oncogenic abnormalities in breast cancer.

DNA tumor virus oncoproteins and retinoblastoma gene mutations share the ability to relieve the cell's requirement for cyclin D1 function in G1

The retinoblastoma gene product (pRB) participates in the regulation of the cell division cycle through complex formation with numerous cellular regulatory proteins including the potentially oncogenic cyclin D1. Extending the current view of the emerging functional interplay between pRB and D-type cyclins, we now report that cyclin D1 expression is positively regulated by pRB. Cyclin D1 mRNA and protein is specifically downregulated in cells expressing SV40 large T antigen, adenovirus E1A, and papillomavirus E7/E6 oncogene products and this effect requires intact RB-binding, CR2 domain of E1A. Exceptionally low expression of cyclin D1 is also seen in genetically RB-deficient cell lines, in which ectopically expressed wild-type pRB results in specific induction of this G1 cyclin. At the functional level, antibody-mediated cyclin D1 knockout experiments demonstrate that the cyclin D1 protein, normally required for G1 progression, is dispensable for passage through the cell cycle in cell lines whose pRB is inactivated through complex formation with T antigen, E1A, or E7 oncoproteins as well as in cells which have suffered loss-of-function mutations of the RB gene. The requirement for cyclin D1 function is not regained upon experimental elevation of cyclin D1 expression in cells with mutant RB, while reintroduction of wild-type RB into RB-deficient cells leads to restoration of the cyclin D1 checkpoint. These results strongly suggest that pRB serves as a major target of cyclin D1 whose cell cycle regulatory function becomes dispensable in cells lacking functional RB. Based on available data including this study, we propose a model for an autoregulatory feedback loop mechanism that regulates both the expression of the cyclin D1 gene and the activity of pRB, thereby contributing to a G1 phase checkpoint control in cycling mammalian cells.

Gene therapy--its potential in the management of oral cancer

Gene therapy is an important new approach to the treatment of many diseases. This review summarises the methods that are available for developing gene therapy, and demonstrates that oral cancer is probably susceptible to these approaches.

Chromosome 13 transfer provides evidence for regulation of RB1 protein expression

The human retinoblastoma susceptibility gene (RB1) located on chromosome 13 has been shown to function as a growth/tumor suppressor gene in a large number of human cancers. Although constitutive expression has been observed in most cultured cells and normal tissues, overexpression of RB1 protein has not been well documented. Perhaps regulating the level of normal RB1 protein expression is one of several ways of controlling its function. To test this hypothesis, we transferred normal copies of chromosome 13 via microcell fusion into the human fibrosarcoma cell line HT1080. Microcell hybrids were generated that contained one, two, or three extra copies of the transferred fibroblast chromosome 13. Compared to the parental cell line, the hybrids were completely unaltered with respect to several properties in vitro and in vivo, including morphology, growth rate, and tumor formation. Northern blot analysis revealed a stepwise increase in RB1 mRNA expression which increased in proportion to the number of alleles present in each cell line. Although RB1 protein exhibited correct nuclear localization and was phosphorylated in a normal cell cycle-dependent manner in the hybrids, the increased level of protein expression in each hybrid was nearly identical and did not increase beyond a threshold amount, although mRNA expression continued to increase. These results demonstrate that HT 1080 cells can tolerate an increase level of RB1 protein, but that expression beyond a certain level may be down-regulated. These transfer studies provide evidence for regulation of RB1 protein expression and may suggest an alternative form of monitoring and controlling normal RB1 functioning.

Tumor suppressor genes, growth factor genes, and oncogenes in hepatitis B virus-associated hepatocellular carcinoma

A series of changes in the genes that control hepatocyte growth, or interference with the protein products of these genes, appears to have an important role in the etiology of hepatocellular carcinoma (HCC). Mutations of the p53 tumor suppressor gene have been identified in 30-50% of HCC patients in some geographic areas. Abnormalities of the RB tumor suppressor gene have been found in 20-25% of HCCs, including 80-86% of HCCs with p53 mutations. Overexpression of transforming growth factor alpha (TGF-alpha), insulin-like growth factor II (IGF-II), and the oncogenes N-ras, c-myc, and c-fos have been found in high percentages of HCC patients. The cumulative effect of these changes may be more important than the order in which they occur. Some of these changes may explain the mechanism(s) by which the hepatitis B virus participates in the development of HCC.

Absence of transforming growth factor-beta responsiveness in the tamoxifen growth-inhibited human breast cancer cell line CAMA-1

Tamoxifen has been an effective antiestrogen in suppressing breast cancer growth which is estrogen-responsive or dependent. Early studies have provided circumstantial evidence that transforming growth factor-beta (TGF-beta) may be an autocrine mediator of tamoxifen action. Therefore, it is both fundamentally important and clinically relevant to investigate the relationship between tamoxifen and TGF-beta. In this study, we demonstrated that CAMA-1 cells, which are sensitive to tamoxifen inhibition, did not respond to TGF-beta growth inhibition. The type I and II TGF-beta receptors were undetectable by the radio-ligand affinity labeling technique. Despite the presence of a normal TGF-beta type II receptor gene, the mRNA transcript of the gene was undetectable by the extremely sensitive Intron-differential RNA/PCR method. The possibility that the lack of TGF-beta receptors might be intimately linked to the absence of normal retinoblastoma (Rb) gene products, as suggested by previous studies of retinoblastoma cells, was further investigated. The lack of TGF-beta receptor expression was found due to reasons other than the absence, deletion or abnormality of the Rb gene because a normal Rb gene and its hyper- and hypo-phosphorylated protein products were detected in CAMA-1 cells. In conclusion, our results suggest that the TGF-beta system is not obligatory for antiestrogen growth inhibition of CAMA-1 cells.

Mechanistic considerations in chemopreventive drug development

This overview of the potential mechanisms of chemopreventive activity will provide the conceptual groundwork for chemopreventive drug discovery, leading to structure-activity and mechanistic studies that identify and evaluate new agents. Possible mechanisms of chemopreventive activity with examples of promising agents include carcinogen blocking activities such as inhibition of carcinogen uptake (calcium), inhibition of formation or activation of carcinogen (arylalkyl isothiocyanates, DHEA, NSAIDs, polyphenols), deactivation or detoxification of carcinogen (oltipraz, other GSH-enhancing agents), preventing carcinogen binding to DNA (oltipraz, polyphenols), and enhancing the level or fidelity of DNA repair (NAC, protease inhibitors). Chemopreventive antioxidant activities include scavenging reactive electrophiles (GSH-enhancing agents), scavenging oxygen radicals (polyphenols, vitamin E), and inhibiting arachidonic acid metabolism (glycyrrhetinic acid, NAC, NSAIDs, polyphenols, tamoxifen). Antiproliferation/antiprogression activities include modulation of signal transduction (glycyrrhetinic acid, NSAIDs, polyphenols, retinoids, tamoxifen), modulation of hormonal and growth factor activity (NSAIDs, retinoids, tamoxifen), inhibition of aberrant oncogene activity (genistein, NSAIDs, monoterpenes), inhibition of polyamine metabolism (DFMO, retinoids, tamoxifen), induction of terminal differentiation (calcium, retinoids, vitamin D3), restoration of immune response (NSAIDs, selenium, vitamin E), enhancing intercellular communication (carotenoids, retinoids), restoration of tumor suppressor function, induction of programmed cell death (apoptosis) (butyric acid, genistein, retinoids, tamoxifen), correction of DNA methylation imbalances (folic acid), inhibition of angiogenesis (genistein, retinoids, tamoxifen), inhibition of basement membrane degradation (protease inhibitors), and activation of antimetastasis genes. A systematic drug development program for chemopreventive agents is only possible with continuing research into mechanisms of action and thoughtful application of the mechanisms to new drug design and discovery. One approach is to construct pharmacological activity profiles for promising agents. These profiles are compared among the promising agents and with untested compounds to identify similarities. Classical structure-activity studies are used to find optimal agents (high efficacy with low toxicity) based on good lead agents. Studies evaluating tissue-specific and pharmacokinetic parameters are very important. A final approach is design of mechanism-based assays and identification of mechanism-based intermediate biomarkers for evaluation of chemopreventive efficacy.

Molecular biology of breast cancer

BACKGROUND

A gene responsible for an inherited predisposition to breast and ovarian cancer has been localized to the long arm of chromosome 17 and termed BRCA1. As well as being closely linked to breast/ovarian cancer cases, this gene may be involved in up to 45% of site-specific breast cancers. The identification and cloning of the BRCA1 gene is imminent, and will facilitate the screening and counselling of families at risk of breast cancer, and in the longer term may open up new therapeutic possibilities. The tumour suppressor gene TP53 is mutated in 25%-40% of cases of sporadic breast cancer, and is associated with an aggressive tumour phenotype and poor prognosis in both node-positive and node-negative cases. The pattern of mutations in this tumour suppressor gene shows a higher than expected frequency of G to T transversions, mostly restricted to the highly conserved domain in exons 5 to 8. In many, but not all cases, point mutation of one allele is accompanied by deletion of the remaining normal allele at chromosome 17p13. Abnormalities of TP53 appear to be relatively early events in tumorigenesis, being present in ductal carcinoma in situ lesions. The retinoblastoma gene RB1 shows a variety of abnormalities in about 20% of breast cancers, and there may be an association with TP53 mutations. Other abnormalities which occur with a particularly high incidence in breast cancer include allele loss at chromosome 1p/1q, 3p, 6q, 11p, 16q and 18q. The ERBB2 oncogene encodes a transmembrane receptor tyrosine kinase whose ligand has recently been claimed to be the heregulin family in man.(ABSTRACT TRUNCATED AT 250 WORDS)

Tumor suppressor genes and their roles in breast cancer

Tumor suppressor genes have been identified by the occurrence of mutations in many families with hereditary forms of cancer, exposed during development of the tumor by loss of heterozygosity. They have a number of diverse functions. For example, both the RB gene of retinoblastoma and the p53 gene, which is commonly mutated in breast and colon cancer among others, produce proteins involved in distinct steps of cell cycle control, while the nm23 product prevents metastasis. Here we review the data developed until now on the possible presence and role of mutations in these and other tumor suppressor genes in breast cancer. A more complete understanding of the tumor suppressor genes could not only provide diagnostic information, but could lead to specific gene therapy to replace suppressor functions lost in individual tumors.

Cellular oncogenes and suppressor genes as prognostic markers in cancer

Activation of cellular or c-oncogenes and loss of function of suppressor genes appears to be the key event in the formation of most human cancers. Altered forms of these genes or their protein products have the potential to provide a new generation of cancer markers. As cancer markers, the most useful application of c-oncogenes and suppressor genes so far, has been in providing prognostic information. The correlation of N-myc gene amplification with poor prognosis in neuroblastoma was one of the first examples of prognostic data supplied by a c-oncogene. Most, but not all investigators, find that either amplification or increased expression of c-erbB-2 gene correlates with poor prognosis in breast cancer. Other potential prognostic markers in breast cancer include amplification of the c-myc gene, and increased expression of both EGFR and p53 protein. Although c-oncogenes and suppressor genes have the potential to supply prognostic information in a broad range of cancers, many of the results are still preliminary with conflicting conclusions.

Efficient transcriptional activation of many simple modular promoters by simian virus 40 large T antigen

Simian virus 40 (SV40) large T antigen is a multifunctional protein which plays central roles during both lytic and transforming infections by SV40. It is a potent transcriptional activator and increases expression from the SV40 late promoter and from several cellular promoters. To understand better the transcriptional activation activity of large T antigen, we examined its ability to transactivate a set of simple modular promoters containing one of four upstream activation sequences coupled with one of three different TATA box sequences originally constructed and studied by Taylor and Kingston (Mol. Cell. Biol. 10:165-175, 1990). Large T antigen activated transcription from all of these simple promoters. The identity of the TATA box was a more important determinant of the final level of gene expression than was the identity of the upstream activating sequence element. We also determined the ability of a set of mutant SV40 large T antigens to activate a subset of these promoters. Several mutant SV40 large T antigens which had reduced ability to activate the complex SV40 late and Rous sarcoma virus long terminal repeat promoters showed reduced transcriptional activation activity on all of the modular promoters tested. We used a set of promoter derivatives of the human U6 small nuclear RNA promoter containing different TATA boxes and found that wild-type large T antigen could activate transcription from all of them, although to widely different levels of expression.

Clinical and genetic patterns of neurofibromatosis 1 and 2

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Lessons from our children

While the incidence of cancer is increasing among both children and adults, mortality rates have decreased for children, while they have increased for adults. Of children diagnosed with cancer today, 80% are predicted to be long-term survivors. Although there are differences between children and adults with respect to the tumor types, biology, and outcome, there are common lessons which we can learn from our children regarding the genetics of cancer, its management and treatment, and the importance of longitudinal studies of the survivors. Specific pediatric cancers, such as retinoblastoma, have led to the recognition of tumor suppressor genes, now also observed among adult tumors including sarcomas, breast, lung, and bladder cancer. The presence of the tumor suppressor gene provides an understanding for the incidence of second malignant tumors among patients with heritable diseases. Furthermore, cancer prone families, such as those with the Li-Fraumeni syndrome, also carry the p 53 tumor suppressor gene; the presence of which greatly increases the risk of developing invasive cancer. Childhood cancer is rare; it represents only 1% of the total US cancer problem. However, 53% of all children with cancer, but only 2% of all adults, are studied via the NCI cooperative group mechanism. For some specific childhood tumors such as rhabdomyosarcoma and Wilms' tumor, as many as 70-85% of all cases are managed via NCI sponsored trials. Essentially all pediatric cancer is treated by interdigitating radiation with surgical resection and systemic chemotherapy. This approach has contributed to high cure rates. Finally, our understanding of the late effects of being a cancer survivor have come from longitudinal studies of children. The most severe long-term effects related to radiation in childhood pertain to growth and development, infertility, and second malignant tumor induction. Here the children treated for Hodgkin's disease have taught us the dose and volume effects on axial skeletal and soft tissue growth. Infertility issues are also treatment-related and may often be obviated by using gonadal shielding. The risk of secondary leukemia is related to dose and class of specific chemotherapeutic agents administered; it is 5.5% among children receiving 6 cycles of MOPP. There is a 22-fold risk at 30 years of age of solid tumor induction following radiotherapy for children with Hodgkin's disease. These serious concerns have been offset by current therapeutic approaches of using lower doses and smaller volumes of radiation with fewer cycles of less toxic chemotherapeutic agents. Childhood cancer ranks high among number of person-years of potential life saved annually.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of oncogenes and tumor suppressor genes in human breast cancer

Oncogenes (c-erbB-2, c-myc, and some genes linked to the 11q13 lesion), tumor suppressor genes (retinoblastoma gene, p53) and an antimetastatic gene (nm23/nucleoside diphosphate kinase) play important roles in breast cancer progression. Amplification of c-erbB-2, c-myc, and int-2, and expression of RB, p53(mutant), and NDP kinase were determined in 77 primary breast cancer specimens. nm23-H1 allelic loss was also studied. c-erbB-2 and c-myc amplification, loss of RB expression, p53(mutant) expression, and nm23-H1 allelic loss were also found in non-invasive carcinoma. int-2 amplification was significantly correlated with lymph node status (P = 0.02) and a significant association was found between p53(mutant) expression and tumor size (P = 0.04). c-erbB-2 amplification was strongly associated with disease-free and overall survival in multivariate analysis (P = 0.002). All of the c-erbB-2 amplified cases and all but one of the int-2 amplified cases in node-positive patients had relapsed within 2 years post resection. The cancer cells may acquire new proliferative pathways sequentially as a result of multiple genetic alterations which enable them to bypass the estrogen-dependent proliferation.

Abnormalities of retinoblastoma gene structure in human lung tumors

The retinoblastoma (Rb) gene is associated with the pathogenesis of several types of human cancer, including retinoblastoma, osteosarcoma, soft tissue sarcomas, and lung, breast and bladder carcinomas. Loss of heterozygosity is a common mode in allelic inactivation of Rb and other tumor-suppressor genes. We investigated DNA from 15 human lung tumors for loss of heterozygosity of the Rb locus using a polymerase chain reaction (PCR) based restriction fragment length polymorphism assay. Of informative cases we found loss of heterozygosity in 2 out of 3 squamous cell carcinomas and 1 out of 2 adenocarcinomas of the lung. We also found structural rearrangements in two out of fourteen Hind III digested lung tumors examined at the 5' region of the human Rb gene using Southern blot hybridization analysis. Since these two tumors were classified as stage III it is possible that the alteration of Rb gene is involved in the progression of this type of cancer. Using specific primers for exons 15, 16, 21 and 22 of the Rb gene, we carried out amplification of these exons by polymerase chain reaction. None of these tumors showed a deletion of exons 15, 16, 21 and 22.

Loss in expression of the retinoblastoma gene product in human gliomas is associated with advanced disease

The retinoblastoma susceptibility gene (RB), the prototype of the class of tumor suppressor genes, is inactivated in a number of human malignancies. We investigated a possible role of RB in human brain tumors. Immunoprecipitation revealed frequent loss of RB protein expression in glioma cell lines (8/24), which was accompanied by lack of RB encoded transcripts. Among seventeen primary brain tumors studied by Western blotting, loss of Rb protein expression was observed in WHO grade 3 and 4 gliomas (3/10). However, none of the low grade gliomas and the other primary brain tumors investigated lacked RB protein expression. These data suggest a role for RB in glial malignancy, and loss of Rb expression appears to be associated with glial tumor progression.

The adenovirus E1A-associated kinase consists of cyclin E-p33cdk2 and cyclin A-p33cdk2

The adenovirus E1A oncoproteins form stable complexes with several cellular proteins. Association of E1A with these proteins has been shown to be important for the oncogenic potential of E1A. Several of these proteins have been identified and include the product of the retinoblastoma gene and a key cell cycle regulatory protein, cyclin A. E1A also associates with a potent histone H1 kinase. The two major components of this activity are the cyclin E-p33cdk2 and cyclin A-p33cdk2 complexes. Both the cyclin E-p33cdk2 and cyclin A-p33cdk2 complexes have been implicated in regulatory events controlling entry into or passage through DNA synthesis. Although the architecture of such interactions remains unclear, it is likely that by targeting such complexes, adenovirus is affecting some aspect of cell cycle control.

Retinoblastoma (Rb) gene product expression in lymphomas. Correlation with Ki67 growth fraction

The retinoblastoma susceptibility gene (Rb) has been characterized as a tumour suppressor gene. Rb protein is involved in cell-cycle control, regulating gene transcription. The absence of Rb protein in inherited retinoblastoma has been proved to be the result of inactivation of both Rb alleles through mutation or deletion, according to the general model for suppressor genes. The frequent detection of Rb gene alterations in human tumours (retinoblastoma, osteosarcoma, bladder carcinoma, small-cell lung carcinoma) and the correlation with clinical outcome found in some tumours prompted us to study Rb gene expression in lymphoid tumours in an attempt to determine whether Rb gene expression is related to histological type and degree of aggressivity in human lymphomas. To establish normal levels of Rb protein, its expression was analysed in vitro on cytospin preparations from normal and pokeweed mitogen (PWM) or phytohaemagglutinin (PHA)-stimulated peripheral blood lymphocytes (PBLs), using a monoclonal antibody (PMG3-245). Rb protein expression in vivo was quantified using a computer analysis system (CAS) on frozen sections from reactive and neoplastic lymphoid tissue. As a control of tissue preservation, and to compare Rb expression and growth fraction, the tumours and cells were labelled simultaneously with the Ki67 monoclonal antibody. Normal and stimulated lymphocytes showed a gradual increase of Rb protein during progression of the cell cycle, with a peak in the M phase. G0-G1 cells had no detectable levels of Rb protein, suggesting that the Rb gene may act as a 'status quo' cellular growth fraction control mechanism. In reactive lymphoid tissue, Rb protein was mainly expressed in germinal centres (lymph nodes, tonsils) and cortical thymocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Prognostic indicators in early breast cancer

Approximately 115,000 new cases of axillary node negative breast cancer were diagnosed in this country last year. Since about 20-30% of these patients will ultimately relapse and die of their disease, adjuvant systemic therapy has been advocated for this group to decrease the relapse rate and prolong survival. However, although most clinical trials have demonstrated a modest impact on disease recurrence, the available data have failed to show consistent improvements in overall survival and does not justify the generalized use of systemic treatment in this patient subgroup. For this reason, a plethora of prognostic factors have been described to identify those patients with a higher risk of recurrence to concentrate therapeutic options in this specific group. Of all the disease prognosticators studied, tumor size, nuclear grade, and proliferative indexes appear to correlate well with tumor recurrence. In addition, biologic characteristics of primary tumors such as the presence of hormone and growth factor receptors, secretion of specific polypeptides and proteases, expression of proto-oncogenes, and abnormalities in tumor suppressor genes have been shown to be potentially useful as prognostic indicators in patients with early breast cancer. Despite these provocative data, larger clinical trials are necessary before incorporating these parameters in the routine evaluation of patients with axillary node negative breast cancer.