Pathology and genetics of tumors of soft tissue and bone

The E6 oncoprotein encoded by human papillomavirus types 16 and 18 promotes the degradation of p53

The cell cycle is composed of a series of steps which can be negatively or positively regulated by various factors. Chief among the negative regulators is the p53 protein. Alteration or inactivation of p53 by mutation, or by its interactions with oncogene products of DNA tumour viruses, can lead to cancer. These mutations seem to be the most common genetic change in human cancers.

The p53 tumour suppressor gene

Deletions or mutations of the retinoblastoma gene, RB1, are common features of many tumors and tumor cell lines. Recently, the RB1 gene product, p105-RB, has been shown to form stable protein/protein complexes with the oncoproteins of two DNA tumor viruses, the adenovirus E1A proteins and the simian virus 40 (SV40) large T antigen. Neither of these viruses is thought to be associated with human cancer, but they can cause tumors in rodents. Binding between the RB anti-oncoprotein and the adenovirus or SV40 oncoprotein can be recapitulated in vitro with coimmunoprecipitation mixing assays. These assays have been used to demonstrate that the E7 oncoprotein of the human papilloma virus type-16 can form similar complexes with p105-RB. Human papilloma virus-16 is found associated with approximately 50 percent of cervical carcinomas. These results suggest that these three DNA viruses may utilize similar mechanisms in transformation and implicate RB binding as a possible step in human papilloma virus-associated carcinogenesis.

https://science.sciencemag.org/content/sci/243/4893/934.full.pdf

The human papilloma virus-16 E7 oncoprotein is able to bind to the retinoblastoma gene product

Merkel cell carcinoma (MCC) is a rare but aggressive human skin cancer that typically affects elderly and immunosuppressed individuals, a feature suggestive of an infectious origin. We studied MCC samples by digital transcriptome subtraction and detected a fusion transcript between a previously undescribed virus T antigen and a human receptor tyrosine phosphatase. Further investigation led to identification and sequence analysis of the 5387-base-pair genome of a previously unknown polyomavirus that we call Merkel cell polyomavirus (MCV or MCPyV). MCV sequences were detected in 8 of 10 (80%) MCC tumors but only 5 of 59 (8%) control tissues from various body sites and 4 of 25 (16%) control skin tissues. In six of eight MCV-positive MCCs, viral DNA was integrated within the tumor genome in a clonal pattern, suggesting that MCV infection and integration preceded clonal expansion of the tumor cells. Thus, MCV may be a contributing factor in the pathogenesis of MCC.

/pdf/clonal-integration-of-a-polyomavirus-in-human-merkel-cell-1x6ky8rqes.pdf

Clonal integration of a polyomavirus in human Merkel cell carcinoma.

https://www.cell.com/cell/pdf/0092-8674(88)90559-4.pdf

SV40 large tumor antigen forms a specific complex with the product of the retinoblastoma susceptibility gene

SV40 large T antigen binds preferentially to an underphosphorylated member of the retinoblastoma susceptibility gene product family.

An N-Terminal transformation-governing sequence of SV40 large T antigen contributes to the binding of both p110Rb and a second cellular protein, p120

A series of replication-competent simian virus 40 (SV40) large T antigens with point and deletion mutations in the amino acid sequence between residues 105 and 115 were examined for the ability to immortalize primary cultures of mouse and rat cells. The results show that certain mutants, including one that deletes the entire region, are able to immortalize. However, consistent with previous data, the immortalized cells are not fully transformed, as judged by doubling time, sensitivity to concentrations of serum, and anchorage-independent growth. The region from 106 to 114 has structural features in common with a region involved in transformation by adenovirus E1a protein (J. Figge, T. Webster, T.F. Smith, and E. Paucha, J. Virol. 62:1814-1818, 1988) and influences the binding of the retinoblastoma gene product to large T (J.A. DeCaprio, J.W. Ludlow, J. Figge, J.-Y. Shew, C.-M. Huang, W.-H. Lee, E. Marsilio, E. Paucha, and D.M. Livingston, Cell 54:275-283, 1988). Together, these results imply that the sequence from 106 to 114 forms part of a domain that is essential for transformation of established cells, is dispensable for immortalization, and is not required for SV40 replication. The results also indicate that the ability of SV40 large T to immortalize primary cells is independent of its ability to bind to the retinoblastoma gene product.

Identification of a region of simian virus 40 large T antigen required for cell transformation.

Simian virus 40 large T antigen (T) can transform cultured cells, but the mechanisms by which it functions are not entirely understood. Several lines of evidence have suggested that the amino-terminal approximately 130 residues of T may be sufficient to confer the transforming capability. Oligonucleotide-directed mutagenesis was used to generate a series of deletion and substitution mutants within the amino-terminal 82 residues of T, the segment which is shared with simian virus 40 small t antigen (t). Results of stability and transformation assays of these mutants strongly suggest that the 1-to-82 region of T contains sequences which govern T transforming activity and affect in vivo stability. Instability and a defect in transforming activity could be separated from one another genetically. Thus, the 1-to-82 region appears to contain a specific region that contributes to the transforming function of the protein. This segment operates by means other than the simple binding of pRb and/or p107.

Eva Paucha

Papers

SV40 large tumor antigen forms a specific complex with the product of the retinoblastoma susceptibility gene

SV40 large T antigen binds preferentially to an underphosphorylated member of the retinoblastoma susceptibility gene product family.

An N-Terminal transformation-governing sequence of SV40 large T antigen contributes to the binding of both p110Rb and a second cellular protein, p120

Identification of a region of simian virus 40 large T antigen required for cell transformation.

The T/t common region of simian virus 40 large T antigen contains a distinct transformation-governing sequence.