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Department of Medical Oncology, Hospital Universitario Puerta de Hierro, Madrid, Spain

Most studies evaluating the expression status of p73 isoforms in cancer patients are based on the quantitative analysis of their mRNA levels by utilizing sets of primers that specifically amplify each variant.^1-6 This approach is largely due to the lack of available high-affinity, high-isoform specificity antibodies. At best, antibodies discerning between Np73 and TAp73 forms are commercially available.⁷ In addition, the biologic meaning of the subcellular location (nuclear and/or cytoplasmic) of the p73 proteins remains unclear.

Bozzetti et al found TAp73 and Np73 protein immunostaining in 47% and 45% of analyzed cases, respectively. TAp73 staining was confined to the nucleus and cytoplasm in 40% and 56% of cases, respectively. One case (4%) showed both nuclear and cytoplasmic location. Np73 immunostaining was seen mainly in the cytoplasm (93% of the 30 positive cases). In our study,⁹ the status of the various p73 isoforms in the tumor and normal counterpart tissue of 113 colon and 60 breast cancer patients was assessed by evaluating the mRNA expression levels by quantitative real-time reverse transcriptase polymerase chain reaction (RT-PCR). To establish a correlation between mRNA and protein expression levels, 25 and 14 colon and breast cancer cases, respectively, were analyzed by immunohistochemistry. As p73 variants show nuclear activities, both TAp73 and Np73 proteins were considered positive in tissue samples showing nuclear staining in more than 10% of the epithelial cells, and cells expressing only cytoplasmic staining were excluded. p73 protein accumulates in the nucleus on DNA damage,^7,8 and its expression has been described as mainly nuclear in proliferating cells and only occasionally nuclear in differentiated cells.^7,10,11 In our breast cancer series,⁹ mRNA quantitative expression correlated with immunostaining in 11 of the 14 breast cases. Regardless of their nuclear positivity, all the analyzed cases showed negative cytoplasmic immunostaining for TAp73. For Np73, six of the 10 cases showing negative nuclear staining and two of four samples with positive nuclear staining showed very weak cytoplasmic staining. The remaining two cases showing nuclear staining also harbored clear cytoplasmic staining. Studies analyzing the biologic meaning of p73 isoforms' subcellular location are needed in order to clarify whether their cytoplasmic location correlates with specific cell functions in vivo or, in contrast, corresponds to a functionally inert p73. At present, p73 shuttle from nucleus to cytoplasm is associated with its degradation.¹² In addition, in view of the dissimilar and inconclusive results found in the literature,^13-16 larger studies using faithful antibodies to analyze the subcellular location of p73 variants in normal tissues, breast cancer and other tumor types must be assessed to verify whether p73 location is tumor specific and/or histology specific.

In our series,⁹ correlations were found between vascular invasion and Ex2p73 overexpression (P = .05). A trend was also observed for Np73 (P = .06). Trends were also found between Ex2p73, Ex2/3p73, and TAp73 and the hormone receptors. Bozzetti et al did not find such correlations in their series. As the antibodies used by Bozzetti et al did not recognize the Ex2p73 and Ex2/3p73 isoforms, the correlations we found for these variants could not be analyzed. Larger breast cancer series than those analyzed by Bozzetti et al and our group should be evaluated to confirm the possible role of specific p73 variants as markers for patient suitability for hormone therapy.

Bozzetti et al found a significant correlation between oncogenic and suppressor p73 variant expression levels, as we and others described elsewhere for several tumor types.^2,9 This might indicate that both p73 promoters are regulated by the same stimuli.

In our breast cancer series an association was found between wild-type p53 status and overexpression of the Np73 variant (P = .04). Similar associations between overexpression of specific p73 variants and wild-type p53 were also observed in our colon series.⁹ Such an association was also seen in other studies of breast cancer¹⁷ and other tumor types.² Bozzetti et al did not find such an association in their series. As alterations translated into gain or lack of redundant functions in tumor cells may not confer extra growth advantages during the carcinogenic process, cells harboring redundant functional alterations are less likely to be selected during tumorigenesis. Larger series evaluating this association in breast cancer and other tumor types should be done to clarify the existence of such an association. In addition, whether p73 variants participate/interfere in vivo with pathways other than the p53 one and thereby confer extra functions needs to be thoroughly analyzed. Studies evaluating whether alterations such as p73 variant overexpression or aberrant p73 subcellular location contribute to the oncogenesis process are necessary.

It is clear that acquisition of inactivating alterations in p53 is pivotal in the carcinogenesis process. Currently, p53 immunohistochemical status is widely included in the medical records of cancer patients, although its mutational status has no practical clinical use, since it does not correlate exactly with the tumor phenotype. Factors that may modify tumor suppressor p53 activity, such as expression levels of specific p73 variants, should be assessed in future studies to evaluate whether their inclusion in the medical records of patients along with p53 provides valuable additional information and could be used as molecular markers of patients' outcome. Little is known about p73 functions other than those that mimic p53 activities. Further studies to evaluate p73 variants functions and the biologic meaning of the heterocomplexes formed among tumor suppressor and oncogenic p73 variants and p53 protein are needed, if their involvement in human carcinogenesis is to be fully understood.

AUTHORS' DISCLOSURES OF POTENTIAL CONFLICTS OF INTEREST

The authors indicated no potential conflicts of interest.

ACKNOWLEDGMENTS

Supported by Instituto de Salud Carlos III Grant No. CP03/00032 and Spanish Ministry of Education and Science Grant No. 2004-01002.

REFERENCES

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This Article Full Text (PDF) Purchase Article View Shopping Cart Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Alert me to new issues of the journal Save to my personal folders Download to citation manager Rights & Permissions Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Domínguez, G. Articles by Bonilla, F. Search for Related Content PubMed Articles by Domínguez, G. Articles by Bonilla, F. Social Bookmarking                            What's this?