In this study, high-risk HPV DNA was detected in eight of 20 (40%) metaplastic carcinomas of the mammary gland. HPV-16 was found in seven of these (87.5%), whereas HPV-18 was present in the remaining case (12.5%). Distribution of the HPV genotype is in accordance with previous Mexican studies in which HPV-16 was the commonest HPV detected among breast carcinoma [
20,
22]. According to surrogate immunohistochemical profile, nearly all cases fall into the category of “triple negative” tumors, which form part of the spectrum of basal-like breast carcinomas. To the best of our knowledge, this is the first study to search for high-risk HPV DNA in metaplastic carcinomas of the female mammary gland among Latin American women, and the second reported among other female populations worldwide. The range of association of HPV and conventional breast cancer has been reported as between 0 [
23,
24] and 86% [
25]; such differences have been attributed to several factors including variations in the sensitivity of the PCR methods employed for detecting HPV DNA sequences, according to the quality of DNA and tissue preservation, variations in the prevalence rate of HPV infection among different countries and among different regions of the same country, as well as differences in socioeconomic status among worldwide population. Interestingly, in this study the prevalence of HPV-positive metaplastic carcinomas were higher in comparison with previously reported in Mexican non-metaplastic carcinomas (40 vs. 10%) [
20]. HPV viral loads have not been extensively studied in breast carcinoma; in previous studies [
20,
26], the estimated viral loads for HPV-16 in breast neoplastic and non- neoplastic adjacent tissues were low (<1copy/cell), rendering it unlikely that even integrated HPV is involved in breast carcinogenesis. In esophageal squamous cell carcinoma, another malignant neoplasm probably associated with HPV infection, it is also unlikely that low viral loads have a leading role in the mechanism of carcinogenesis as in cervical cancer [
27]. Notwithstanding the low viral loads found again in the present study, these were higher than in the non-metaplastic carcinomas reported previously [
20,
26]. It was also suggested that high-risk HPV DNA could be acquired and integrated into mammary cells, once breast neoplastic transformation takes place, probably during early events of neoplastic development (i.e. preclinical stage), thus modifying the course of breast carcinoma [
20]. Interestingly, there are contradictory findings regarding the presence of HPV in non- malignant breast conditions: HPV traces have been found in two mammary fibroadenomas [
28] and HPV-18, in three normal breast reduction specimens [
29], whereas in other studies, HPV DNA has not been isolated from benign breast conditions or mammoplasty specimens [
22,
30]; its presence in the breast fluids has been a controversial issue, as well [
31‐
33]. On the other hand, HPV DNA is also detected in normal mammary gland tissues adjacent to breast carcinomas [
20,
34]. In an experimental study, the cell’s invasive and metastatic abilities were induced by transfecting two non-invasive breast cancer cell lines (MCF7 and BT20) with E6/E7 of HPV-16, in comparison with the same non-transfected, non-invasive breast cancer cell lines [
35]. Contrary to this finding but of great interest in this study, is the fact that HPV-positive were smaller than HPV-negative metaplastic breast carcinomas, a feature reported to be associated with a better prognosis in Australian women, mainly because these are early stage tumors [
36]. Indeed, the smaller size of the HPV- positive metaplastic breast carcinomas comprises the only variable that reached statistical significance in this and in similar previous studies carried out in Mexican women [
20,
22], suggesting that HPV could modify, as previously stated, the course of metaplastic breast carcinoma and, as a paradoxical effect, by means of improving the clinical outcome. This paradoxical effect is also present in HPV-associated oropharyngeal [
37] and lung carcinomas [
38]. In the case of lung carcinomas, favorable prognosis of HPV-associated carcinomas has been attributed to high Langerhans-cell infiltration [
38]. The route of HPV infection of the breast tissue remains unsolved. Because the life cycle of HPV occurs in epithelial layers, bloodstream viremia is not an expected event. However, high-risk HPV DNA has been isolated in the peripheral circulating mononuclear cells from females harboring cervical cancer [
39], from pediatric patients infected with Human immunodeficiency virus (HIV) and healthy donors [
40], and from healthy males [
41], raising the possibility that HPV DNA could reach the transformed breast cells from a previous or transient cervical HPV infection, even in women with a subclinical infection, through the circulating blood. We previously suggested that the presence of HPV DNA sequences in breast tissues and breast carcinomas could be related with changes in the level of expression of integrins, particularly with that of the α-6 integrin [
20], but also of Heparan sulfate proteoglycans (HSPG) [
42], which are putative candidates for HPV cell receptors [
43].
On the other hand, it was also suggested that metaplastic carcinomas, together with less differentiated breast carcinomas, represent the progenitor/stem-cell end of the spectrum in which breast carcinomas could be arranged, according to different protein expression patterns and (cyto)genetic alteration patterns [
44]. Within the MCF7 breast-cancer cell line, a stem cell-like subpopulation has been characterized by overexpression of the adhesion molecule α-6 integrin [
45]. Considering this, it is expected that carcinomas arising from progenitor/stem cells, are more likely to express higher levels of α-6 integrin and HSPG, thus rendering a higher frequency of HPV-DNA among the neoplastic cells. Indeed, syndecan-1 and syndecan-4, two cell surface HSPG that have recently been characterized as markers of aggressiveness in breast carcinoma, are overexpressed in estrogen-negative and highly proliferative carcinomas of the mammary gland [
41,
46,
47]. Metaplastic breast carcinomas are consistently negative for ER, PgR and Her-2/
neu but positive for EGFR, as in nearly all of our cases. It is noteworthy that the majority of HPV-associated carcinomas reported in the literature are closely related with the “triple-negative” profile. In the study of Kroupis
et al. [
48] HPV-positive carcinomas tend to be poorly differentiated carcinomas (grade III), less ER-positive, and more proliferative carcinomas. Finally, metaplastic breast carcinoma is more common among African-American and Hispanic women in the U.S. [
2] and the mean age among Hispanic women there is higher than in the Mexican patients of this study (61.2 vs. 49 years), a feature probably related with the exposure to an unfavorable and riskier environment at an earlier age, including HPV infection. Limitation of the study must be seen in the fact that the sample size is low, due to the rarity of metaplastic breast carcinoma, as in Mexico, as worldwide. Because of the small number of cases in this study, levels of statistics significance should be interpreted cautiously.