CRC is a deadly illness of great concern in Hispanic populations and the island of Puerto Rico is a great site to study this ethnicity. During 1998–2002, in both sexes, Puerto Ricans had CRC incidence and mortality rates similar to those for US Hispanics, but their rates were lower than those for non-Hispanic whites and non-Hispanic blacks. However, Puerto Rican men and women with ages 40–59 years had a greater risk of incidence and mortality than their US Hispanic counterparts [74
Evaluation of colon cancers for MMR defects may have significant therapeutic implications. In patients with stage II or III CRC, adjuvant 5-fluorouracil chemotherapy provides survival improvement in patients with MMR-competent tumors while the survival of patients with MMR-defective tumors does not improve with the adjuvant chemotherapy [75
]. Also, loss of tumor MMR function may predict improved outcome in stage III colon cancer patients postoperatively treated with a weekly bolus irinotecan, fluorouracil, and leucovorin regimen as compared with those receiving weekly bolus of fluorouracil and leucovorin [78
HNPCC is characterized by young onset CRC [8
], an increased risk for gynecologic, urinary tract, and gastrointestinal cancers [9
], and most commonly occurs in men [89
]. In HNPCC, a single mutation is inherited in the germline, and MSI occurs only after inactivation of the other allele [90
]. Most sporadic MSI-H cancers are caused by methylation and silencing of the MLH1
], most probably develop in women, and have their origin within serrated polyps [89
]. Colorectal tumors with MSI are associated with location proximal to the splenic flexure and poor histological differentiation [67
MSI occurs in approximately 20% of CRC cases [23
]. These tumors typically fail to express MMR protein as seen on immunohistochemistry [28
]. In the present study, only 4.3% of 164 patients lacked protein MMR expression. Lack of protein expression as a surrogate for MSI is clinically important to patients. A patient with absence of MMR protein expression may be a member of a Lynch syndrome kindred, hence results may have implications for themselves and other family members. Furthermore, individuals belonging to a HNPCC-kindred have a higher risk of developing other non-colorectal cancers such as endometrial, ovarian, and gastric [9
]. Of the seven tumors in our study that lacked MMR protein expression, six tumors lacked expression of MSH2, which usually indicates a germline mutation in the MSH2
gene, and one patient had absence of MLH1 with MSI-H.
The tumor with absence of MLH1 protein expression with MSI-H may have a either a germline mutation or hypermethylation of the MLH1
promoter. Additionally, this patient had normal BRAF
gene, which may be suggestive of HNPCC-associated MLH1
germline mutation. Virtually 100% of individuals with HNPCC do not carry the BRAFV600E
], whereas 68% of those without HNPCC do [14
]. Moreover, BRAFV600E
mutation is associated with sporadic CRC with MSI [30
promoter hypermethylation leads to gene inactivation [96
] and, if present, may be secondary to CpG island methylation phenotype (CIMP), progressing via the MSI-H pathway. CpG islands are regions rich in cytosine-guanosine dinucleotide repeats at the 5′ region of approximately half of all human genes. Methylation of cytosine residues within CpG islands of promoters and proximal exons is associated with loss of gene expression [97
]. CIMP tumors have similarities to tumors with MSI-H, such as right-sided location and poor histological differentiation [98
]. However, BRAF
mutation is frequently seen in sporadic MSI-H CRC, associated with DNA methylation secondary to CIMP-high (3–4 methylated in tumor markers are methylated) status [99
]. For these reasons, it is unlikely that a MLH1
promoter hypermethylation secondary to CIMP may have occurred. However, none of the patients underwent germline genetic testing.
In our study of Hispanics from Puerto Rico, the frequency of MMR protein-negative tumors was far lower than in other studies of unselected CRC patients in other countries or ethnicities, especially regarding MLH1-protein negative expression () [37
]. Moreover, the frequency of MMR protein-negative tumors in our study was lower than in Spain, including MLH1-protein negative expression [57
Comparison of our study with others in other countries or ethnicities for mismatch repair protein expression in unselected colorectal cancer patients
As seen in other studies [4
], colorectal tumors with lack of MMR protein expression, suggesting MMR deficiency, were associated with a proximal location in the colon and poor histological differentiation. However, opposing previous publications [4
], we did not observe associations of MMR protein-negative tumors with gender, earlier age at diagnosis of cancer, positive personal and family history of cancer, or earlier tumor stage, when compared to tumors with MMR protein-positive expression.
We acknowledge that our study has several limitations. First, the study was retrospective, which limited the information to that present in the medical record. This limited our ability to accurately evaluate family history of cancer and other factors, like environmental exposure. Second, we were only able to include patients that had tumor blocks available for analysis. However, the percentage of patients excluded from pathological analysis was small and did not statistically differ with regards to basic demographic characteristics from the patients included in this investigation. As such, we believe our results are representative of the referral population seen at our Center. Third, the sample size was relatively small. The sample size may have limited the power of the study to identify important differences according to MMR protein expression status. This is also a pilot retrospective study and a larger prospective study is undergone to verify our findings. Fourth, only MLH1
protein expression were evaluated. However, over 90% [20
] or most [14
] of genetically characterized HNPCC cases are accounted for by germline mutations in any of them.
Additionally, a fifth limitation is that all patients were not subsequently evaluated for MSI to confirm the suggestive findings from immunohistochemistry. However, for screening of HNPCC among patients with CRC, immunohistochemistry is almost equally sensitive as MSI [33
]. Indeed, MSI testing is not available in the majority of routine pathology service laboratories, and these rely on immunohistochemistry to detect loss of MMR protein expression as a surrogate marker for the presence of MSI [105
]. In consequence, immunohistochemistry is a valid tool to identify patients at risk for HNPCC and patients with sporadic microsatellite instable CRC [40
] when evaluated by experienced pathologists [35
] like the authors. Additionally, many consider immunohistochemistry and MSI testing as complementary. In a sequential approach, immunohistochemistry is done first and, if informative, may result in cost savings. In the other hand, if immunohistochemistry is not informative, MSI testing can then be performed [15
]. Nonetheless, this investigation provides insightful information about the frequency and clinical and pathological characteristics associated with MMR protein deficiency in Hispanics from Puerto Rico, which may be ethnically related.
To our knowledge, the present study is the first one to evaluate the frequency of MMR protein-negative expression in unselected Hispanic CRC patients from Puerto Rico using the relatively inexpensive [35
] prescreening method of immunohistochemistry and its association with clinical and pathological characteristics.
In conclusion, this investigation of unselected Hispanic CRC patients from Puerto Rico revealed a low frequency of MMR protein-negative tumors. Similar to other populations, Hispanic MMR protein-negative tumors were proximally located and exhibited poor histological differentiation. The results may be relevant to the evaluation and management of Hispanic patients with CRC from Puerto Rico in immigrant as well as native populations. The relatively low frequency of MMR protein-negative CRC in unselected Hispanic patients from Puerto Rico may be a reflection of, or explain, the lower CRC incidence rate among US Hispanics as compared to non-Hispanic whites and blacks. Our study is significant on shedding light on the scarce literature on MMR protein expression in CRC.in Hispanic populations.