A total of 507 patients with invasive breast cancer from Nigeria and Senegal were included in the study. In the first cohort of 378 patients, the mean age was 44.8 years, and only 12% were older than 60 years. Almost all tumors were larger than 2.0 cm, and more than two thirds of the cases were lymph node positive. More than 80% of the cases were of intermediate and high grade, with the majority being of ductal histology. IHC analysis revealed that 24%, 20%, and 17% of tumors were positive for ER, PR, and HER2, respectively. The majority of tumors were basal-like or unclassified subtypes (27% and 28%, respectively). Only 29% were luminal A or B, and 15% were HER2-positive/ER-negative subtype.
shows the distribution of clinicopathologic characteristics by the five molecular subtypes. Prevalence of these molecular subtypes was independent of study site, age, and tumor size. Lack of association with age may due to narrow range of age distribution in this relatively young cohort. Subtype was highly significantly associated with histologic grade and the four tumor morphometric parameters (all P < .0001). HER2-positive/ER-negative subtype had the highest grade, followed by basal-like. Luminal tumors had the lowest grade, with unclassified tumors in between. The molecular subtypes also differed by histologic type: metaplastic and medullary tumors were in the basal-like, HER2-positive/ER-negative, or unclassified categories, whereas almost all lobular tumors were classified into luminal A and B subtypes. The association between subtype and lymph node metastasis was marginally significant in the crude analysis but not significant after adjusting for histologic grade.
Patient Demographic and Clinical Characteristics by Established Subtypes
Because of concern for antigen degradation of archived specimens, 10 additional IHC markers were examined. We found at least one of the 10 markers was positive for all hormone receptor–negative tumors, and at least five markers were positive for 83% of the hormone receptor–negative tumors. shows the distribution of all these markers was significantly different between molecular subtypes. Specifically, luminal A subtype tumors were less likely to express proliferation markers, such as Ki67, than other subtypes. P53 mutation was more likely to be present among basal-like, HER2-positive/ER-negative, and luminal B subtypes. BCL2 overexpression was observed among luminal A tumors, whereas BCLXL overexpression was observed among basal-like and HER2-positive/ER-negative tumors.
Immunohistologic Chemistry Markers by Established Subtypes
Hierarchical clustering analysis of all 15 IHC markers demonstrated that the established IHC-based subtype classification could be validated among tumors from West Africa, but nearly one third of the tumors were unclassified (). The two large branches probably represent tumors with good and bad prognosis. The dendrogram also suggests that the unclassified tumors can be further divided into two distinct clusters. Interestingly, one cluster was under the “good” prognosis branch, characterized as BCL2 positive and Cyclin D1 positive, and the other cluster was under the “bad” prognosis branch, with a molecular portrait of vascular endothelial growth factor (VEGF) –A positive, VEGF-C positive, BCLXL positive, and Cyclin E positive.
Fig 1 Hierarchical clustering of 378 invasive breast tumors from West Africa using 15 immunohistochemical markers. LumA, luminal A; LumB, luminal B; HER2, human epidermal growth factor receptor 2; ER, estrogen receptor; PR, progesterone receptor; EGFR, epidermal (more ...)
On the basis of the cluster analysis, we categorized the unclassified cases into two subgroups: 86 patients in the VEGF-positive subgroup (VEGF-A positive or VEGF-C positive) and 21 patients in the VEGF-negative subgroup (negative for both VEGF-A and VEGF-C). The VEGF-positive subgroup had significantly worse histologic grade (42% grade 3 and 56% grade 2) than the VEGF-negative subgroup (5% grade 3 and 29% grade 2; P < .0001). The VEGF-positive subgroup also had significantly higher mitotic index, mean nuclear area, and microvessel density and lower fraction of fields with tubular differentiation than the VEGF-negative subgroup (all P < .0001). The two subgroups were similar in tumor size, lymph node status, and histologic type. Interestingly, the VEGF-negative subgroup had better grade, mitotic index, mean nuclear area, and microvessel density than the luminal A subtype (all P ≤ .05), suggesting that these VEGF-negative tumors are less likely to represent ER-positive cases that were false negatives.
To replicate our initial findings, an additional 129 African tumors were studied. The proportion of ER-positive, PR-positive, and HER2-positive tumors was 27%, 17%, and 16%, respectively. In this second cohort, the proportion of the five molecular subtypes was 33% luminal A, 3% luminal B, 23% basal-like, 14% HER2 positive/ER negative, and 27% unclassified, which were quite similar to those in the first cohort.