Detailed characterisation of metaplastic breast carcinoma

Abstract

Metaplastic breast carcinoma (MBC) encompasses a heterogeneous group of tumours exemplified by the intermingling of adenocarcinoma with metaplastic elements such as squamous, spindle, chondroid, osseous or rhabdoid component. This rare breast cancer subtype has not been well studied, and the aim of my thesis was to gather immunohistochemical data on one of the largest known cohorts of MBC- the Asia-Pacific MBC consortium. Inferences from this data were then made between the various metaplastic subtypes and distinct clinical outcomes (Chapter 3). Mixed MBC was the most frequently occurring WHO histologic type, with the combination of the carcinoma and squamous morphology commonly found. Co- occurrence of specific histologies was highly statistically significant. Fewer morphological components and EGFR negativity were associated with favourable prognosis.The targeting of immune checkpoint regulators is becoming increasingly important in treating breast cancer and to date, little is understood of the MBC tumour immune microenvironment and for the potential application of these treatments in MBC. The attachment of Program death- 1 (PD-1) to its ligand, PD-L1 leads to the activation of the PD-1 and PD-L1 pathway, resulting in immune evasion and dampening of immune response. Another immune evasion technique utilised by cancer cells is the expression of Forkhead box protein (FOXP3), a transcription factor for regulatory T-cells. To comprehend the relationships that exist between immune cell types embedded within the tumour stroma and tumour, the protein expression of PD-L1 and FOXP3 were evaluated and Tumour infiltrating lymphocytes (TILs) were assessed. No significant association between the amount of lymphocytic infiltrate and WHO types, or the number and type of morphologies were found. WHO Types 5 and 6 presented with the least amount of infiltrate, suggesting a restricted immunogenic background. A broad range of Immunohistochemistry score (IHC) for PD-L1 staining in MBC tumour cells was identified, as were significant relationships between PD-L1 tumour expression and increasing TILs presence. There was a statistically significant association between PD-L1 positive stromal tumour infiltrating lymphocytes (sTILs) expression and the WHO type 4 Squamous phenotype. FOXP3 intra-tumoural infiltrating lymphocytes (iTILs) independently prognosticate MBC.The behaviour of MBCs with respect to the phenotype and distribution of metastatic spread has not been fully defined. Also, there is scarcity of information on the differentiation of the histologic subtypes in primary and matched secondary tumours. Chapter 4 details the investigation of the histomorphologic lymph node profile in MBC; through the assessment of the distant and local recurrence status of MBC tumours; and, the evaluation of metastatic dissemination in autopsy cases. This work identified differences with TNBC tumours in terms of lower incidence of lymph node (LN) metastasis, local recurrence (LR) and distant recurrence (DR).Mutations in Neurofibromin 1 (NF1) are considered to be a driver of breast carcinogenesis, and NF1 mutations were shown to be enriched in MBC. To investigate whether drugs could target this genotype, the effect of selumetinib, a MAP/ERK kinase (MEK) 1/2 inhibitor drug on metaplastic-like and mesenchymal cell lines was evaluated (Chapter 5). The effect of XAV- 939, a WNT inhibitor, to metaplastic-like and mesenchymal cell lines was evaluated following evidence to suggest that WNT signalling could modulate PD-L1 expression. Thus, it was hypothesised that targeting WNT signalling with XAV-939 treatment, would alter PD-L1 expression. The subcellular localisation of β-catenin was used as a marker of the WNT pathway. The data shows that both selumetinib and XAV-939 significantly inhibited the growth of metaplastic-like and mesenchymal cell lines.This study provides data on the immunohistochemical expression of a wide range of markers based on a large number of MBC cases. It also broadens our knowledge on the tumour immune microenvironment, LN and recurrent characteristics present in this rare type of malignancy. The study also provides a baseline for further exploration of drug compounds as therapeutic options in MBC tumours.