Advances in her2-expressive breast cancer research

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Focus on "HER2 low expression" breast cancer

Human epidermal growth factor receptor 2 (HER2) is a receptor tyrosine kinase encoded by the ERBB2 gene. HER2-positive breast cancer caused by ERBB2 gene amplification is highly aggressive and has a poor prognosis, accounting for about 15% to 20% of all invasive breast cancers. With the continuous emergence and wide application of anti-HER2 drugs such as trastuzumab and patuzumab, the prognosis of HER2-positive breast cancer patients has improved significantly. In recent years, TKI such as lapatinib and antibody-conjugated drugs (ADCs) such as T-DMI have also been used in the treatment of PATIENTS WITH HER2-positive breast cancer.

Accurate detection and assessment of HER2 status is essential to determine the population benefiting from anti-HER2 therapy. After the 2018 American Society of Clinical Oncology (ASCO)/American Society of Pathologists (CAP) updated guidelines for the detection of breast cancer HER2, the "HER2 low expression" breast cancer subtype is receiving increasing attention due to the efficacy shown by emerging drugs. In addition, topics related to HER2 detection and treatment, including HER2 heterogeneity and HER2 mutations, are also very active areas. Recently, Melissa Krystel-Whittemore et al. published in the journal Diabetic Histopathology introduced the research progress of HER2 expression-related breast cancer in the definition, detection and treatment of "HER2 low expression" breast cancer, ERBB2 mutation, her2 intratumor heterogeneity and other aspects. The following is a brief introduction to the content for everyone to exchange and learn.

What is "HER2 Low Expression" Breast Cancer?

In general, guidelines define breast cancer with a HER2 IHC score of 0, 1+, or 2+ and a FISH test negative as HER2 negative. In recent years, however, a new definition has been defined, defining HER2 IHC1+ or HER2 IHC2+/ISH – two types of breast cancer as "HER2 low expression" breast cancer, which accounts for 45%-55% of all invasive breast cancers. Recent studies have shown that this type of breast cancer can benefit from antibody-conjugated drug (ADCs) therapy that targets HER2.

IHC is the most common method of HER2 detection. HER2 IHC 0 and 1+ are often combined to be HER2 negative. With the introduction of the concept of "HER2 low expression" breast cancer, it is particularly important to distinguish between HER2 IHC 1+ and 0. There are many factors that can affect the accurate diagnosis of IHC staining scores. For example, HER2 IHC is sensitive to cold ischemia timing and formalin fixation time, and all specimen fixation factors can lead to a HER2 IHC score of 0 instead of 1+, false-negative.

The biological mechanism of HER2 low expression has not been fully elucidated. Existing theories suggest that the undiminished LOW EXPRESSION OF HER2 of the ERBB2 gene may be mediated by the cross-signaling pathway of hormones and EGFR pathways. The expression of the HER2 protein may also be a mechanism of resistance reflected in endocrine therapy. In addition, chemotherapy and radiation therapy have also been shown to enhance the expression of HERB by activating the NFkB pathway to increase the expression of ERBB2.

Recently, Schettini et al. compared the clinicopathological features of HER2-negative (IHC0) and HER2-low-expression (IHC1+ or IHC2+/ISH-) tumors in 3,689 patients in 13 databases. Studies have shown that patients with "HER2 low expression" breast cancer are older, have larger tumor volumes, and higher rates of lymph node metastasis than HER2-negative breast cancers. Another study by Denkert et al. found that the Ki67 proliferation index of "HER2 low expression" breast cancer was lower than that of HER2-negative breast cancer.

In addition, Schettini and colleagues evaluated gene expression data from 1320 patients and found that 34 out of 55 genes had different expressions between "HER2 low expression" breast cancer and HER2-negative breast cancer. Among the genes associated with ERBB2 and Luminal, the ESR1, FOXA1, AR, and BCL2 genes were significantly upregulated in "HER2 low expression" breast cancer compared with HER2-negative breast cancer. Among the cell proliferation-related genes, MKI67, CCNB1, MYBL2, and CCNE1 were relatively reduced in "HER2 low expression" breast cancer. Interestingly, when stratified in hormone receptor states, the upregulation of these Luminal-related genes and the downregulation of proliferation-related genes can be observed in HR-positive HER2-low-expression breast cancers compared to HER20 breast cancers, but there is no such difference in triple-negative HER2-low-expression breast cancers. Moreover, erbb2 gene expression in HR-positive HER2-low-expression breast cancer was higher than in triple-negative HER2-low-expression breast cancer.

It is important to note that "HER2 low expression" breast cancers are not generally classified as HER2-enriched subtypes, but their ERBB2 expression is indeed higher than that of HER2-negative breast cancers. This highlights an important difference that may exist between true HER2-positive breast cancer and "HER2 low-expression" breast cancer, as the underlying tumor phenotype is not defined by the expression of one gene, but by the expression or regulation of multiple genes. For example, Denkert and colleagues in the GeparSepto study of 556 breast cancer patients NGS sequencing analysis found that compared with HER2-negative breast cancer, "HER2 low expression" breast cancer piK3CA and TP53 mutations were significantly reduced, while analyzing the data in the GeparOcto study found that BRCA1/2 mutations were more frequent in HER2-negative breast cancer patients than "HER2 low expression" breast cancer.

The current results are inconsistent as to whether there is a difference in prognosis between patients with HER2-negative breast cancer and patients with "HER2 low expression" breast cancer. Schettini et al. found no difference in overall survival between "HER2 low expression" breast cancer patients and HER2-negative breast cancer patients. The results from Denkert et al. show that the pathological complete response rate for "HER2 low expression" breast cancer is lower than that of HER2-negative breast cancer. Denkert et al. studies also show that the disease-free survival and overall survival of patients with "HER2 low expression" breast cancer are significantly higher than those of HER2-negative breast cancer. However, there was no difference in disease-free survival and overall survival between patients with HER2 IHC 1+ and HER2 IHC 2+ breast cancer.

What are the treatment options for "HER2 low expression" breast cancer?

In a phase III clinical trial called NSABP-B47, patients with "HER2 low expression" breast cancer were not effective in combining adjuvant therapy with trastuzumab on the basis of chemotherapy. However, some novel HER2-targeting therapies have significant efficacy in "HER2-low-expression" breast cancer.

The first is to target the HER2 drug ADC. ADCs are made up of monoclonal antibodies, connectors, and small molecule cytotoxic drugs that can strike cancer cells like missiles. T-DM1 is an ADC consisting of trastuzumab and DM1, but is not effective in patients with "HER2 low expression" breast cancer. Recent studies have shown that next-generation ADC drugs have better efficacy in "HER2 low expression" breast cancer. The results of related studies show that the new generation of ADC drugs releases the drug carrier through the herr2-expressed tumor cells entering the cell, and the drug carrier plays a role in killing the tumor cells that do not express HER2 (i.e., the bystander effect) due to the permeable cell membrane entering the surrounding cells, and these mechanisms do not directly inhibit the signaling of the HER2 receptor. Several studies are underway in a phase III, randomized, multicenter trial of the efficacy of next-generation ADC drugs in patients with "HER2 low expression" breast cancer.

In addition, since the advent of trastuzumab, anti-HER2 drugs have become an important means of breast cancer treatment. However, due to the limited efficacy of trastuzumab against "HER2 low expression" breast cancer, novel anti-HER2 monoclonal antibodies, such as margetuximab, are being studied for this population. Although the results of the Phase II trial of margetuximab have not yet been published, the current data have shown initial efficacy. Bispecific antibodies are monoclonal antibodies against two different epitopes. At present, research on the bispecific antibody ZW25 is underway, and although in vivo studies have shown it to have antitumor activity in "HER2 low expression" breast cancer, the current study is still in phase I.

In addition, vaccines against the HER2 receptor are also being studied, particularly nelipepimut-S. Phase I and Phase II clinical trials have shown that patients with any level of HER2 expression (IHC1+, 2+, or 3+) develop immunity to HER2 after vaccination. While phase I and phase II trials did show the benefits of HER2 vaccination, subsequent phase III trials did not directly demonstrate the clinical effect of vaccination in disease-free survival, overall survival, and asymptomatic recurrence survival in patients with "HER2 hypoexpression" breast cancer.

ERBB2 mutation

Although most of the alterations in ERBB2 in breast cancer are gene amplifications, there may also be missense mutations in ERBB2. These mutations are most common in exons 19-20, which encode the tyrosine kinase domain, and the other most common mutation is in the extracellular domain, encoded by exon 8. PIK3CA mutations are the most common genetic mutations that occur with ERBB2 mutations, with CDH1 mutations being the second most common. Simultaneous ERBB2 and CDH1 mutations are most common in invasive lobular carcinoma. When looking at HER2 non-amplified breast cancer, breast cancers with ERBB2 mutations have worse recurrence-free survival rates than breast cancers without these mutations. Studies have shown that tyrosine kinase inhibitors such as neratinib are most active in patients carrying erbb2 tyrosine kinase domain mutations. Other new tyrosine kinase inhibitors, such as poziotinib and pyrrolotinib, are also conducting related clinical trials.

ERBB2 mutations can also be seen as a mechanism of resistance to anti-HER2 therapy, particularly ERBB2 L755S mutations. In vitro models of cell lines have shown that dual HER1/2 kinase inhibitors can be used to reverse drug resistance in this condition. In addition, ERBB2 mutations are also seen as a resistant mechanism for endocrine therapy in patients with ER+/HER2 non-amplified invasive breast cancer treated with fulvestrant or aromatase inhibitors.

What does intratumor heterogeneity of HER2 staining mean?

According to the CAP guidelines, HER2 heterogeneity is that the level of ERBB2 gene amplification and HER2 protein expression varies within the tumor, while the proportion of cells expanded in the intramutum with HER2 is ≥5% and <50%. There are usually three modes of intratumoral heterogeneity of HER2 in breast cancer: the first: the "polytype", which refers to two different tumor clones within the tumor, the two clones are separate, one clone of HER2 is amplified and the other HER2 is not amplified; the second: "mosaic" type, showing tumor cells with different HER2 expression states mixed together; the third, "dispersed", refers to the her2-negative cell population, there are isolated HER2 amplification cells.

Invasive breast cancers with HER2 heterogeneity but no overall amplification detected by FISH often have a larger tumor volume, higher histological grade, and frequency of lymph node metastasis. In addition, patients with her2 heterogeneity were shorter in disease-free and overall survival, and responded poorly to HER2-targeted therapy. In addition, a study by Filho et al. showed that tumors with HER2 heterogeneity but no pathological complete remission (pCR) had more ERBB2 non-amplified cells than tumors that acquired pCR. In neoadjuvant therapy for breast cancer with HER2 heterogeneity, the combination therapy with T-DM1 and paltozumab is less effective than chemotherapy plus HER2 monoclonal antibody dual-target therapy. This suggests that the therapeutic effect of T-DM1 on HER2 heterogeneous breast cancer may not be strong enough, and chemotherapy may be necessary to obtain a comprehensive therapeutic response.

Therefore, it is important to report the percentage of HER2 amplified cells in the tumor in pathological testing to guide the selection of the right treatment. Retesting is also necessary when HER2 heterogeneous tumors metastasize or disease progression, as HER2-amplified clones may become the dominant clones, potentially effective for traditional anti-HER2 therapies.

Uncommon HER2 staining pattern

Mammary invasive micropammar carcinoma (IMPC) is a special type of mammary invasive carcinoma that exhibits a unique morphology in which tumor cell populations are arranged in vascular-free communities surrounded by a matrix, and tissue retraction is visible around epithelial tumor cells. The morphology of IMPC can affect the staining of HER2 IHC. IMPC's HER2 staining pattern often shows as cup-shaped or U-shaped basal staining, which can lead to "incomplete" membrane staining of HER2. In 2013, asCO HER2 detection guidelines were updated to state that IMPC with moderate to strong but incomplete membrane staining in 10% of tumor cells > should be classified as HER2 IHC2+, not IHC1+.

In a recent study by Perron et al., the authors found that the most common HER2 IHC staining pattern in IMPCs was the basal surface type. In addition, IMPCs with moderate to strong staining but incomplete membrane staining are classified as HER2 IHC2+ according to the latest IMPC HER2 staining guidelines. This highlights the importance of identifying basal staining patterns in IMPC, otherwise these cases would have been classified as IHC 1+ (negative) and HER2 amplification would have been missed on FISH testing.

Conclusion:

"HER2 low expression" breast cancer is a distinct subtype of breast cancer, although in the past it was considered HER2-negative breast cancer. There is evidence that the "HER2 low expression" subtype is specific in terms of molecular basis as well as response to treatment compared to HER2-negative breast cancer, for which we should try novel treatments rather than the standard anti-HER2 drug (trastuzumab) for the treatment of HER2-amplified tumors. In addition, for breast cancer with ERBB2 mutations, tyrosine kinase inhibitors have shown some efficacy, and ERBB2 mutations are also seen as a resistant mechanism for anti-HER2 drug therapy. Tumors with HER2 heterogeneity, which may be driven by chromosomal instability, do not respond to standard treatment regimens as well as HER2 amplified tumors, and have worse overall survival. Finally, it is also important to recognize the effect of tumor histology on HER2 staining, as observed in IMPC. Taken together, the findings underscore the importance of ongoing and new research explorations that contribute to understanding the biology, treatment, and prognosis of HER2-associated breast cancer.

Bibliography:

[1] Diagnostic Histopathology Volume 28,Issue 3,March 2022,Pages 170-175.

*This article is supported by AstraZeneca and is intended for medical professionals only

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Valid until: 2023-4-24

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