Canonical ErbB-2 isoform and ErbB-2 variant c located in the nucleus drive triple negative breast cancer growth

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ErbB-2/HER2, a member of the ErbBs family of receptor tyrosine kinases is a major player in the breast cancer (BC) scenario. ErbB-2 overexpression in the cytoplasmic membrane (MErbB-2) or ERBB2 gene amplification defines an intrinsic BC subtype, namely ErbB-2-enriched (ErbB-2E), for which successful targeted therapies are available [1-3]. 

Compelling findings, including ours, demonstrated that MErbB-2 migrates to the nucleus (NErbB-2) of ErbB-2E BC cells where it binds promoters/enhancers of target genes and functions as a transcription factor (TF) or as a coactivator to promote BC growth, metastasis and resistance to anti-MErbB-2 therapies [4-12]. Furthermore, our clinical studies revealed a role of NErbB-2 as an independent prognostic factor of poor clinical outcome in ErbB-2E tumors [13].

In the course of these clinical studies we unexpectedly found high NErbB-2 expression in tumors which do not express clinically significant levels of receptors for estrogen and progesterone and lack MErbB-2 overexpression/ERBB2 gene amplification, that is, triple negative breast cancers (TNBC). TNBCs are typically high-grade tumors which most frequently develop in young women and are associated with poor prognosis [14]. Gene expression (GE) profile studies in TNBC cohorts revealed that this subtype is indeed a heterogeneous group and defined four different GE profile clusters [15, 16]. Clinical biomarkers and targeted therapies for this disease remain elusive, so chemotherapy has been the standard of care for early and metastatic TNBC [17].

Our serendipitous discovery of NErbB-2 presence in TN tumors generated great interest in the members of our collaborative team composed by basic scientists from our lab and by surgeons, pathologists and oncologists from major hospitals in Argentina and Chile. A young survivor from TNBC, collaborating with us as advocate also most heartily encouraged us to pursue this investigation. We agreed to embrace the challenge and discovered that NErbB-2 correlates with poor prognosis in primary TN tumors, disclosing NErbB-2 as a novel biomarker for TNBC.

Next, our lab team, with the outstanding contributions by María Florencia Chervo, a bright doctoral fellow, sought out to explore the biological significance of NErbB-2 in multiple in vitro and in vivo TNBC models. We found that ErbB-2 is indeed present and functional in the nucleus of human cell lines from all four TNBC subtypes. Our discovery on the presence of an ErbB-2 isoform, distinct from the wild-type (WT) ErbB-2, in TNBC cells led us to study ErbB-2 mRNA alternative splicing events using a PCR-sequencing approach in collaboration with Dr. Petrillo (School of Sciences, University of Buenos Aires), combined with an RNA interference (RNAi) strategy. We found that TNBC cells express either the canonical, WT ErbB-2, encoded by transcript variant 1, or the non-canonical ErbB-2 isoform c, encoded by alternative variant 3, or both. These ErbB-2 isoforms function in the nucleus as transcription factors. Evicting both from the nucleus or silencing isoform c only, blocks TN cell and tumor growth. This reveals not only NErbB-2 canonical and alternative isoforms role as targets of therapy in TNBC, but also isoform c dominant oncogenic potential.

In summary, our discoveries challenge the present scenario of drug development for personalized BC medicine that focuses on wild-type proteins, which conserve the canonical domains and are located in their classical cellular compartments.

Figure 1. Model of nuclear ErbB-2 action governing TNBC growth (taken from Fig. 7 of the paper). TNBC may express at the nucleus either WTErbB-2, encoded by the canonical transcript (T1) or ErbB-2 isoform c, encoded by the alternative ErbB-2 transcript variant 3 (T3), or both. Nuclear ErbB-2 isoforms bind DNA at ErbB-2 response elements (HAS sites) and regulate transcription of Erk5, a kinase mediating nuclear WTErbB-2 and isoform c proliferative effects on TNBC. We propose novel therapeutic strategies: eviction of NErbB-2 in TNBC expressing isoform c and WTErbB-2, or silencing isoform c in TNBC that only displays said isoform.

More information: Dr. Patricia V. Elizalde, Laboratory of Molecular Mechanisms of Carcinogenesis and Molecular Endocrinology, Instituto de Biología y Medicina Experimental IBYME-CONICET. e-mail: patriciaelizalde@ibyme.conicet.gov.ar

 

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Patricia V. Elizalde, PhD

Principal Investigator , Argentina Council of Scientific and Technical Research (CONICET)

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