RRAS2: A Prognostic Biomarker Driving Postpartum Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) accounts of 20% of all breast cancer cases is a highly aggressive subtype of breast cancer characterized by the absence of estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 (HER2). A unique and perplexing aspect of TNBC is its increased incidence among women in the post-partum period. While parity is generally protective against breast cancer in the long term, recent studies have found a paradoxical transient increase in breast cancer risk immediately following childbirth. This post-partum breast cancer which is often of the triple-negative subtype is associated with worse survival outcomes and a higher propensity for metastasis. However, the biological mechanisms driving this increased risk is poorly understood which complicates efforts to develop effective preventive and therapeutic strategies. Even after extensive research, it is still a challenge to find specific molecular drivers in TNBC that could serve as therapeutic targets. Moreover, it has been difficult to find genetic mutations that drives TNBC which led researchers to investigate other potential contributors to TNBC pathogenesis, including alterations in gene expression and signaling pathways that may not involve traditional oncogenic mutations. To this account, RRAS2 which is a member of the RAS-related GTPase family has emerged as a gene of interest. RRAS2 is rarely mutated not like the classical RAS oncogenes (KRAS, NRAS, HRAS), which are frequently mutated in various cancers. However, preliminary evidence suggests that its overexpression may play a significant role in breast cancer development, particularly in TNBC. Given the gaps in understanding the molecular basis of post-partum TNBC and the urgent need for new therapeutic targets, the study aimed to investigate the role of RRAS2 overexpression in breast cancer initiation and progression, as well as its potential as a biomarker for identifying high-risk patients. New study published in Molecular Cancer and conducted by Dr. Claudia Cifuentes, Clara Oeste, Isabel Fernández-Pisonero, Alejandro Hortal, Carmen García-Macías, Jeanne Hochart, Regina Rubira, Lydia Horndler, Carlos Horndler, Xosé Bustelo, and led by Professor Balbino Alarcón from  the Consejo Superior de Investigaciones Científicas de Madrid in Spain explored the hypothesis that unmutated RRAS2, when overexpressed could act as a key oncogenic driver in TNBC especially in cases associated with recent pregnancy.

The authors generated a conditional knock-in mouse model that overexpressed wild-type human RRAS2 specifically in mammary epithelial cells and observed that all female mice overexpressing RRAS2 developed TNBC, but intriguingly this development was dependent on pregnancy. Furthermore, they showed that overexpression of RRAS2 led to the formation of breast ductal adenocarcinomas which highlights RRAS2 as a potent oncogenic driver in mammary tissue under specific physiological conditions, such as those induced by pregnancy.

The team also investigated the underlying mechanisms by which RRAS2 overexpression contributes to tumorigenesis. They conducted histological analyses of the mammary glands from RRAS2-overexpressing mice at various stages of their reproductive cycle and found that during pregnancy there was a marked increase in the proliferation of alveolar buds, however, after weaning, these structures did not regress as they normally would but instead the mammary tissue exhibited sustained hyperproliferation and delayed involution. This abnormal persistence of alveolar buds and the associated hyperproliferation likely create a microenvironment conductive to tumor development. To investigate further the molecular changes associated with RRAS2 overexpression, the authors performed RNA sequencing on tumors from the RRAS2-overexpressing mice and found significant upregulation of pathways involved in cell proliferation, extracellular matrix remodeling, and survival while apoptosis pathways were downregulated. Moreover, the researchers also identified increased activity in pro-oncogenic signaling pathways including the PI3K/Akt/mTOR and Wnt/β-catenin pathways which are known to play critical roles in cancer development and progression.

In parallel with their experimental mouse model studies, the authors performed a cohort of human breast cancer sample analysis and found that RRAS2 was overexpressed in 68% of breast cancer samples with the highest expression observed in TNBC cases and in young women who had recently given birth. Further analysis of the human breast cancer samples showed that a single-nucleotide polymorphism (SNP) in the 3′ untranslated region of the RRAS2 gene known as rs8570 was associated with increased RRAS2 expression. This SNP was more frequently found in breast cancer samples compared to blood from healthy donors. In addition, the authors found a significant number of patients who had the RRAS2 gene amplified, i.e. higher number of copies of the gene, not only in the breast tumors but also in their blood. The blood in breast cancer patients is a non-tumoral tissue. The increased frequency of the SNP allele and the pre-existing amplification of the gene in blood  indicate a potential genetic predisposition to breast cancer associated with RRAS2. According to the authors, the presence of rs8570 SNP together with RRAS2 amplification supported their hypothesis that genetic factors combined with physiological changes during and after pregnancy contribute to the development of TNBC..

In conclusion, Professor Balbino Alarcón and colleagues successfully demonstrated the correlation of the RRAS2 overexpression as a key driver of TNBC post-partum and provided a new molecular target for therapeutic intervention. They suggested that targeting RRAS2 or its downstream signaling pathways such as the PI3K/Akt/mTOR and Wnt/β-catenin pathways could provide new avenues for the development of targeted therapies potentially improve patient outcome. Moreover, the authors highlighted the importance of considering physiological changes associated with pregnancy in breast cancer’s risk and development which could lead to more personalized screening strategies and early interventions for women at risk potentially improving early detection and treatment outcomes. Additionally, the identification of a specific SNP in the RRAS2 gene associated with increased expression suggests a potential genetic biomarker for breast cancer susceptibility that could be used in the future to identify individuals at higher risk specially in populations with a high prevalence of TNBC. In a statement to Medicine Innovates, Professor Alarcon said “We hope that this research will be soon translated into useful biomarkers for risk assessment and therapy guidance“.