By Lina carolina 
Breast cancers can be classified into subgroups that hint at the aggressiveness of the cancer and the likelihood that the patient will experience a recurrence years after their initial diagnosis. Now, researchers at Stanford Medicine have revealed that these complex subgroups can be distilled into three overarching categories based on fundamental structural variations within their DNA. This groundbreaking classification, published in the prestigious journal Nature, centers on the amplification of cancer-driving genes known as oncogenes and the presence of tiny, independent circles of DNA called extrachromosomal DNA (ecDNA). These genomic anomalies are established early in the cancer’s development and persist as the disease progresses and spreads.
The implications of this discovery are profound. By understanding the role these structural variations play in tumor evolution, scientists believe they can develop more precise diagnostic tools and novel targeted therapies. This refined classification system promises to empower physicians to better identify patients who will benefit most from aggressive early interventions, while also allowing others to safely defer more intensive treatments.
"My lab has had a long-standing interest in understanding how aggressive breast tumors arise, why they are resistant to therapy and why they are prone to recur in distant organs," stated Dr. Christina Curtis, the RZ Cao Professor and a professor of oncology, genetics, and biomedical data science at Stanford Medicine, and the senior author of the study. "This research shows that breast tumors develop key structural variants that set the tumor on its course very early in its development. In short, some are born to be bad. It emphasizes the importance of robust biomarkers and of intervening early in the course of the disease."
The research team, led by postdoctoral scholars Dr. Kathleen Houlahan and Dr. Lise Mangiante, former research assistant Cristina Sotomayor-Vivas, and graduate student Alvina Adimoelja, meticulously analyzed the genomic architecture of nearly 2,000 breast cancers across various stages of development. Their findings offer a paradigm shift in how breast cancer is understood and potentially treated.
Unraveling the Genomic Architecture of Breast Cancer
For decades, breast cancer classification has primarily relied on the presence or absence of specific protein receptors on cancer cells. The three main subtypes traditionally identified are:
- Hormone-Receptor Positive (HR+) Tumors: These cancers express receptors for estrogen or progesterone. Therapies for HR+ breast cancer aim to reduce estrogen production, block its binding to receptors, or degrade these receptors. The majority of HR+ cancers are estrogen-receptor positive (ER+).
- HER-2 Positive Tumors: These cancers overexpress the HER-2 protein receptor. While aggressive, they can be effectively treated with targeted therapies that inhibit HER-2 activity. These account for approximately 15% to 20% of all breast cancer cases.
- Triple-Negative Breast Cancers (TNBC): These tumors lack all three of the aforementioned receptors (estrogen receptor, progesterone receptor, and HER-2). TNBC constitutes about 10% of newly diagnosed cases and is often considered the most challenging to treat, with a tendency for earlier recurrence.
While this receptor-based classification has guided treatment decisions, it has not always fully captured the underlying biological complexity or accurately predicted long-term outcomes.
A Decade of Discovery: From Subgroups to Structural Variants
Dr. Curtis’s research journey began over a decade ago with a focus on the evolutionary trajectory of breast cancer. In 2012, her team employed machine-learning techniques to compare the DNA and RNA sequences of patients’ healthy cells with those of their tumors. This innovative approach provided a detailed molecular profile, highlighting genetic alterations and their impact on gene expression. This study identified 11 clinically significant subgroups, a significant expansion beyond the existing receptor-based classification, each with varied prognoses. However, the precise clinical utility of these numerous subgroups remained to be fully elucidated.
A subsequent large-scale study involving 75,000 individuals with ER+ breast cancer revealed a disquieting reality: even after five years of hormone therapy, and within the lowest-risk clinical groups, breast cancer recurrences continued to occur. This prompted Dr. Curtis and her colleagues to investigate the underlying reasons and explore whether their previously defined subgroups could offer a more nuanced risk stratification.
By 2019, their research demonstrated a powerful correlation: when receptor status was layered with their subgroup classification, it could predict which HR+ tumors were prone to recur long after initial diagnosis and treatment. Notably, four of the eight ER+ subgroups exhibited a significantly higher likelihood of recurrence, even a decade or two post-diagnosis. This revealed that approximately one-quarter of women with HR+, HER-2-negative breast cancer faced a nearly 50% chance of recurrence decades later. This elevated risk profile even surpassed that of some triple-negative breast cancers and mirrored the pre-trastuzumab (Herceptin) era for HER2-positive cancers, before the advent of targeted therapies dramatically improved outcomes.
Furthermore, this approach also proved valuable for triple-negative breast cancers, identifying patients unlikely to experience recurrence beyond five years and those at higher risk. This patient stratification is crucial for determining the necessity of aggressive early treatment or intensified long-term monitoring, while also enabling others to potentially avoid harsher treatment regimens.
However, a critical question remained: what were the fundamental genomic drivers behind these observed differences among the subgroups? "We wanted to take a step back," Dr. Curtis explained. "Each of the four higher risk subgroups has copy number events — duplications or amplifications of specific oncogenes involving different regions of the genome. These patterns of genomic copy number change were similar to that seen in HER2-positive disease. If we look at these tumors in an unbiased way and deconstruct these different types of mutations, what could we learn about their processes that give rise to these characteristic events? Would we discover something different?"
The Three Pillars of Genomic Variation
The Stanford team’s latest research focused on the genomic architecture – the intricate patterns of mutations and structural variations within a cancer cell’s DNA. By examining nearly 2,000 breast cancers, ranging from early-stage ductal carcinoma in situ (DCIS) to advanced metastatic disease, they successfully categorized these diverse tumors into three distinct groups based on their genomic peculiarities.
Group 1: Localized Amplifications and Extrachromosomal DNA
This group strongly encompasses the high-risk HR+ subgroups and the HER-2 positive subgroup. These tumors are characterized by complex but localized amplifications of oncogenes, coupled with the presence of extrachromosomal DNA (ecDNA). These ecDNAs are small, circular DNA molecules that exist independently of the main chromosomes and are often enriched with oncogenes. Recent studies have increasingly implicated ecDNAs as key drivers of cancer growth and evolution, as they frequently bypass normal cellular regulatory mechanisms.
"Here we have two different molecular subtypes, which we treat differently in the clinic but that strongly overlap in their patterns of chromosomal instability," Dr. Curtis noted. This finding suggests a shared underlying genomic vulnerability despite differing receptor expression. The presence of amplified oncogenes and ecDNA points to a mechanism of rapid and aggressive gene dosage gain, fueling uncontrolled cell proliferation.
Group 2: Global Genomic Instability
Triple-negative breast cancers, particularly those with a propensity for recurrence, often fall into this category. These tumors exhibit a more widespread genomic instability, accumulating alterations across the entire genome. A subset within this group also displays deficiencies in their ability to repair DNA damage, further contributing to their chaotic genetic landscape. "The whole genome shows scars," Dr. Curtis described. "It’s not limited to particular oncogenes." This global instability suggests a more fundamental breakdown in DNA maintenance processes, leading to a broader range of mutations.
Group 3: Genomic Stability
In stark contrast, the "garden-variety" HR+, HER-2-negative breast cancers, which typically carry lower risks of recurrence, possess relatively stable genomes. These tumors show fewer structural variations and a more ordered genetic structure, aligning with their generally more favorable prognosis.
Early Origins, Enduring Impact
A crucial aspect of this research is the finding that these defining structural variations are established very early in tumor development, often preceding observable clinical symptoms. These foundational genomic alterations are then maintained as the tumors grow, invade surrounding tissues, and metastasize to distant organs. This persistence underscores the importance of these early events in dictating the tumor’s ultimate behavior and its response to treatment.
Furthermore, the researchers observed correlations between these genomic categories and the tumor’s microenvironment, specifically how immune cells infiltrate and interact with the tumor. This suggests that the underlying genomic architecture can influence the body’s own defense mechanisms against cancer.
Towards Precision Medicine: New Avenues for Therapy
The identification of these distinct genomic patterns opens up exciting new avenues for targeted therapeutic interventions. The researchers propose several potential strategies based on their findings:
- Targeting DNA Repair Deficiencies: Approximately 13% of patients with DNA repair-deficient ER+ breast cancers could potentially benefit from existing drugs designed for inherited breast cancers associated with BRCA1 and BRCA2 mutations. This highlights the power of identifying shared vulnerabilities across different breast cancer subtypes.
- Exploiting Focal Amplifications and ecDNA Vulnerabilities: Tumors that rely heavily on amplified oncogenes and ecDNA may be susceptible to novel compounds that specifically target these drivers or the resulting replication stress. This could involve drugs that inhibit the replication of ecDNA or disrupt the mechanisms by which amplified oncogenes promote cell division.
- Intervening in Mutational Processes: Other therapeutic approaches might focus on directly targeting the mutational processes that propagate these early, often "catastrophic" genomic events. Understanding the specific pathways involved in generating these alterations could lead to the development of drugs that halt or reverse their progression.
"These early, sometimes catastrophic mutational events happen decades prior to the diagnosis of the tumor, emphasizing opportunities for earlier interventions," Dr. Curtis emphasized. "Despite the complexity of their genomes, there are constraints and only so many evolutionary paths for a tumor to follow. We now have an understanding of how and when these complex alterations arise and their accompanying vulnerabilities."
Broader Implications and Future Directions
The Stanford study’s findings have far-reaching implications for the clinical management of breast cancer. By moving beyond receptor status to a deeper understanding of genomic architecture, clinicians can gain a more accurate picture of a patient’s prognosis and tailor treatment strategies with greater precision. This could lead to:
- Improved Risk Stratification: More accurate identification of high-risk patients who require aggressive upfront treatment and intensive follow-up.
- De-escalation of Treatment: The ability to identify lower-risk patients who can safely avoid the toxicity and side effects associated with certain aggressive therapies, such as intensive chemotherapy or extended endocrine therapy.
- Development of Novel Therapies: The research provides a roadmap for the development of new drugs that target specific genomic vulnerabilities identified in each of the three main categories.
- Early Intervention Strategies: The understanding that key genomic alterations are established early suggests potential for earlier diagnostic screening and preventative measures, though this remains a long-term goal.
The research was supported by grants from the National Institutes of Health (CA261719 and CA252457) and the Breast Cancer Research Foundation, underscoring the significant investment in understanding and combating this pervasive disease. Dr. Curtis, a member of Bio-X, the Stanford Cancer Institute, and a Chan Zuckerberg Biohub investigator, continues to lead efforts at the forefront of cancer genomics and artificial intelligence. This latest discovery marks a significant leap forward in the quest to decipher the intricate biology of breast cancer and translate that knowledge into tangible benefits for patients.
