ROS1 holds the key to cancer treatment?
"A breakthrough study illuminates how the subcellular location of ROS1 fusion proteins can dictate oncogenic signaling, opening new avenues for targeted therapies."
Cancer is often driven by the aberrant hyper-activation of receptor tyrosine kinases (RTKs), which are crucial for cell growth and differentiation. This activation results from various genetic alterations, including gene rearrangements that lead to the formation of fusion proteins. Among these, fusions involving the anaplastic lymphoma kinase (ALK) and ROS proto-oncogene 1 (ROS1) have gained significant attention.
In these fusions, the kinase domain of the RTK is joined in cis to various N-terminal partners. Kinase inhibitors (TKIs) like crizotinib have proven effective in treating cancers driven by these fusions; however, resistance remains a significant challenge, highlighting the need for a deeper understanding of their oncogenic mechanisms.
A crucial aspect of these oncoprotein kinase fusions is how their subcellular localization influences their oncogenic properties. Many oncoprotein fusion kinases, such as ALK and ROS1 variants, acquire subcellular localization signals from their N-terminal partners. This raises the question of whether abnormal subcellular localization significantly contributes to their ability to promote cancer.
Testing and Treatment for ROS1-Positive Lung Cancer
ROS1 rearrangements are detected using complementary diagnostic approaches, each with specific advantages and limitations. The Frontiers review says testing is mandatory at the metastatic stage in non-squamous non-small cell lung cancer (NSCLC) and for non-smokers with squamous-cell carcinoma. Treatment options include ROS1-targeted therapies, care for disease progression and brain metastases, and clinical trials. ROS1 rearrangements occur in approximately 2% of NSCLC cases, and targeted tyrosine kinase inhibitors (TKIs) have been developed to treat this molecular subtype.
From ROS1 Biology to Targeted Treatment
ROS1 fusions can drive cancers with diverse cellular origins in both adults and children, although ROS1 inhibitors are approved only for ROS1 fusion-positive NSCLC, according to the 2020 Nature review. A 2024 review examined ROS1 biology, diagnostic methods, and the effectiveness and clinical applications of ROS1 inhibitors, drawing on articles retrieved from PubMed/MEDLINE and Web of Science. A separate 2025 review describes the discovery of ROS1 as an oncogenic driver and the subsequent development of targeted TKIs; it reports that ROS1 rearrangements occur in approximately 2% of NSCLC cases.
How Does Subcellular Localization Regulate Oncogenic Signaling?
Researchers investigated the differential functional properties of various ROS1 oncoprotein fusions. They engineered a genetically controlled system to express common ROS1 fusion oncoproteins found in patient tumors, including CD74-ROS1, SDC4-ROS1, and SLC34A2-ROS1. A topological analysis indicated that these fusions result in a membrane-anchored, cytoplasmic-facing kinase domain. All three ROS1 fusions showed constitutive kinase activation, as measured by ROS1 phosphorylation.
- SDC4-ROS1 and SLC34A2-ROS1: Fusion oncoproteins localized to endosomes, activating the MAPK pathway.
- CD74-ROS1: Variants localized to the endoplasmic reticulum (ER), showing compromised activation of MAPK.
- FYVE-CD74-ROS1: Re-localization of CD74-ROS1 from the ER to endosomes restored MAPK signaling.
Fusion Partners and Signaling
The cited research describes how the N-terminal fusion partner can affect the subcellular localization of ROS1 fusion oncoproteins, with implications for cell signaling. One report places SCD4-ROS1 and SLC34A2-ROS1 on endosomes, where they activate the MAPK pathway, while CD74-ROS1 variants are found in the endoplasmic reticulum with impaired MAPK activation. A study of a novel NOTCH1–ROS1 fusion in pediatric angiosarcoma contrasts its findings with intracellular ROS1 fusions, which it describes as having dimerization-independent activity.
Evidence Gaps and Treatment Limits
Evidence linking ROS1 fusion partners to response to crizotinib remains limited: one study notes that very few studies have investigated this relationship because ROS1-positive disease is rare. ROS1 rearrangements account for 1%–2% of NSCLC cases, and targeted TKIs have improved outcomes for patients in this molecular subset. Testing for ROS1 positivity can involve tumor tissue or blood, and the American Lung Association lists FISH analysis among the available tests.
Crizotinib Outcomes Across Evidence Sources
A real-world meta-analysis of crizotinib for advanced ROS1-fusion NSCLC reported a pooled objective response rate (ORR) of 70.6% across nine studies, median real-world progression-free survival (rwPFS) of 14.5 months across 11 studies, and overall survival (OS) of 40.2 months across nine studies. In the PROFILE 1001 trial, the ORR was 72% (95% confidence interval [CI], 58%–83%), including six confirmed complete and 32 confirmed partial responses; median duration of response was 24.7 months (95% CI, 15.2–45.3). These figures come from different evidence settings and measure different outcomes, so they should not be treated as directly interchangeable. Targeted therapy is described as preferred first-line treatment for metastatic ROS1-positive NSCLC, with other treatments considered when targeted therapy is unavailable or stops working.
What Does This Mean for Future Cancer Treatments?
These findings demonstrate that the specific N-terminal fusion partners in gene rearrangements involving RTKs can directly control MAPK signaling by influencing subcellular localization. Understanding the subcellular localization of aberrant gene fusion oncoproteins could provide additional avenues for cancer treatment. Targeting these specific localizations may offer more effective and personalized therapeutic strategies, especially in overcoming resistance to existing treatments like crizotinib. Further research into how subcellular localization regulates fusion oncoprotein activity could lead to new biomarkers and therapeutic targets, improving outcomes for cancer patients.
Targeted Inhibitors and Clinical Outcomes
A review of ROS1-rearranged NSCLC reports that these rearrangements account for 1%–2% of cases and that ROS1 TKIs have significantly improved outcomes for affected patients. It identifies crizotinib and entrectinib as first-generation inhibitors and reports objective response rates exceeding 60% for them. The review frames these therapies as an important treatment advance for this molecular subset.
Research Priorities Across Disease Stages
A 2024 review discusses how clinicians select initial and subsequent agents for patients with advanced ROS1-positive lung cancer. It also offers a framework for considering treatment in earlier-stage disease, including non-metastatic cases. The authors highlight areas where further research could support continued progress in outcomes.
Unanswered Questions About Prior Chemotherapy
ROS1 rearrangements occur in approximately 1%–2% of NSCLC and define a molecular subset described as markedly sensitive to TKIs. The cited report notes that ROS1 inhibitors demonstrate high response rates. However, it says the potential effect of prior systemic chemotherapy on subsequent ROS1-TKI efficacy had not been systematically evaluated.
Real-World Evidence Remains Limited
A retrospective study at Beijing Chest Hospital examined medical records for 99 patients with ROS1-rearranged NSCLC diagnosed from February 2017 to January 2026. The study’s background describes ROS1 rearrangement as rare but significant and notes that real-world treatment data across different fusion subtypes remain limited. Another retrospective real-world study set out to compare first-line crizotinib, described as a standard-of-care TKI, with newer ROS1 agents; its report included 49 patients with advanced metastatic ROS1-expressing NSCLC. These studies illustrate ongoing efforts to characterize treatment patterns and outcomes in a rare patient population.