Researchers Identify Three Subtypes of Mismatch Repair Deficient High-Grade Gliomas

Researchers have identified three distinct subtypes of mismatch repair deficient (MMRD) high-grade gliomas, providing crucial insights into tumor development and varying patient responses to immunotherapy. These findings are informing therapeutic approaches and have prompted proposals for reclassifying these aggressive brain tumors.

The study, led by Dr. Uri Tabori and his team at The Hospital for Sick Children (SickKids), was recently published in Nature Genetics.

Understanding High-Grade Gliomas

High-grade gliomas are aggressive brain tumors, frequently impacting children and young adults. In certain instances, these tumors are driven by mismatch repair deficiency (MMRD), a condition that leads to hypermutation—a rapid accumulation of genetic mutations within tumor cells—and resistance to standard treatments such as chemotherapy and radiation. Tumors with MMRD are referred to as primary mismatch repair deficient high-grade gliomas (priMMRD-HGG).

Due to the exceptionally high number of mutations observed in priMMRD-HGG, treatment strategies have increasingly shifted towards immunotherapy. This approach aims to leverage the body's own immune system to target and eliminate cancer cells.

Despite improved survival rates with immunotherapy, clinicians had noted significant variations in treatment responses, along with differences in patient age of onset and imaging results, which prompted this detailed investigation.

Key Research Findings

The research team meticulously analyzed genomic and clinical data from priMMRD-HGG cases to unravel the mechanisms behind these observed variations.

The study revealed three distinct molecular pathways that correlated with the clinical observations, forming a basis for developing more targeted therapies and exploring potential future vaccine strategies.

The team successfully classified 162 priMMRD-HGG cases from 152 patients, utilizing extensive data from the International Replication Repair Deficiency Consortium. These cases were grouped into three distinct subtypes:

This subtype represents the most common form, accounting for 62% of the studied tumors. It is uniquely characterized by the presence of both MMRD mutations and polymerase proofreading deficiency (PPD).

These tumors have shown high sensitivity to immunotherapy. A pioneering clinical trial, U-R-Immune Glioma, is currently exploring an immunotherapy-first approach for these patients, which may allow for sparing initial radiation therapy and its associated side effects.

Comprising 19% of the gliomas examined, this subtype features MMRD mutations but crucially lacks PPD or IDH1 gene alterations.

Single-agent immunotherapy has been observed to be less effective for this group, while dual-agent or combination therapies show significant promise. The OPTIMISE trial is an adaptive trial specifically designed to target these particular genetic variations.

This subtype accounts for the remaining 19% of gliomas and is defined by the presence of both MMRD mutations and an IDH1 gene variation.

Tumors in this group typically exhibit poor responses to immunotherapy when administered alone. Researchers are actively working towards a clinical trial that would combine targeted immunotherapies with an IDH1 inhibitor specifically for these patients, aiming to overcome their immune-cold nature.

Implications and Future Directions

Beyond immediately advancing patient care, these significant findings have led the research team to propose a reclassification of these tumors by the World Health Organization (WHO).

Specifically, they suggest reclassifying priMMRD-3 as a distinct subtype of astrocytoma, and priMMRD-1 and priMMRD-2 as specific subtypes of pediatric high-grade glioma.

This proposed reclassification aims to more accurately reflect their unique molecular and clinical behavior, potentially facilitating future research and fostering international collaborations to tackle these challenging cancers.

Additionally, the research has initiated groundbreaking investigations into a potential vaccine strategy known as "immune interception." The study found that while these tumors exhibit high mutation rates, the mutations are not random and share commonalities across patients. These shared mutations could potentially be intercepted to prevent tumor progression earlier in the disease course. This work serves as the foundational pillar for a new immune cancer interception program at SickKids, aiming for proactive and tailored treatments.