Glutamate metabotropic receptor 4 in breast cancer: a potential and specific target for chimeric antigen receptor therapy.

BACKGROUND

Despite the revolutionary success of chimeric antigen receptor (CAR) therapy in hematologic malignancies, its application in solid tumors is hindered by the scarcity of tumor-specific membrane antigens rigorously validated in clinical specimens. Here, we identified glutamate metabotropic receptor 4 (GRM4) as a novel target with dual advantages: breast cancer (BC)-predominant membrane expression and restricted normal tissue distribution, potentially circumventing on-target off-tumor toxicity.

METHODS

Through integrative multi-database analysis (DESeq2/edgeR/limma differential screening, CellMarker filtration, the Human Protein Atlas database validation), GRM4 was prioritized. Its expression was validated in non-malignant organs [immunohistochemistry (IHC)], BC cell lines [western blot (WB)/quantitative polymerase chain reaction (qPCR)/immunofluorescence (IF)], 158 BC clinical samples with paired para-cancerous tissues (IHC). Subcellular localization, tumor proportion score, and subtype-specific distribution were analyzed. Clinical correlations and survival outcomes were evaluated using chi-square tests and Kaplan-Meier analysis.

RESULTS

Membrane expression was confirmed by IHC in 35.44% of clinical cases, and its presence in breast cancer cell lines was validated by WB, qPCR, and IF. GRM4 exhibited tumor-specific membrane/cytoplasmic expression in 80.38% of BC patients (127/158) across all subtypes (≥70% positivity), with 51.27% showing >50% tumor cell positivity. Critically, GRM4 was absent in normal breast/para-cancerous tissues and confined to the brain in non-malignant organs. While GRM4 correlated with advanced clinical stage (p=0.025) and age (p=0.026), it was independent of overall survival (p=0.449).

CONCLUSIONS

GRM4 emerges as a novel CAR-associated target for breast cancer, demonstrating tumor-specific overexpression, brain-restricted normal expression, and pan-subtype applicability with potential on-target off-tumor effect.