Circulating MicroRNAs do not provide a diagnostic benefit over tissue biopsy in patients with brain metastases.

Brain metastases (BMs) are frequent and devastating complications of systemic malignancies, necessitating accurate diagnosis and origin identification for effective treatment strategies. Invasive biopsies are currently required for definitive diagnosis, highlighting the need for less invasive diagnostic approaches and robust biomarkers. Circulating microRNAs (miRNAs) have demonstrated potential as sensitive and specific diagnostic biomarkers in various cancers. Thus, our objective was to identify and compare miRNA profiles in BM tissue, cerebrospinal fluid (CSF), and plasma, with a specific focus on liquid biopsies for diagnostic purposes. Total RNA enriched for miRNAs was isolated from histopathologically confirmed BM tissues (n = 30), corresponding plasma samples (n = 30), and CSF samples (n = 27) obtained from patients with diverse BM types. Small RNA sequencing was employed for miRNA expression profiling. Significantly differentially expressed miRNAs were observed in BM tissues, enabling the differentiation of primary origins, particularly breast, colorectal, renal cell carcinoma, and melanoma metastases. The heterogeneity observed in lung carcinomas also manifested in the corresponding BMs, posing challenges in accurate discrimination from other BMs. While tissue-specific miRNA signatures exhibited the highest precision, our findings suggest low diagnostic potential of circulating miRNAs in CSF and blood plasma for BM patients. Our study represents the first analysis of miRNA expression/levels in a unique set of three biological materials (tissue, blood plasma, CSF) obtained from the same BM patients using small RNA sequencing. The presented results underscore the importance of investigating aberrant miRNA expression/levels in BMs and highlight the low diagnostic utility of circulating miRNAs in patients with BMs.