German dairy cows expose heterogenous bovine meat and milk factor DNA repertoires in milk and reveal Rep expression in mammary tissue

Abstract Bovine Meat and Milk Factors (BMMFs) are plasmid-like DNA elements isolated from bovine blood and milk. Histological studies demonstrated increased expression of a BMMF-encoded protein (Rep) in colorectal cancer tissues, suggesting BMMFs as potential carcinogenic biomarkers. However, the limited BMMF DNA repertoire identified in cancer patients in the context of the high number of known sequences, and BMMF detection in non-bovine foods challenge the hypothesis that early-life meat and milk consumption might contribute to cancer. Therefore, we investigated BMMFs at individual cow level, with immunohistochemical detection of BMMF-Rep in mammary glands of 27 dairy cows and characterisation of BMMF DNA and Rep in milk from 18 cows via PCR and immunoblotting. BMMF Rep expression was detected in lactocytes or interstitial tissue in 59% of animals, albeit with limited spatial distribution. BMMF DNA was present in 16 of 18 individual milk samples, with 26 sequences identified comprising both animal-specific and shared BMMF repertoires. However, only one isolate showed 99% identity with a cancer-identified isolate. Three milk samples were BMMF Rep-positive. This study provides a first hint that bovine milk, although frequently containing BMMF DNA, might not be a major source of protein and DNA of BMMF types found in cancer patients.