miRNA in native and processed cow's milk and its implication for the "farm milk effect" on asthma. Cow's milk miRNA and asthma

Abstract Background Children regularly consuming unprocessed cow’s milk directly obtained from a farm have a lower risk of asthma than children consuming industrially processed milk. Cow’s milk contains microRNA (miRNA), a molecule class with immunoregulatory properties, which has been implied in the development of asthma. Aims and objectives To test whether industrial processing of cow’s milk alters its miRNA contents and composition and whether affected miRNA species target asthma genes. Methods Three batches of milk from three small farms were subjected to different industrial procedures resulting in 24 different milk samples. RNA was extracted and sequenced for small reads by next genome sequencing, and sequences were verified by RT-qPCR for selected miRNAs. Experimentally validated targets of differentially expressed miRNA species were extracted from the mirTarbase v4.0 database focusing on 139 candidate genes for asthma, atopy, and respiratory infections. Results miRNA sequences were reduced in all high-heat treated milk samples (boiled, ultra-high heat treated, and heat treated for extended shelf-life) but did not differ between farms. Expression levels of members of the let-7 family and other miRNA species were strongly affected by heat treatment and fat contents, e.g. log 2-fold change for let-7a-5p: 4.01 and 1.54, respectively. High-heat treatment affected predominantly miRNAs interfering with the genes STAT3, PTGS2, IL-6, IL-13, IFNG, and IFNGR1. Also skimming reduced the expression of miRNAs interfering with STAT3. Conclusion Industrial processing of milk, particularly high-heat treatment, reduces miRNA species interfering with asthma genes. This mechanism might be involved in the beneficial effect of unprocessed milk on asthma.