German Cows Show Distinct BMMF DNA Profiles in Milk
Researchers examining milk from individual dairy cows found that the animals carried different collections of small circular DNA elements known as Bovine Meat and Milk Factors, or BMMFs. Of 18 milk samples, 16 contained BMMFs, and the analysis recovered 26 sequences. Six cows had repertoires with no overlap with those found in the other animals, while ten of the distinct types appeared in only one cow.
After sequences with genetic similarities were grouped, 11 were classified as newly isolated elements and others matched sequences described previously. The researchers said the findings showed greater variation than studies based on commercial milk can reveal, because commercial products generally combine milk from many animals.
Small circular DNA elements
BMMFs are described as plasmid-like DNA elements, usually circular, that can encode Rep, a protein involved in replication. Some of their characteristics resemble structures found in bacteria. Research groups associated with the German Cancer Research Center, or DKFZ, have studied the elements since the previous decade to examine their origins, natural reservoirs and behaviour in cells and tissues.
The study also examined mammary-gland tissue from 27 dairy cows. Using antibodies against Rep, the researchers detected signals in milk-producing cells or nearby tissue in about 59% of the animals. The distribution was limited and localised. In milk itself, relevant Rep signals were found in only three of the 18 samples in an immunoblot using one of the antibodies tested.
Findings related to cancer research
Earlier studies had identified related proteins and sequences in tissues near human tumours, particularly in research on colorectal cancer. In the new study, however, only one sequence from the individual milk samples showed roughly 99% identity with a BMMF isolate previously found in data connected with human cancer. Most of the elements detected in the cows were genetically different from those reported in human-tissue studies.
The researchers did not conclude that milk was an important source of health risk, and the study did not confirm or rule out a role for BMMFs in carcinogenesis. A joint assessment by Germany's Federal Institute for Risk Assessment and the Max Rubner-Institut said that, based on the available evidence, adverse health effects from BMMFs or their DNA in meat, dairy products or other foods were not expected. Related elements have also been reported in foods of both animal and plant origin.
Detection methods and study limits
The team used highly sensitive procedures for DNA isolation, amplification and sequencing, including rolling-circle amplification and polymerase chain reaction. These methods can detect small amounts of DNA that less sensitive approaches might miss. The authors therefore described the results as a combination of high genetic diversity and low DNA abundance; detection in 88.9% of milk samples did not indicate a high concentration.
To reduce the possibility of cross-contamination, milk was collected from individual cows through an automated milking system, with cleaning between animals and separate filters for each sample. The sample remained limited: 18 cows were assessed through individual milk, and 27 through mammary-tissue analysis. The researchers characterised the work as exploratory and hypothesis-generating. They identified animal genetics, age, microbiota, environment, feed, production systems, health history and interactions with microorganisms as factors for further investigation.
