Bookmarkers beware: Bookmarks to pages other than the home page may not work after we change servers in April Editor's Choice | This Week in BMJ | Press releases BMJ No 7126 Volume 316 News Saturday 17 January 1998 Urine may be better than milk for "biopharming" Scientists in the United States have produced mice that can produce urine rich in human growth hormone. The scientists, from the Department of Agriculture in Maryland, have produced the mice by replacing the protein coding region of a gene that is expressed only in the epithelial cells lining the bladder with the genetic sequence that codes for human growth hormone (Nature BioTechnology 1997;16:75-9). The next step will be to see if larger farm animals can be similarly modified to produce human proteins. If such technology is developed, the commercial production of biologically useful proteins from the urine of genetically modified farm animals could become a viable option.   Polly and her identical sisters Photo: ROSLIN INSTITUTE Three body fluids - milk, blood, and urine - have been suggested as potentially suitable media for protein production in genetically modified animals. So far, proteins have been produced from both milk and blood. The most recent example of this was Polly, the genetically cloned sheep that has been designed to produce human factor IX in her milk. However, only female animals can be used for protein production in milk, and the process also depends on lactation taking place. Protein production in urine should therefore confer several advantages over production in milk: not only could the whole herd be used, but production would start at birth and could continue throughout the life span of the animal. Researchers also believe that isolating and purifying proteins from a salt solution such as urine will be much more straightforward than retrieving proteins from milk, which contains fat and other contaminants. "Biopharming" is clearly a cheaper way to produce biological molecules than using tissue culture techniques, although it takes a long time to produce a suitable herd of transgenic farm animals. In 1995 scientists identified a family of genes (uroplakins) which are expressed specifically on the surface of bladder epithelial cells. Researchers then reasoned that they should be able to modify these bladder specific genes to produce "foreign" proteins that would be secreted into urine and could then be purified. To do this, David Kerr and his colleagues replaced the protein coding section of one of the uroplakin genes in mice with the genetic sequence needed to express human growth hormone. They found that this hybrid gene was still expressed only in bladder epithelial cells but that it now also controlled the gene for human growth hormone. The researchers identified packets of human growth hormone that were fused on to the apical surface of bladder epithelial cells and found that high concentrations of human growth hormone were secreted into the urine from these cells. In addition, both male and female mice inherited the hybrid gene. The researchers chose to work with human growth hormone because it is easily detected and measurable in very small quantities with radioimmunoassay techniques. The concentration of human growth hormone measured in the mouse urine was 200-500 ng/ml, which is about 200 times greater than the concentration released by the animal's own pituitary gland. Robert Wall, one of the research team, anticipated that if larger farm animals could be induced to produce human growth hormone in a similar fashion, the volume of urine would obviously be greater but the concentration of protein would probably be the same. Using the mammary gland as a "bioreactor" to produce human growth hormone would, he suggested, provide a far higher yield of protein, but it would be much more complicated to purify the hormone from milk. This is not the first time that animal urine has been harvested for medical use. Oestrogen compounds have been retrieved from the urine of pregnant mares for use in hormone replacement therapy for many years. As well as potential biomedical uses, there are also several practical agricultural applications that could come out of this technology, said Dr Wall: "Disposing of animal waste is one of the greatest agricultural problems in the world, so one application could be to try to develop a way of modifying animal urine so that it does not cause detrimental effects to the environment." Abi Berger BMJ At the Roslin Institute in Midlothian, Scotland, scientists are still waiting to see if Polly, the genetically cloned sheep, will produce human factor IX in her milk. According to Dr Harry Griffin, spokesman for the institute, by inducing premature lactation in Polly, they should have the answer within six months. And if the experiment is successful and Polly does produce factor IX, Dr Griffin suggests it would be another two to three years before a large enough flock of sheep could be engineered to produce sufficient factor IX to be used in clinical trials. Researchers at Roslin are now concentrating their efforts on refining the process known as "gene targeting" in larger farm animals. Up to now, genetic material has been added to eggs, not always with the knowledge of where in the chromosome these foreign genes become incorporated. With accurate gene targeting, it will become possible not only to add foreign genes but also to substitute one gene for another, in addition to knocking out exact genes. The whole process will become much less "hit and miss." One important application for gene targeting will be to produce animal models for genetic diseases that more closely mimic humans affected by the disease. Although mouse models currently exist for conditions such as cystic fibrosis, they are considered disappointing in their usefulness in modelling cystic fibrosis in humans. According to Dr Griffin, much more useful will be sheep with cystic fibrosis that have exact gene substitutions or genes knocked out--these will exist within one or two years. In parallel with developing gene targeting in sheep, pigs are also receiving attention. Transgenic pigs are to be used to cope with the shortfall in organs needed for human transplantation in the future. Pig organs intended for use include the heart, kidney, and pancreas. To prevent immediate rejection of such organs, transgenic pigs that contain a human protein called complement inhibitory factor are being produced. This protein coats the pig organs and reduces the risk of rejection. Working to remove specific genes from pigs is also expected to have a substantial impact on the future success of xenotransplantation. Home | Current issue | Past issues | Classified ads | Career Focus | Feedback Collections | About this site | About the BMJ | BMA | Medline