Meet the humanized mouse. It’s not the talking mouse of movies but an extension of using mice as a model system to study human disease and function. In these mice strains whole swaths of the mouse genome have been replaced with the corresponding sections of the human genome. In these animals where the swap has been made the human genes effectively replace their mouse counterparts, resulting in a human molecular system that operates within the mouse.
Genes however are not enough. For immune system replacements, genetic changes are augmented with organ grafts. Mice strains exist that essentially have a human immune system operating within them. Such strains open an experimental opportunity that would simple not be available otherwise. With such a strain a researcher can gain insights into how a human system would respond because though they are working with mice, those mice contain a reconstituted human system within. Model organisms standing in as a proxy for human subjects must always be evaluated with the knowledge that human responses may differ. That is till true even with these humanized systems, but we are far closer to a human response than before.
- Nat Rev Genet. 2011 Dec 16;13(1):14-20. doi: 10.1038/nrg3116. Genomically humanized mice: technologies and promises. Devoy A, Bunton-Stasyshyn RK, Tybulewicz VL, Smith AJ, Fisher EM. Go To PubMed
- Nat Rev Immunol. 2007 Feb;7(2):118-30. Humanized mice in translational biomedical research. Shultz LD, Ishikawa F, Greiner DL. Go To PubMed
interesting articles with so many dynamic problems wrapped up into a potentially great solution to one big issue!
Shutlz paper- The NOD-scid mice are described as preferable in part because of decreased immune response allowing for a more successful graft process…can this success be attributed in part to the induced disease-state (diabetes). Diabetes being inherently an immunodeficient state. Or, is the decreased immune response attributable to a direct scid-mutation-related affect on the immune system? Both?
Devoy paper- Uses SOD1 as an example of mouse/human protein identity sharing of 83% and critical discrepancy between a tryptophan residue location at codon 32 only appearing in humans. When humanization or xenotransplant is utilized in chicks, rather than mice, is this to address an issue like the one noted? Can similar hurtles be expected in these types of experiments (unreliability of a xenotransplant in chicks due to something like leakiness)?
How is this likely to effect drug development? As the mouse models become more and more humanized and part of the normal routine in drug development, are we likely to see not only quicker development of new drugs and therapies, but also fewer human studies and quicker FDA approval? Also, since the life span of a mouse is much smaller than that of a human, would the oberservation of long term effects in the mouse be comparable to the long term effects in the human?
Although the utilization of humanized mice has been shown to be promising in creating disease models indicative of those found it humans, one key issue that seems to arise is that of epigenetics.
Some human disorders are associated with genomic imprinting where both parents contribute different epigenetic patterns for specific loci in their germ cells. So if these humanized mice contain both their own genome as well a portion of the human genome then mate, how is genomic imprinting in the mice affected? If one of the alleles in the progenic mice now contains human elements, how is genetic silencing affected and how does this affect the stability of the mouse genome?
X-chromosome inactivation, one example of the epigenetic phenomenon, can also be an issue when sex-linked diseases are the focus of study in these humanized mice. How are epigenetic mechanisms such as DNA methylation affected if the chromosomes are not homologous since they may contain elements from two different species?
In the conclusion of “Genomically humanized mice: technologies and promises” , Devoy et al. describe a potential concern in which it is questionable if human DNA sequences can be correctly and efficiently read by mouse transcriptional machinery. However, if human non-coding and coding regions can be effectively knocked-in mimicking disease states, it seems plausible that researchers would be able to knock-in human sequences using the same technologies that would code for our own transcriptional machinery proteins which would alleviate this concern.
Transplanting the human genome into a mouse appears to be a fool-proof method for studying the human system in a mouse and to provide a model that represents humans but does not require humans as subjects. However, extracting a model from one organism and transplanting into another is not without limitations. This method can definitely help us to determine alot of information in regards to toxicants and their effects on biological systems. But it does not necessarily represent the actual human system in its original form. It is similar to a transplanted organ. It can be done, but there are so many factors to control and to observe to accurately determine if all the subtle and less obvious interplay of proteins and other molecules that would act differently in the transplanted organism than in the new one. So with a transplanted organ, there are many cases where it fails even though it is a properly functioning organ. Because life in one system does not always represent life in the other system in its exact form.
In clarity, this is like the discovery of using knockout mice to study certain genomic phenotypes and patterns. It works very well and has advanced research in numerous ways. But there are areas that we need to watch out for that to ensure we do not extrapolate too much without getting the whole picture. For example, activity of T-cells and cytokines are sensitive to their environments and their activity can be altered through the presence of a new system. In conclusion, proceed with caution as we do not want to assume too much from mice-human models. We need comparison studies before we can completely migrate to the idea of a human-in-mice models.
This is a great model organism to use for drug development and various treatments but there are some ethical concerns that will come into play, especially for studies being conducted in the US. Stem cell research had great potential but religious groups have really tainted the view of the general public when it comes to using human stem cells.
Any time you use human cells/genes in research there will be controversy that could overshadow the actual research and many people may never even care to see the results of the study. In addition to that, research is often federally funded and if legal issues arise, funding tends to get cut.
Though using the humanized mouse as a model and bridge between the in vitro studies and the in vivo analysis seem to be ideal, it is still a skeptical approach in the fact that is still not a 100% representative of the entire human genomic environement, and thus may not answer some of the complex questions involved in drug development and disease prevention.
Some of the genes’ manipulations/mutations (knockout and knock-in mice) involved in the different proctocols may have non-observable (at a naked eye) but yet significant implications in the true function and effect of diverse therapeutic agents and their development/optimization.
I beleive it could provide great insight in comparative studies; but not altogether validate the diverse hypothesis risen. In addition, its ethical ambiguities could still stir reluctance in the practice and application of the method.
I beleive that all currently known limitations should be taken into greater consideration, and should be overcomed or figured out before moving forward in excitement of this tool for the study of the humans’ health and disease states.
Although we can successfully introduce human genes into the mouse genome to make humanized mouse, but many post-transcriptional regulations might interfere with the expression of desired proteins. Specially, how the micro-RNA pool of mouse is going to regulate the expression of the human genes or how newly synthesized micro-RNAs from the human genes are going to regulate the expression of existing mouse genes, will be crucial factors in further advancement of these techniques. Nevertheless these humanized mouse models are great tool to study human diseases and further technical advancements will led to more close and accurate models for human diseases.
It is clear that benefits in gene therapy using genomically humanized mice have already been seen, however given that these models are essentially novel, are we too quick to applaud its benefits? Notwithstanding that the results already obtained seem promising, I believe that more long-term studies are required, particularly since we are dealing with chimeras and not animals which are complete representatives of the human genome.
Ethical considerations are also important – exactly how far are we allowed to go in our quest for answers? How much genetic manipulation is too much, and are there any international committees/ laws established to govern/ limit the use or abuse of genomic humanization – especially since the model can be used to study disease/ the pathogen process?
Although there may be many limitations in using a humanized mice model, there seem to be many benefits that come along with this method. It will help provide a better understanding of how systems may work in the human body without disturbing the ethical issues. There may be concerns about the efficiency of the incorporation of human DNA sequences to be read by mice transcriptional machinery, since there is limited information. Due to the little information we have in using these models, research should be expanded to gain more knowledge of these methods.
These methods will definitely provide a new input for use in gene therapy. It will enhance our understanding of the expression of human sequences along with which mutations are involved in certain diseases.
One factor that may cause a problem in using these methods are the involvement of epigenetics. But with continued studies and advancements, I believe humanized mice models can become very informational to understand functions of the human genome.
Question in Devoy et. al: I am not quite understanding the method of RMCE or RMGR. Can anyone clarify?
The humanization of mice is an innovative approach to therapeutics in curing a multitude of diseases without using actual humans. I understand that 100% of the genome is not replicated into the mouse but it is still a start. Considering that this is a fairly new discovery, I’m sure it will continue to develop into a more sophisticated project with 100% accuracy.
It is fascinating to see that mice are not normally permissive to hepatitis C virus, but they have been able to replicate this ability from a human into a mouse. That goes to show that this humanization can go as far as making an organism susceptible to a virus that it originally had defenses for.
Cancer is a big fear in the world. Using the NOD-scid Il2rg mice as a model will allow scientists to discover where these tumors arise. They will be able to determine whether they start as early as the stem cell level or if it comes later on. But, will tumor growth time be able to be translated into a human and how will the origination of these tumors be proven on a human?
As mice go on to procreate, will there be a change in genes that will cause the mouse to revert back to its original genomic pattern or will it continue to keep the humanized genome?
As a generally new technology, the humanized mouse models are able to provide insights, granted not everything, into an in vivo model where other methods would not be as successful. I feel this technique presents an incredible opportunity to further develop new techniques in producing more accurate transgenic techniques as well being able to pinpoint mutations and target therapies. Being able to introduce part of the human genome is an innovative and exciting technique. Shultz et al.’s review shows the advantages and the development of each limitation. It is great to see how each problem has been dealt with and thereby also providing an additional strain of humanized mouse than can be used for another disease or investigation.
Many have mentioned a fear of the ethical boundaries, however I feel the benefits way out weight the risks. I do not see where the issue of ethics comes into play. In the case of cancer or infectious viruses, this technique is able to show a more complete scenario of the stages, from infection/initial growth to the complete formation of either a tumor or the efficacy of vaccines/therapies.
The only aspect I would be how else can it be confirmed that the correct portions of the genome have been transferred? What are the long-term affects? With time and more research I believe this will provide a significant amount of insight and information.
Humanization of the mouse facilitates evaluation of complex biological processes, particularly those associated with disease onset. Given that very few proteins are conserved between humans and mice it is necessary to create a model that will express human proteins. Additionally, infectious diseases that that occur in humans are not observed in mice. As a result, diseased states that normally occur in humans can be assessed in mice without provoking strong ethical responses. However, different strains of mice yield different responses. Hence it is necessary to evaluate which strains are valid for the particular study of interest. This method is relatively new and thus the long-term drawbacks have yet to be determined. What are the effects of continuously replacing the mouse genome with the human genome? What effect will this have on mice as a species and will this affect the validity of the research and results obtained with these animals?
This seems to be a very neat technique specially in case of diseases like Cancer, AIDS (or any other infectious disease), auto immune diseases or studies regarding organ transplant complications, as in all the above mentioned problems, direct study cannot be conducted on humans. The only visible problem is that of the innate immunity of the mice producing immune cells like Natural killer cells, there is one more problem, once the mice gets humanized, certain immune cells are not produced like Certain type of B cells, because they require certain environment which is only found in the human body. Over coming these problems, humanized mice has all the potential to be an excellent model to study many human diseases in vivo.
The ability to take the human immuno responce system and tranfer it into a mous is a ground breaking discovery. This allows scientists to test autoimmune and genetic diseases on mice instead of having to do their research in an autopsy. Scientists took the genetic information of a human and put it into mice. The only negative side is that mice have a difference gene expression and RNA. The proteins are very different in the two species, to have a truly ideal situation more than just the genes and immune system need to be swapped.