Animal Models in Auditory Research

Most audiologists know that hearing impairment is the most common congenital sensory impairment. It affects up to 1 in 500 newborns, 1 in 300 children by the age of 4, and 1 in 1000 newborns will have genetically inherited hearing loss. In addition to aging, hearing loss in adults stems from exposure to chronic infection, noise, chemicals and other factors as well as aging.  Age-related hearing impairment is the most common sensory deficit among aging individuals, running rampant in nursing homes and assisted living centers. 

While there have been great strides in auditory research in recent years greatly contributing to the knowledge about the hearing system, 95% of the research is conducted with animal models using mice.  Although humans and animals (technically “non-human animals”) may look different, at a physiological and anatomical level they are remarkably similar.  Animals, from mice to monkeys, have the same organs (heart, lungs, brain etc.) and organ systems (respiratory, cardiovascular, nervous systems etc.) which similarly perform the same functions. These similarities mean that nearly 90% of the veterinary medicines used to treat animals are the same as, or very similar to, those developed for the treatment of  humans. There are minor differences, but these differences are far outweighed by the similarities.

The differences can offer important clues to disease and methods of treatment – for instance, if we knew how a mouse can be cured of hearing loss, this might lead to a treatment for human hearing loss.   According to Speaking of Research (2017), humans share approximately 99% of their DNA with mice, and moreover researchers can use “knockout” mice to work out what effect individual human genes have in our body. This “knockout procedure is done  by “turning off” one of the genes in a mouse, common to a human, and seeing what effect this has on the mouse. By recreating human genetic diseases using these methods scientists can begin to look for treatments.

 Those who defend the use of animals in research contend that nonhuman animals are enough like humans to make them scientifically adequate models of humans, but different enough to make it morally acceptable to experiment on them. Some researchers, however,  are concerned that animal research has limitations insofar as its application to humans.  Ethical objections notwithstanding, instilling suffering to sentient species are inherent with animal models.  To some this suffering is not worth the information obtained from these animals when considered in light of the differences from humans in both size and physiology, genetic differences, and variations in biological targets limit the ability of data collected from an animal model to be translated to people.  According to NAVS (2017), when animals are used in studies of human diseases, the artificial way in which the disease is induced in the animal is far removed from the way diseases occur naturally in people, limiting the value of such studies. The validity, usefulness, expense and ethics of scientific experiments that rely upon animal models are increasingly being called into question—not only by animal advocates, but by those in the scientific community—which is why it is essential for research scientists to develop and utilize models that better reflect human biology and offers the best chance possible to improve human health and well-being.

What Do the Hearing Experts Say?

 In the hearing research, prestigious researchers and institutions such as Stanford’s Initiative to Cure Hearing Loss (SICHL), an arm of the Department of Otolaryngology, have many progressive animal investigations in progress.  SICHL project researcher Dr. Alan Cheng feels that hair cell regeneration holds a lot of promise in the cure of hearing loss.  For example, does inducing hair cell regeneration restore hearing all by itself, or are there other things that need to happen, and how are they all related?  Dr. Cheng says that, “We must know more about the general environment of the inner ear after hair cell loss or noise damage occurs, which is very different from the conditions inside a healthy ear….. And these changes may very well differ between a young versus aged inner ears. Knowing more about what goes on in terms of gene activity and protein production after hearing loss will go a long way towards identifying areas of intervention where we can encourage hair cell or neuronal regeneration.”  While this fundamental hair cell research goes on in earnest, Dr. Cheng feels that there is some disconnect between animal models and that the study of hair cell regeneration that would mirror the real causes of hearing loss in humans and finding more appropriate models is essential to finding clues to hair cell regeneration.  Agreeing with Dr. Cheng, Dr. Larry Lustig, of University of California, San Francisco, feels that “Researchers need to develop a viable animal model of genetic deafness that more closely matches common forms of genetic deafness seen in humans. The mouse model we used for our study had genetic mutations in a gene that is critical for hair cells to transmit signals to the brain. While we were able to replace the mutated gene with the correct form, and had some recovery of hearing in the mice, this isn’t the kind of genetic deafness people have, so better animal models are very much needed.”  Dr. Allen Ryan, University of California – San Diego, also agrees that there is a need for better models of study.  Dr. Ryan states that “in that all current experiments and studies are in mice or other animal models, with the best result in birds or very immature mammalian inner ears.”

Since the beginning of the 20th Century, Nobel Prizes have charted the world’s greatest medical advances.  Of the 106 Nobel Prizes for Physiology or Medicine, 94 were dependent upon research using animal models and virtually every one for the past 30 years.  While some researchers are looking for other models, the research goes on to better understand the process of hearing, find a cure for deafness, and improve the plight of those with hearing impairment. Until a better model comes along, animals will remain the mode of operation for these endeavors.

A Comment to this Post –

Dr. Jont Allen commented on this post as an experienced animal researcher.  He comment was, “One problem with this analysis is that it only reports on a limited set of hearing disorders. Middle ear research is also of grave concern, and there animal models have been and continue to be, very useful.  In my view, cochlear hair cell research/regeneration is a very small piece of the research pie, and unlikely to help people with cochlear damage hear better. For example, even if you have new hair cells, how do they get connected to the auditory nerve?”  Thanks to Dr. Allen for his comment, check out his web site at www.auditorymodels.com

References

NAVS (2017).  Alternatives to animal research. National Anti-Vivisection Society.  Retrieved February 14, 2017.

Parmet, S. (2016). When Will We Have a Cure for Hearing Loss?  American Hearing Research Foundation.  Retrieved February 12, 2017.

Staff (2017).  The animal model:  Biological similarity of humans and other animals.  Speaking of Research.  Retrieved February 15, 2017.

Stanford Medicine (2017).  The Sichl Mission.  Stanford Initiative to Cure Hearing Loss. Retrieved February 12, 2017.

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Pearson Education (2011).  Animal Testing.  Retrieved February 15, 2017.