Somatosensation and Pain

The MRC Mouse Genetics Network provides a dramatic new opportunity to address the problem of pain in a range of human pathologies. Pain is on the increase. Amazingly, about half the population have an ongoing pain issue, and more than 3 million British citizens have daily inadequately-treated debilitating pain that makes their lives deeply problematic. Many painful condition like arthritis are associated with aging, and as we live longer, the number of people in pain is steadily increasing.
We are all familiar with aspirin-like drugs as effective anti-inflammatory pain killers, as well as the utility of the very potent opioid drugs that are limited in their use by side effects that can lead to death - many thousands a year in the United States. Why do we have no new effective pain -killers in an analogous way to the plethora of new anti-cancer drugs that are proving so effective? There are several reasons. Firstly, some drugs that work well in animal models don't work well in humans. Despite the fact that all effective pain killers do work in mice, there is no way to be absolutely sure that an effective rodent pain killer (tested in genetically identical mice) will work in genetically distinct humans. Secondly, we are beginning to realise that there are different mechanisms at play in different pain states (for example arthritis or cancer pain), and even different pain mechanism between males and females. Thus by understanding more about pain mechanisms, we have a much better chance of developing useful analgesic drugs. The work proposed by the MRC National Mouse Genetics Network addresses these questions and by incorporating a large number of excellent pain groups, provides substantial added value over more classical research programmes. We are only planning to work on targets that have been demonstrated to play a key role in human pain, migraine or headache. This can be done by identifying genes that are responsible for rare pain conditions in humans (gain or loss of pain). We have close to 100 families with such heritable conditions. We will model these pain syndromes in genetically modified mice and find out exactly how the loss of pain occurs, and whether it provides a straight-forward target for classical drug development using medicinal chemistry and molecular screens. The nerves that innervate all tissues of the body (sensory neurons) are required for almost all pain conditions, so we will be focusing on these damage-sensing neurons. Neuroimmune interactions are key regulators of pain, and so we have specialist groups focussing on these pain mechanisms, that nonetheless act through sensory neurons. If we can identify particular types of sensory neurons responsible for particular pains, we have developed the technology to switch off electrical activity in these sets of neurons with a drug that allows the nerves to recover when it is no longer administered.
It may be difficult if, for example, there is no obvious biological activity associated with the key gene altered in human pain states to devise an obvious drug development program. In this case we can use the information obtained from DNA sequencing to mimic the pain-free state by gene therapy - introducing the very mutations that cause a lack of pain in our original patients.
These studies are not solely focused on pain. It is known that sensory neurons regulate the immune response, and are themselves influenced by the microbiome (gut bacteria). They play a key role in responses to infection, injury and a general homeostatic function that may be compromised in various metabolic disorders. Thus the genetically-modified mice that will underpin our attempts to understand and treat pain will also be valuable for other research groups involved in the MRC National Mouse Genetics Network, with significant clinical relevance to many diseases.

We propose to map key genes for human pain using insights from rare pain free people, model and dissect the mechanisms in mice, and devise strategies for developing new analgesic drugs. Human pain insensitivity is an extremely rare condition. Harmonization of clinical data and medical history is performed by standardized questionnaires and a detailed clinical workup. Sequencing is carried our using Long Read Oxford Nanopore whole genome sequencing (one sample per PromethION flow cell to generate ~100 gigabases of sequence data , equivalent to 30x coverage). Whole genome sequencing will identify both exonic and intronic mutations and genomic structural rearrangements. Mutations in known and novel genes will be identified. Cell-based assays will help prove pathogenicity of mutations ahead of mechanistic studies in mouse models.
Generating orthologous mouse models is time consuming and expensive and we plan to use viruses to rapidly interrogate potential targets before selecting the genes that are the most promising in terms of understanding and treating human pain. We will use adeno-associated viral (AAV) delivery in wild-type C57BL/6 mice followed by pain behavioural assays or functional testing of mouse tissues. Intrathecal delivery of AAVs offers an efficient route to transduce DRGs whilst intraperitoneal injection in pups enables wide-ranging expression of mutant genes across tissues. These tests exploit CRISPR based gene silencing techniques, or overexpression of dominant mutations to explore pain responses. On the basis of viral data we will then make appropriate mouse lines and comprehensively phenotype them. The viral data may also suggest gene therapy routes and identify sets of key sensory neurons that may be silenced with chemogenetics to effect analgesia. Reporter lines will be made for the community whilst training courses and collaborative studies with other hubs are described in the full application.