The Genetics of Neuropathic Pain from Model Organisms to Clinical Application

Abstract

Neuropathic pain (NeuP) arises due to injury of the somatosensory nervous system and is both common and disabling, rendering an urgent need for non-addictive, effective new therapies. Given the high evolutionary conservation of pain, investigative approaches from Drosophila mutagenesis to human Mendelian genetics have aided our understanding of the maladaptive plasticity underlying NeuP. Successes include the identification of ion channel variants causing hyper-excitability and the importance of neuro-immune signaling. Recent developments encompass improved sensory phenotyping in animal models and patients, brain imaging, and electrophysiology-based pain biomarkers, the collection of large well-phenotyped population cohorts, neurons derived from patient stem cells, and high-precision CRISPR generated genetic editing. We will discuss how to harness these resources to understand the pathophysiological drivers of NeuP, define its relationship with comorbidities such as anxiety, depression, and sleep disorders, and explore how to apply these findings to the prediction, diagnosis, and treatment of NeuP in the clinic.

Figure 1

A Comparison of the Genetic Investigation and Functional Analysis of NeuP in Human and Animal Models The study of NeuP in humans has the obvious advantage of verbal feedback of the individual’s subjective experience. The development of relatively non-invasive neurophysiological techniques has allowed us to further probe for diagnostic and functional biomarkers in NeuP patients. The house mouse (Mus musculus) and fruit fly (Drosophila melanogaster) are genetically modifiable organisms commonly used in studies of nociception and pain processing. High-throughput genetic and phenotypic analysis (e.g., nociception) can be achieved in flies, as well as fish and worms, whereas complex behavioral traits such as cognitive affect can be assessed in rodents. Appreciation of plasticity in the fly nervous system has led to the application of several injury and chemically induced neuropathy models to this organism (Khuong et al., 2019). Adult flies are also capable of learned conditioning to thermal stimuli such that they subsequently avoid that area of the chamber even in the absence of heat (Wustmann et al., 1996). Mutants displaying spontaneous avoidance behavior that mimics the stimulus-elicited avoidance response in wild-type larvae remain to be investigated as a possible measure of ongoing aversive sensation (Heiman et al., 1996). Abbreviations: CCI, chronic constriction injury; CRISPR/Cas9, clustered regularly interspaced palindromic repeats/CRISPR-associated protein-9 nuclease; DN4, Neuropathic Pain Diagnostic Questionnaire; EMS, ethyl methanesulfonate; ENU, N-ethyl-N-nitrosourea; GWAS, whole-genome association study; LANNS, Leeds Assessment of Neuropathic Symptoms and Signs; NPSI, neuropathic pain symptom inventory; QST, quantitative sensory testing; SNI, spared nerve injury; SNL, spinal nerve ligation; TALENs, transcription activator-like effector nucleases); TMP/UV, trimethylpsoralen/ultra violet; ZFN, zinc-finger nuclease.

Figure 2

The Challenges of Conducting Genome-wide Association Studies in NeuP

Figure 3

A Venn Diagram of Genes Reaching Study Specific or Suggestive Significance in Human Candidate Gene and Genome-wide Studies So Far in NeuP and the Overlap of Biological Pathways These genes have been summarized in a recent systematic review of NeuP by Veluchamy et al. (2018), where the inclusion criteria were any study analyzing genetic variants in people with NeuP compared to people without NeuP. The number of genes and our understanding of their contribution within these pathways, in the context of NeuP, is likely to change as more studies are published.

Figure 4

Na_V1.7 Variants Contributing to NeuP Na_V1.7 is the most studied ion channel with genetic association to NeuP and represents the canonical example of how variants can drive disease pathogenesis. Schematic illustrates the cardinal biophysical changes in Na_V1.7 function associated with rare inherited (IEM and PEPD) and common NeuP states painful diabetic peripheral neuropathy (PDPN) and idiopathic small fiber neuropathy. IEM-associated variants uniquely exhibit hyperpolarized voltage of activation, whereas PEPD variants display impaired fast inactivation, which results in a persistent current in the majority of cases. Similar to PEPD, PDPN variants also demonstrate impaired fast inactivation. Small fiber neuropathy-associated variants show mixed properties, with impaired slow and fast inactivation as well as enhanced resurgent currents being described in different variants. Paroxysms of pain can be initiated by a range of triggers and are modulated by genetic and environmental factors, e.g., genetic modifiers, temperature, and metabolic state of diabetes mellitus patients (Blesneac et al., 2018, Mis et al., 2019). Convergence of these factors will determine the resultant degree of primary afferent excitability, which is important in the initiation and maintenance of NeuP. In this way, clinical outcome is dependent on the basic characteristics of the mutation interacting with modifying factors, explaining why patients with identical mutations exhibit diverse clinical phenotypes and why some mutations only result in a pain phenotype upon a secondary insult.