MOGILab was the first to systematically study pain genetics in the mouse, and although pain genetics is now largely performed by GWAS (genome-wide association studies) in large human cohorts or in humans with rare inherited disorders, mouse studies remain vitally important to elucidate the mechanism of action of genes identified in humans. By showing that the results of preclinical pain studies are robustly affected by the chosen strain, we have encouraged the field to consider their subjects carefully and to use more than one strain whenever possible. We have cautioned that although pain-relevant genes are now more easily identified, there are likely hundreds or even thousands of “pain genes”. This represents a major challenge to any goal of explaining clinical variability, although the heuristic value of identifying such genes remains high.
Selectively Bred Lines
Early studies performed by Dr. Mogil in the labs of Dr. John Liebeskind (grad school advisor) and Dr. John Belknap (postdoc advisor), used mouse lines selectively bred for high and low stress-induced analgesia and high and low opioid analgesia. These studies were performed in collaboration with Dr. Przemyslaw Marek and Dr. Benjamin Kest, using the HA/LA lines developed by Dr. Bogdan Sadowski and/or the HAR/LAR lines developed by Dr. Belknap, and were aimed at determining genetic correlations among traits, and estimating the number of genes responsible for the phenotypic selection.
Inbred Strain Surveys
In 1999, a landmark dual paper was published describing the sensitivity of 11 inbred mouse strains on 12 common pain tests, and the genetic correlations among them. The number of pain tests was expanded in 2002, 2003, 2004, and 2014 (Young et al., Pain, 155:868-880). Similar “strain surveys” were conducted on response to opioid and non-opioid analgesics, anesthesia, itch, immediate-early gene expression, and opioid side-effects like tolerance and dependence. Major conclusions from these studies, many performed by Dr. Sonya (Wilson) Lehto, include:
- Inbred strains of mice and rats differ robustly on all pain tests and to all analgesics
- Heritability (h2) values vary widely by phenotype, but the median is nearly 50%
- Different modalities of pain (e.g., thermal versus mechanical) have a distinct genetic basis
- Genetic sensitivity to analgesics depends on the modality of pain being inhibited more than the identity of the drug itself
- C57BL/6 is an outlier strain, and is not an ideal choice for pain experiments
Transgenic Knockout Mice
Dr. Mogil was co-first author of the first transgenic knockout mouse study of relevance to pain, in which mice lacking expression of beta-endorphin (via a point mutation of Pomc) were found to lack opioid stress-induced analgesia. Since then, MOGILab has worked towards elucidating the role in pain of the following genes using mutant mice or another genetic knockdown technique (in chronological order):
- Pomc (β-endorphin)
- Pnoc (orphanin FQ/nociceptin)
- Mc1r (melanocortin-1 receptor) (also see here)
- Calca (calcitonin gene-related peptide)
- Accn3 (acid-sensing ion channel 3)
- Ccr2 (C-C chemokine receptor 2)
- Kcnj9 (G-protein coupled inwardly rectifying potassium channel 3)
- Atp1b3 (sodium-potassium pump, beta3 subunit)
- Gnao1 (G protein, αo subunit)
- Adamts5 (A Distintegrin and Metalloproteinase with Thrombospondin Motif 5)
- Otr (oxytocin receptor)
- Avpr1a (vasopressin-1A receptor) (also see here)
- Tyrp1 (tyrosinase-related protein 1)
- Tlr4 (toll-like receptor 4)
- Kcc3 (potassium-chloride cotransporter 3)
- P2rx7 (purinergic P2X ion channel 7)
- Pcsk6 (proprotein convertase subtilisin/kexin 6; PACE4)
- Comt (catechol-O-methyltransferase) (see also here)
- Yy1 (yin-yang 1 transcription factor)
- Rptor (raptor)
- Rps6k1/2 (ribosomal protein S6 kinase β1/2) (also see here)
- Cacna1a (calcium voltage-gated channel, α1A subunit)
- Oprm1 (μ-opioid receptor) (also see here and here)
- Bdnf (brain-derived neurotrophic factor)
- Foxn1 (forkhead box protein N1) (also see here and here)
- Rag1 (recombination activating gene 1) (also see here, here, and here)
- P2rx4 (purinergic P2X receptor 4)
- Lmx1b (LIM homeobox transcription factor 1beta)
- Chrna6 (nicotinic acetylcholine receptor, α6 subunit)
- Eifebp1 (eukaryotic initiation factor 4FE binding protein 1) (also see here)
- Wnk1 (WNK lysine deficient protein kinase 1)
- Eif2a (eukaryotic initiation factor 2A)
- Pkr (double-stranded RNA-dependent protein kinase)
- Perk (PKR-like ER kinase)
- Gcn2 (general control non-derepressible 2)
- H2 (major histocompatibility complex, class 2)
- Eif4e (eukaryotic initiation factor 4E)
- Mnk (MAP kinase-interacting serine/threonine kinase 1)
- Mmp9 (matrix metalloprotein 9)
- Oprd1 (δ-opioid receptor) (also see here)
- Oprk1 (κ-opioid receptor) (also see here)
- Cd4 (cluster of differentiation 4)
- Mras (muscle RAS oncogene homolog)
- Kit (proto-oncogene c-KIT)
- Adrb1/2 (adrenergic receptor, β1/2)
Linkage Mapping Findings
Before genetic association studies in humans became feasible with the advent of mapped single nucleotide polymorphisms (SNPs), gene mapping (i.e., ascertaining the location and thus the identity of genes responsible for the inherited portion of phenotypic variability) occurred largely in mouse models via DNA markers called microsatellites. Using a variety of techniques—ranging from simple F2 hybrid crosses to the use of advanced mapping populations such as congenic lines, recombinant inbred lines, recombinant congenic lines, and SNP-based haplotype mapping—MOGILab has provided evidence for the following genes as being responsible for quantitative trait loci (QTLs) in variability related to pain or pain inhibition (in chronological order):
- Oprm1 (μ-opioid receptor)→morphine analgesia (also see here)
- Oprd1 (δ-opioid receptor)→thermal pain
- Mc1r (melanocortin-1 receptor)→stress-induced and kappa-opioid analgesia
- Htr1b (serotonin-1B receptor)→morphine analgesia
- Calca (calcitonin gene-related peptide)→thermal pain
- Kcnj9 (G-protein coupled inwardly rectifying potassium channel 3)→analgesia
- Atp1b3 (sodium-potassium pump, β3 subunit)→inflammatory pain
- Gnao1 (G protein, αo subunit)→morphine dependence
- Mapk8 (c-Jun N-terminal kinase 1)→inflammatory pain
- Tyrp1 (tyrosinase-related protein 1)→inflammatory pain
- Avpr1a (vasopressin-1A receptor)→inflammatory and capsaicin pain
- P2rx7 (purinergic P2X ion channel 7)→neuropathic pain
- Yy1 (yin-yang 1 transcription factor)→inflammatory pain
- Chrna6 (nicotinic acetylcholine receptor, α6 subunit)→neuropathic pain
In a number of cases these QTLs were found to be sex-specific. One particular association, of the Mc1r gene with stress-induced analgesia and κ-opioid analgesia, was of great interest to the media, as this gene is responsible for red hair.
Human Association Findings
Largely in collaboration with Dr. Roger Fillingim and Dr. Luda Diatchenko, MOGILab has participated in providing evidence for the involvement of the following human genes (or other genetic elements) in pain:
- MC1R (melanocortin-1 receptor)→κ-opioid analgesia and pain sensitivity (also see here)
- OPRM1 (μ-opioid receptor)→pressure pain sensitivity (interacts with sex and ethnicity)
- WNK1/HSN2 (WNK lysine deficient protein kinase 1)→hereditary sensory and autonomic neuropathic type 2
- AVPR1A (vasopressin-1A receptor)→capsaicin pain
- P2RX7 (purinergic P2X ion channel 7)→neuropathic pain
- PCSK6 (proprotein convertase subtilisin/kexin 6; PACE4)→osteoarthritis pain
- COMT (catechol-O-methyltransferase)→heat/capsaicin pain (interacts with sex)
- CHRNA6 (nicotinic acetylcholine receptor, α6 subunit)→neuropathic pain
- EREG (epiregulin)→temporomandibular disorder
- EGFR (epidermal growth factor receptor)→temporomandibular disorder
- MRAS (muscle RAS oncogene homolog)→temporomandibular disorder
- miR-19b (microRNA-19b)→chronic widespread pain after trauma
- DCC (netrin-1 receptor)→multi-site chronic pain
- APOE (apolipoprotein E)→multi-site chronic pain
- TP53 (tumor protein p53)→effect of chronic pain on lifespan (male only)
Current Projects
Based on unusually strong GWAS evidence in the UK Biobank for the association of a gene called SPOCK2 (Freidin et al., Pain 162:1176-1187, 2021) with chronic pain (and also because Dr. Mogil is a fan of Star Trek), we are currently evaluating the potential role of Spock2 in pain in the mouse.