Genetic Association Studies of Pain Phenotypes in Adult SCD Patients

Vasoocclusive episodes (VOEs) are the hallmark of sickle cell disease (SCD) responsible for > 90% of health care utilization. Several studies have elucidated the spectrum of pain syndromes in SCD and have led to the recognition of chronic, neuropathic, and centralized pain in these patients. We previously reported that adult SCD patients exhibited significant hyperalgesia in response to pressure pain algometer testing compared to age, race, and sex matched controls. While demographic and anthropometric factors were associated with self-reported pain, distress, and pressure pain threshold, genetic associations were not tested. Thus, we studied genetic polymorphisms in 167 adult patients with SCD (SS and Sb⁰-thal). Data were collected on number of hospitalizations, ED visits, and opioid use (morphine equivalents) in the past 12 months. Daily pain, subjective distress, and interference with daily activities were measured using daily diaries (as in the PiSCES study) over a period of 6 months. Subjects underwent mechanical pain testing with a pressure algometer at 3 anatomic sites (masseter, ulna, and trapezius) during steady state. Blood samples were obtained for ascertainment of SCD genotype and DNA extraction. Genotyping was performed at the University of Florida Genomics Facility using the Algynomics (Chapel Hill, NC) Pain Research Panel, a custom made chip utilizing the Affymetrix MegAllele technology. The panel assesses 3,295 SNPs representing 358 genes known to be involved in systems relevant to pain perception, nociceptive transmission, inflammation, mood, and affect.

Individuals with >10% missing data were excluded from analysis. Hardy-Weinberg Equilibrium was checked for the genotypes at each SNP with a chi-squared test. A total of 328 rare SNPs with minor allele frequencies (<1%) were dropped. The final pain phenotypes included experimental pain, distress, interference with daily activities, opioid usage, and hospitalizations / ED visits. Linear regression was fitted to each SNP, regressing each phenotype onto genotype under an additive model, with age, gender, and BMI as covariates. The analysis was performed using Plink. False discovery rate was controlled using the Benjamini & Hochberg method.

Six SNPs in 5 genes (ESR2, KCNJ11, DBH, ATP1A1, CACNA2D2) were significantly associated with pressure pain threshold. ESR2 polymorphisms have been reported to be associated with temporomandibular, synovial, and neurogenic pain. KCNJ11 has been reported to play an important role in opioid sensitivity/analgesia. DBH polymorphisms have been related to analgesia, migraines, and catecholamine and serotonin metabolism. ATP1A1 was reported in association with headaches and paresthesias. CACNA2D2 has been correlated with increased sensitivity to opioids.

In terms of opioid usage, 3 SNPs (CAMK2B, PRKD1, CACNA2D4) had significant associations and have been reported to be related to stress, depression, neuronal connectivity, pain modulation, neurogenic inflammation, and pain transmission.

ED visits were significantly associated with 3 SNPS (TRIM31, PLAUR, PRKG1), which have been reported to be associated with fibromyalgia, nociceptive processing, and inflammation.

Two SNPs (PRL and ADRA1A) were significantly associated with hospitalizations that have previously been related to nociception and hyperalgesia as well as inflammation and allodynia.

To our knowledge, this is the first detailed study of genetic associations with different pain related phenotypes in subjects with SCD. Our results show that SNPs in genes relevant to pain processing, nociception, hyperalgesia, opioid sensitivity, inflammation, and stress are significantly associated with different pain phenotypes and therefore appear functionally relevant in adults with SCD. These findings will hopefully serve as a discovery cohort, which should be validated in an independent group of SCD patients. A better understanding of pathways and networks relevant to SCD pain may ultimately lead to new therapeutic interventions.