Identification of genetic variants contributing to pain-related symptoms in familial and sporadic patients affected by peripheral neuropathy

Peripheral neuropathy (PN) is a debilitating disorder that reduces the patient's quality of life and increases healthcare costs (Cruccu & Truini, 2017). PN is caused by peripheral nerve injury; if the damage is limited to A and unmyelinated C fibres, it is referred to as Small Fiber Neuropathy (SFN) (Hovaguimian & Gibbons, 2011), and it is frequently accompanied by neuropathic pain (NP) (Van Hecke et al., 2014). In the general population, the prevalence of SFN is estimated to be around 53 per 100,000 people. However, patients are frequently underdiagnosed because regular neurologic examinations do not identify abnormalities in small fibres (Peters et al., 2013). SFN is associated with several diseases, including diabetic neuropathy and sarcoidosis, making diagnosis more difficult. If the cause of SFN is uncertain, it is referred to as idiopathic SFN. Recent research suggests that mutations in voltage-gated sodium channels (VGSCs) genes are responsible for SFN due to their function in nociceptive transmission (Faber et al., 2012), which explains 3% of the disease pathogenesis (Devigili et al., 2008). The discovery of mutations in VGSC genes also highlighted the disease's genetic aetiology. Finding the genetic markers of PN patients based on their phenotypes could aid in resolving the pathophysiology of PN and classifying high-risk subjects. The current study is being conducted within the international consortium PAIN-Net, which emerged from a previously European Union-funded project called the PROPANE study. Both initiatives, with slight changes, seek to better understand the genetic architecture of PN to identify high-risk painful PN patients. Consequently, the principal aim of this PhD thesis was to identify new genes and genetic variants associated with pain-related symptoms in patients affected by PN. For this purpose, whole exome sequencing (WES) was performed in twelve families with 34 affected and 19 unaffected members, as well as in 43 sporadic cases diagnosed with PN or SFN, with an early onset of clinical manifestations. Our findings indicate that painful PN is genetically heterogeneous, with more than 40% of cases caused by genes other than VGSC genes. We detected 48 mutated genes in the familial cohort and 77 mutated genes in the sporadic cohort. Most of these genes are associated with neurotransmission rather than ion channel function. In addition to VGSC genes, the ATP7B gene, a copper ion transporter, had a high number of mutations in both familial and sporadic cohorts. ATP7B gene variants suggest that PN symptoms may be exacerbated by dysfunctional copper metabolism. Additionally, we discovered TRP gene variants, specifically TRPM2, that were causative for NP and conclusively segregated into two families. Pathway analysis of qualifying genes obtained from familial and sporadic cohorts revealed the nicotinic acetylcholine receptor signalling pathway as a process that appears to be altered in the disease, and that is shared between familial and sporadic subjects. This result strengthened the notion that synaptic transmission plays a role in the pathogenesis of PN. This genetic study revealed new pain-related genetic markers that may be associated with disease onset and pain modulation. These mutated genes will be researched further to establish the pathophysiological causes of painful PN, facilitate the stratification of high-risk individuals, and assist in developing targeted therapies to alleviate pain.