HIV-Associated Sensory Neuropathy

As advances in the treatment of HIV are now allowing patients a longer life span, further comorbidities become apparent. This includes sensory neuropathy (HIV-SN) which can affect a patient’s quality of life. Here, we review factors influencing HIV-SN in patients receiving antiretroviral therapy that promotes this condition and in the modern era when these therapies have been withdrawn. This has halved the incidence of HIV-SN, but the condition remains significant in the lives of many sufferers. Genetic polymorphisms that influence pathogenesis of HIV-SN have indicated likely mechanisms, but studies of skin biopsies and animal models are needed to confirm the roles of the encoded proteins.


Introduction
Management of HIV patients is now focused on their quality of life as antiretroviral therapy (ART) increases life expectancy. However, with longer lives, a growing number of patients experience a neurological disorder that predominantly affects small fibers. HIV-associated sensory neuropathy (HIV-SN) may arise not only as a result of HIV infection itself but also as a side effect of ART. The clinical pictures triggered by HIV infection or ART are very similar and include neuropathic pain, tingling sensation, and numbness [1-3]. HIV-SN is one of the most common complications of HIV infection.
The incidence and prevalence of HIV-SN vary widely-perhaps because most studies do not distinguish between neuropathy due to HIV itself and due to ART regimens with different risk profiles. Cross-sectional studies including patients receiving ART identify HIV-SN in 16-50% of HIV patients [4][5][6]. ART that includes the non-nucleotide reverse transcriptase inhibitor (NNRTI), stavudine (d4T), is associated with high prevalence of HIV-SN. The prevalence in Melbourne was up to 42%, whereas in Kuala Lumpur and Jakarta, the reported level was lower, 19 and 34%, respectively [7]. Stavudine is no longer in first-line therapy, and the prevalence of HIV-SN is almost halved (14.2%) compared to data from the same clinic in Indonesia when patients received stavudine [8].
In untreated patients, the risk factors for HIV-SN were severe HIV disease marked by low numbers of CD4 + T cells and high viral loads (HIV RNA) in plasma. In the era of ART (including stavudine), the risk factors of HIV-SN included older age, height, <50 CD4 + T cells/mm 3 , malnutrition, and concurrent diabetes [1, 7, 9, 10]. HIV-SN DOI: http://dx.doi.org/10.5772/intechopen.81176 Stimulated skin wrinkling (SSW) test is a method to assess small nerve fiber function using exposure to eutectic mixture of local anesthetic. It has been shown to correlate with intraepidermal nerve fiber density (IENFD) in patients with a sensory neuropathy [24] and has high sensitivity compared to other assessments of small-fiber neuropathy in diabetic patients [25]. Skin wrinkling occurs as a result of vasoconstriction in the glabrous skin, mediated by postganglionic sympathetic fibers [26]. Other assessments that have been used to detect smallfiber neuropathy in HIV-SN patients include quantitative sudomotor axon reflex tests (QSART) [27], quantitative sensory tests (QST) [18], and sympathetic skin responses (SSR) [22,23].
Skin biopsies are the gold standard for the detection of damage to small-diameter sensory nerves, including non-myelinated and myelinated intraepidermal nerve fibers. Lower nerve fiber densities have been demonstrated in patients with HIV-SN [18]. Studies have used several different techniques. The European Federation of Neurological Societies recommended a biopsy of the skin to a depth of 3 mm by using a skin punch biopsy on the distal limbs to calculate the linear density or nerve fibers with a minimum of 50 μm-thick slices, fixed in a 2% solution of paraformaldehyde-lysine-periodate (2% PLP). Immunohistochemical staining techniques recommended are bright-field immunohistochemistry and indirect immunofluorescence [28]. PGP9.5 immunofluorescence allows nerves to be visualized using a confocal microscope [29]. Smaller intraepidermal nerve fiber densities (IENFD) in HIV-SN patients correlated with the clinical and electrophysiological severity [30]. Skin biopsies can also be used to identify cells and mediators that contribute to SN. These are discussed later in this chapter.

Clinical factors influence the risk of HIV-SN
Analyses of the risk factor of HIV-SN require that we consider the condition in three distinct eras-(1) pre-ART, (2) the use of combination ART that included stavudine (d4T), and (3) the use of non-neurotoxic ART. In the pre-ART era, the risk factors for developing HIV-SN included HIV disease severity, low CD4 + T-cell counts, high viral load, and older age [31,32]. In the second era, the risk factors are older age, height, low nadir CD4 + T-cell counts, HIV duration, malnutrition, diabetes mellitus, dyslipidemia, and the use of neurotoxic drugs (usually stavudine; see Table 1; [7,14,15,33,34]). Stavudine is no longer recommended by the WHO as first-line ART and is now rarely used anywhere in the world, but HIV-SN has not disappeared. The risk factors of HIV-SN in patients on ART without stavudine are almost the same as in the pre-ART era-high plasma viral load and older age [8]. Isoniazid is widely used as therapy for tuberculosis and has been recognized as a risk factor for neuropathy for a long time. It remains weakly associated with HIV-SN even though patients receiving isoniazid are also given B6 supplementation to prevent neuropathy. Protease inhibitor (PI) exposure may be a risk factor of HIV-SN. Lopinavir, indinavir, and ritonavir, but not nelfinavir, were associated with neuropathy in one study [35].

Genetic risk factors
The risk of HIV-SN cannot be correlated with a single genetic variant, so candidate genes are discussed separately (see Table 1). It is of interest to determine if any aligns with the greater sensitivity of individuals of African descent [13, 14, 36].

Genes in linkage disequilibrium with TNF or encoding components of pathways regulated by TNF
In patients receiving stavudine, haplotypic combinations of alleles of singlenucleotide polymorphisms (SNP) spanning the tumor necrosis factor (TNF) block in the central major histocompatibility complex (MHC) associate with variations in the prevalence of HIV-SN, but the associations were different in Africans and Asians [12]. For example, a polymorphism in intron 10 of BAT1 (marking an MHC haplotype associated with several inflammatory disorders) and a polymorphism in the promoter region of the TNFA gene (TNF-1031) were associated with an increased risk of HIV-SN in Caucasians [37]. TNF-1031*2 is associated with an increased risk of HIV-SN in Indonesian HIV-positive patients who receive stavudine [15,16]. However, in Africans, different SNP alleles were found in linkage disequilibrium with TNF-1031*2, so TNF-1031*2 was not associated with HIV-SN. These findings link HIV-SN with an unknown SNP in the TNF block marked by (but distinct from) TNF-1031. The link between HIV-SN and inflammation was supported by studies linking IL4 genotypes with HIV-SN in Africans receiving stavudine [13]. The P2X7R receptor is expressed by microglia and may be involved in neuropathic pain, as its ablation or inhibition in animal models of neuropathy can reduce responses to painful stimuli [38]. Conversely, stimulation of P2X7R will increase the release of pro-inflammatory cytokines such as IL-1β, IL-6, and TNFα [39] as well as pro-inflammatory chemokines such as CXCL2 and CCL3, which have been implicated in neuropathic pain [40,41].

The
In animal studies, P2X4R was activated in spinal microglial cells in rats with induced pain [42]. Mice with disrupted P2X4R genes showed reduced pain response in two models of chronic pain (inflammatory and neuropathic) [43]. P2X4R is upregulated after peripheral nerve injury which results in increased activity of DOI: http://dx.doi.org/10.5772/intechopen.81176 mitogen p38 [44]. This process initiates the release of brain-derived neurotropic factor (BDNF). BDNF induces neuronal hyperexcitability through interaction with the TrkB receptor [45,46].
The CAMKK2 gene encodes calcium-/calmodulin-dependent protein kinase 2 (CaMKK2), which acts as a pervasive second messenger of Ca 2+ in many cellular functions such as energy balance, neuronal differentiation, and inflammation [47]. CaMKK2 plays a role in neural plasticity and neurite growth by activating another protein kinase CaMKI [48]. CAMKK2 and P2X4R polymorphisms affect TNFα production in vitro. This suggests a mechanism for their impact on HIV-SN [49]. Hence, polymorphisms in CAMKK2 may affect inflammation or neuronal growth.

Mitochondrial haplotypes and iron metabolism
The process of mitochondrial toxicity induced by ART is not a simple drug toxicity, but mitochondrial DNA (mtDNA) SNP has a role in developing HIV-SN in patients receiving NRTI. SNP in African mtDNA haplogroup L1c and European haplogroup J is associated with decreased prevalence of HIV-SN compared with all other haplogroups [36]. Moreover, Thai persons belonging to mtDNA haplogroup B were more likely to develop HIV-SN [50].
HIV-1 Nef protein may influence iron levels via interactions with the hemochromatosis protein HFE in humans [51]. In an observational prospective study, Kallianpur et al. suggested that disruption of iron homeostasis due to HIV infection might damage neurons and potentially lead to HIV-SN. They presented evidence that the HFE C282Y mutation may be a protective factor in HIV patients using NRTI [52]. They subsequently linked polymorphisms in iron management genes with increased risk (TF, CP, ACO1, BMP6, B2M) and reduced risk (TF, TFRC, BMP6, ACO1, SLC11A2, FXN) of HIV-SN [53].

The pathophysiology of HIV-SN
The pathophysiology of HIV-SN is not completely understood, but there are several promising theories. It remains unclear whether HIV inflicts direct damage in the nerve body of dorsal root ganglia (DRG) or damages nerve fibers; both will lead to the development of distal axonopathies. HIV causes distal axon degeneration, reduction of nerve fiber in DRG, infiltration of inflammation cells, and reduction of the intraepidermal nerve fiber (IENFD) count [2]. As HIV itself cannot directly infect nerve bodies, destruction of neuron in HIV-SN may be caused by neurotoxic agents released by activated macrophage and satellite glial cells (TNF-α, IL-1β, chemokines), viral proteins with neurotoxic properties (gp41, gp120, Tat, Vpr), infection of perineural cells, or combinations of these processes [54][55][56][57][58]. A study in simian immunodeficiency virus macaque model confirmed that HIV infection activates perineuronal inflammatory cells (including macrophages and lymphocytes) in trigeminal ganglia and DRG during the early stage of infection. In the later stage, neuronal damage becomes evident, and regenerative capacity of small epidermal nerve is impaired [59].
HIV infection may cause macrophages to respond to the axonal degeneration (even in mild cases) causing inflammation of the nerves and DRG. Proinflammatory mediators were released by Schwann cells at DRG and may accumulate adjacent to peripheral nerves, activate apoptotic pathways and cause damage to the nerves directly or indirectly (reviewed in [55]). The gp120 virus protein may act directly on chemokine receptors expressed on neurons and cause pain [60]. A histopathology study of skin biopsies from HIV-SN patients on ART without stavudine confirmed the presence of inflammatory macrophages and T cells expressing some chemokine receptors (CX3CR1, CCR2, CCR5), along with reduced IENFD [61].
HIV protein gp120 is a component of the viral glycoprotein sheath. The entry of the HIV virus into cells requires the interaction of gp120 with CD4 glycoprotein and a chemokine receptor (usually CXCR4 and/or CCR5) which may be expressed on neurons or infiltrating inflammatory cells. Several chemokine receptors, such as CCR2, CCR5, and CXCR4, and CX 3 CR1 (fractalkine receptor) are located in primary afferent neurons or secondary neurons of the spinal dorsal horn. Chemokines and gp120 can cause pain through direct effects on chemokine receptors expressed by nociceptive neurons [62]. For example, binding of gp120 to CXCR4 receptors increases the release of CCL5, which binds CCR5 and triggers the release of TNFα and other neurotoxic substances. These interactions activate an influx of Ca 2+ , kinase cascades, and STAT3 signaling leading to the signs and symptoms of HIV-SN. The pathways have been reviewed previously [61,63].
The pathophysiology of HIV-SN in patients on stavudine may reflect damage to the mitochondria of neurons and axons via damage to mitochondrial DNA (mtDNA) [64]. Inhibition of mtDNA gamma polymerase, mtDNA intercalation, and damage in stress response of mitochondria has been demonstrated in vitro in cultures of T-lymphoblastoid cells [65]. This finding is further supported by differences in haplotypes or SNP in mtDNA in Europeans, Hispanics, and Africans that may contribute to differences in the prevalence of HIV-SN [36,52,66,67].

Therapeutic options
Management of HIV-SN aims to avoid further nerve damage and minimize the patients' symptoms especially neuropathic pain. Some studies showed that smoked cannabis is effective and has analgesic value to relieve pain in HIV-SN patients [68,69]. However, due to legal issues in many countries, the recommendation of smoked cannabis has been controversial. Other pharmacological treatments recommended for neuropathic pain are amitriptyline, pregabalin, and gabapentin [70]. However, these medications were not superior to the placebo in HIV-SN patients [71][72][73]. Another option is non-pharmacological treatment such as acupuncture and hypnosis. However, acupuncture was not superior to the placebo to improve pain in HIV patients [74]. A small study showed that hypnosis showed benefit to reduce the pain score in HIV-SN patients [75].

Conclusions and future directions
Despite the withdrawal of the most toxic drugs from recommended ART regimens, HIV-SN remains a common neurological complication of HIV disease. The risk factors of HIV-SN have changed with changes in ART from the patient's age and height to the efficacy of ART and the use of protease inhibitors. Genetic polymorphisms that influence pathogenesis of HIV-SN will provide candidate molecules, which may contribute to pathogenesis, but studies of skin biopsies from patients are needed to confirm the roles of the encoded proteins. Animal models may reveal mechanisms for neuropathy and pain by HIV proteins but do not mimic the complexities of HIV disease in patients.