PLA2G6-associated neurodegeneration (PLAN) is a complex group of neurodegenerative diseases that result from mutations in a gene known as PLA2G6. According to the age of onset and clinical features, PLAN can be mainly classified into four subtypes: infantile neuroaxonal dystrophy (INAD), atypical neuroaxonal dystrophy (ANAD), adult-onset dystonia-parkinsonism (DP) and autosomal recessive early-onset parkinsonism (AREP). The onset of INAD and ANAD occurs in childhood and these diseases manifest as progressive psychomotor deterioration, axial dystonia, spasticity, and ataxia, as well as optic atrophy in some children. Cerebellar cortical atrophy and iron deposition in the globus pallidus and substantia nigra can be detected by Magnetic Resonance Imaging (MRI) in most patients (1, 2). Past research has suggested that PLAN can be classified as neurodegeneration with brain iron accumulation II (NBIA II) (3, 4). However, although there is a phenotypical intersection between NBIA and PLAN, we propose that neither disease can completely include the other. The onset of DP and AREP occurs in adulthood and patients often have normal birth, achieve early age mile-stones and have a normal childhood. Patients with DP or AREP show clinical manifestations of parkinsonian syndrome. These patients are characterized by bradykinesia and tremors with occurrence of dystonia, in addition to cognitive regression as well as gait instability. Some symptoms are similar to those of parkinsonism, but cerebellar cortical atrophy and iron deposition do not occur in these patients. A high clinical variability is exhibited in these phenotypes, but age of onset and clinical manifestations are the main criteria used to make distinctions between the subtypes of PLAN.

In 2006, the PLA2G6 gene was initially cloned in two unrelated Israeli INAD families, both of which included consanguineous marriages (5). In 2010, for the first time, the PLA2G6 gene mutation was associated with parkinsonism (6). At present, the pathogenesis of the PLA2G6 mutation in neurodegenerative diseases remains unclear. Different mutations and even mutations at the same site may cause phenotypic disparities. For example, the presence of pathogenic PLA2G6 gene mutation sites (p. D331Y) reported in AREP patients can be associated with a 70% decrease in enzyme activity (7), and the (His597fx69) frameshift mutation can cause the activity of enzymes to differ from previous cases (8). Here, we speculate whether different mutations can result in the diversity of enzymatic activity, thus causing different clinical phenotypes. In this review, we demonstrate clinical phenotypes with different genotypes in PLAN and discuss the underlying relationships of these symptoms with evidence from genetic studies, with a primary focus on the clinical manifestations and genotypic features supported by neuropsychology research, neuroimaging and molecular genetics. Finally, we explore the link between phenotypes and genotypes for PLAN in the light of current PLA2G6 gene research.

The most common phenotype of the PLA2G6 gene mutation is NBIA II. PLAN mainly includes INAD, ANAD, and two other diseases are present in parkinsonian syndrome, DP and AREP (9). In addition, some patients present sporadic parkinsonism similar to AREP, known as sporadic early-onset parkinsonism (EOP) (10). PLA2G6 mutations that cause phenotypical clinical characteristics are shown in Table 1. Moreover, these mutations are associated with hypothyroidism, schizophrenia, diabetes and other diseases (11–13). On magnetic resonance images, most patients carrying the PLA2G6 mutation showed an iron accumulation in the globus pallidus and/or the substantia nigra in T2-weighted images (13, 14). In pathological examinations of individuals with the PLA2G6 mutation, abnormal α-synuclein proteins and hyperphosphorylation of tau proteins were found, and may progress to become Lewy bodies (LBs), neurofibrillary tangles and neuropil threads (15, 16). Neuronal biopsies of patients' central nervous systems and peripheral nervous systems tissue provided evidence of the presence of axonal distension, swellings and spheroid bodies (17). In other cases, brain tissue sections with Perl's staining showed iron deposition in the globus pallidus or substantia nigra (18), and oculogyric crises were also found in PLAN patients (19). Here, we describe the clinical features of several PLA2G6-associated neurodegenerative diseases.

PLA2G6 mutations have both clinical and genetic heterogeneity. Patients have different types of PLA2G6 mutations, including missense mutations, truncation mutations, and copy number variations. Individuals carrying the PLA2G6 mutation can also have different clinical phenotypes depending on specific genotypic features. The previous reported mutation sites of PLA2G6 are listed in Figure 1 (1, 2, 6–8, 13, 15–18, 22, 23, 30, 34, 38, 41–43, 45, 47, 51–69). Reportedly, in two PLA2G6 mutation families, all three patients carried PLA2G6 mutations (p.R632W) but presented different clinical manifestations than previously reported (13, 43). Shi et al. (7) also suggested that the incomplete loss of enzymatic activity causes AREP. Moreover, expansive copy number variants (CNVs) have been detected in the development of PLAN (27, 57). The crystal structure of iPLA₂β is complex, and the different mutation sites are disparately located on the enzyme (70). According to previous studies, mutation sites in the ankyrin repeat (AR) domains, catalytic(CAT) domains, or any other domains may lead to different enzyme activities. We speculate that the coincident mutation of PLAN may initiate the pathological mechanisms. The pivotal factor that affects the relationships between mutations and clinical phenotypes may be enzyme activity. However, more cases are needed to clarify the relationships between genotypes and phenotypes of PLA2G6.

The PLA2G6 gene is located on 22q13.11 (71), with 17 exons. The protein, encoded by PLA2G6, is a member of the A₂ phospholipase family (PLA₂), known as group VI calcium-independent phospholipase A₂ (iPLA₂β). It is an enzyme functioning in inflammation, immune responses, cell proliferation, apoptosis and remodeling of membrane phospholipids (36, 72).

iPLA₂β is an intracellular and calcium ion-independent protein. This protein was first isolated from the P388D1 cell line and described in 1994 (73). iPLA₂β contains 806 amino acids; human iPLA₂β is 88 KDa and contains an N-terminal domain, ARs domain and CAT domains (70, 74, 75). The iPLA₂β protein encoded by the PLA2G6 gene is an important lipase in the human body which is widely distributed in the tissues of human organs (http://www.proteinatlas.org). In the human brain, iPLA₂β is highly expressed in the substantia nigra, cortex and the hippocampus (76–78). iPLA₂β can hydrolyse the sn-2 acyl chain of phospholipids and the major decomposition products are docosahexaenoic acid (DHA) and lysophospholipids (75). Under the action of cyclooxygenase and lipoxygenase, DHA produces neuroprotectin D1 (NPD1), and NPD1 plays a crucial role in anti-inflammatory processes and immune responses in the brain (79, 80). NPD1 is specifically involved in the catabolism of fatty acids and arachidonic acid (AA)-related inflammatory reactions (81, 82). iPLA₂β exerts an anti-inflammatory function through the action of NPD1, against AA, and has a protective effect on cells in inflammatory reactions. The loss of iPLA₂β's function may affect proteins and processes normally involved in regulating the movement of membranes within axons and dendrites, subsequently leading to mitochondrial abnormalities and synaptic transmission impairment (83–86). Moreover, animal models of PLA2G6 showed neurodegeneration (87) and revealed that iPLA₂β is closely related to dopaminergic cells, axonal development (88), endoplasmic reticulum stress, mitophagy impairment (89), and changes in Ca(2+) signaling (90). Thus, these animals provide great disease models for PLAN.

iPLA₂β is associated with a variety of diseases and medical emergencies, including strokes, spinal cord injuries and neurodegenerative diseases (91–93). However, the pathogenesis of PLA2G6 in neurodegenerative diseases remains unclear and the function of iPLA₂β, resulting from different mutation sites and types, may be the vital factor. It was reported that pathogenic PLA2G6 gene mutation sites (p.A341T, p.G517C) in patients with INAD\NBIA can cause significant decreases in enzymatic activity (94). Moreover, the activity of iPLA2β was reduced by 70% compared to normal functioning, owing to the p.D331Y homozygous mutation (7) and the (His597fx69) frameshift mutation, making the activity of enzymes <6% compared to that of WT iPLA₂β (8). Differential enzymatic activity caused by multifarious mutations may be a key factor in explaining the high clinical variability in PLAN.

PLA2G6 mutations have both genotypic and phenotypic heterogeneity. Here, we summarized the major subtypes of PLAN and analyzed their potential relationships. Mutated forms of PLA2G6 include missense mutations, truncated mutants, fragment deletions, and CNVs. Individuals carrying different PLA2G6 mutations may also display varied clinical symptoms. The subtypes of PLA2G6 mutation-related disorders are INAD, ANAD, DP, and AREP, with distinct characteristics associated with each disorder. In recent years, HSP has also been found to be associated with the PLA2G6 gene mutations. Some mutation sites of HSP also coincide with the mutation sites of the previous four phenotypes. Whether HSP can be considered a PLAN phenotype requires additional research. In the cases of INAD/ANAD, an MRI exhibited iron accumulation in the basal ganglia and globus pallidus, as well as abnormal α-synuclein and hyperphosphorylation of tau proteins in brain tissues, while other cases had relatively moderate MRI features. Moreover, α-synuclein and neurofibrillary tangles pathologies indicate that PLAN may be consistent with idiopathic Parkinson's disease (iPD) to some extent. Interestingly, the view that later onset cases tend to have less tau involvement but still severe α-synuclein pathology may need further discussion. (3, 15, 16, 95).

The link between the phenotypes and genotypes of PLAN suggests that different mutation sites lead to various protein activities. Mutation sites in different domains, for example, ARs or the CAT domains, may have different effects to physiological processes. Based on a previously described case, a PLA2G6 gene mutation site can result in patients presenting several different phenotypes. Potentially, enzyme activity as well as DNA methylation, synergistic genetic processes, or environmental factors may participate in the pathogenesis of PLAN. Based on the information presented in Figure 1, we speculate that there is no obvious rule for mutation site distribution of PLAN, but further studies are required to clarify why there is no mutation localized in exon 9 and whether the mutations can be categorized according to the 3D structure of iPLA₂β (70).

In recent decades, the study on PLA2G6-related disorders has been performed in terms of pathogenesis and iPLA₂β function, which provides hope of a detailed understanding of this disease. iPLA₂β is a vital protein involved in immune responses, inflammatory processes, fatty acid metabolism, oxidative stress, and apoptosis, which may be the pathogenic mechanisms underlying the progression of neurodegenerative diseases. iPLA₂β is involved in the metabolism of DHA and NPD1, which are closely related to human neurocognitive development and anti-inflammatory properties, respectively (96). Whether mental retardation manifesting in young children is caused as a result of iPLA₂β affecting the metabolism of DHA is unclear; a better understanding would perhaps provide new information regarding the treatment of PLAN. In addition, iron metabolism may offer clues regarding disease therapy. In summary, the features of PLAN may provide information regarding the etiology of other neurodegenerative diseases.

YG conceived the study and wrote the manuscript. BT and JG discussed and revised the manuscript. YG prepared the tables. All authors read and approved the final version of the manuscript.