Pantothenate Kinase-Associated Neurodegeneration; PKAN-NBIA1 (PANK2; Online Mendelian Inheritance in Man (OMIM)*606157)

PKAN was first described by two German physicians, Hallervorden and Spatz, so it was named Hallervorden–Spatz disease. However, due to their cooperation with the Nazi party today it is known as PKAN. It is the most common form of NBIA, which includes almost half of the cases (Figure 1). The exact prevalence of PKAN is yet unknown, but it is estimated to be 1–3/1,000,000. ^{Reference Di Meo and Tiranti2,Reference Hayflick, Kurian and Hogarth4} It can occur from early childhood to middle age. There is an association between the age at onset and the disease course. The early-onset form of PKAN has a higher rate of progression than the late-onset form of the disease. ^{Reference Hayflick, Kurian and Hogarth4}

Although this NBIA is reported in many ethnicities and the dominant clinical picture is similar, there have been some variations in different populations. Lee et al. have found that cranial dystonias were less common in Asians than in Caucasians, and Asians tended to have segmental dystonia, whereas Caucasians were more likely to present generalized dystonia. In classic PKAN, dysarthria was more frequent in Asians, but parkinsonism in Caucasians. Pyramidal signs and mental impairment were less prevalent in Asians. ^{Reference Lee, Lu and Chuang7}

Classic Form of PKAN

This form is characterized by an early start and rapid progression. It occurs due to complete loss of function mutations in the PANK2 gene and often develops before the age of 6 years (average age at onset is 3 years old with a median survival of 9.5 years after onset). The main presentation of PKAN is considered to be a gait disturbance. Most of the affected children to the classic form of PKAN show a history of mild growth and developmental delay as well as dyspraxia before dystonia and spasticity. Pigmented retinopathy is a common feature in classical PKAN, but seizures are not. Although PKAN is known as neuroacanthocytosis disorder, the acanthocytosis, abnormal red blood cell morphology, is found in only some patients; since the high sensitivity genetic tests are available in many countries, this feature of erythrocytes is not a common diagnostic criterion. ^{Reference Hayflick, Kurian and Hogarth4,Reference Chang, Zhang, Jiang, Wang and Wu8}

Atypical Form of PKAN

In this form of PKAN, the clinical features are different from the classic form and start at later age (with a median age at onset of 18 years and 2% mortality in the 10 years follow up after onset). ^{Reference Chang, Zhang, Jiang, Wang and Wu8} It is often presented with speech abnormalities. Progression happens at a slower pace, and it is the result of an incomplete loss of function mutations in the PANK2 gene. Other features of this form include dystonia and dysarthria. Among neuropsychiatric impairments in patients with the atypical form of PKAN, impulsivity and obsessive-compulsive disorder are the most common manifestations. ^{Reference Hayflick, Kurian and Hogarth4} Pigmentary retinopathy is rare in this form, though extraocular movement abnormalities are common. ^{Reference Hogarth9}

Genetics of PKAN

The pattern of inheritance is AR and the disease results from mutations in the PANK2 gene encoding for pantothenate kinase 2 within the 20p13 chromosomal position. ^{Reference Di Meo and Tiranti2,Reference Hartig, Hörtnagel and Garavaglia10} Currently, according to Human Gene Mutation Database (HGMD) professional 2019.4, 188 mutations in PANK2 have been reported. Chang et al. have proposed that the proportion of the patients carrying two null alleles is significantly higher in early-onset (classic) patients compared to late-onset (atypical) cases (27.5% vs. 0.8%). The null allele was presumed as “loss-of-function” allele. ^{Reference Chang, Zhang, Jiang, Wang and Wu8}

Pathophysiology of PKAN

PANK2 enzyme is a CoA sensitive sensor in the mitochondrial inner membrane. ^{Reference Leonardi, Rock, Jackowski and Zhang11} PANK2 mutations lead to CoA metabolism impairments as a result of defects in the pantothenate kinase enzyme, leading to the accumulation of some of the CoA pathway substrates such as cysteine and compounds containing cysteine in basal ganglia. The result of these events is the iron accumulation in GP under the influence of rapid, toxic, and spontaneous cysteine oxidation, which leads to free radical production. Since basal ganglia and retina are sensitive to oxidative stress, the involvements of these sites are more prominent in PKAN patients. ^{Reference Gregory and Hayflick12,Reference Bokhari and Bokhari13} A specific pattern of iron accumulation is detected in many PKAN cases, which is called the “eye of tiger” sign (Figure 2A and B). ^{Reference Sethi, Adams, Loring and El Gammal6}

Figure 2: Brain magnetic resonance imaging (MRI) of different subtypes of NBIA from Iran. (A & B) Eye of tiger sign pattern (red arrow) in MRI of pantothenate kinase-associated neurodegeneration (PKAN) patients. (C) cerebellar atrophy (orange arrow) in phospholipase A2-associated neurodegeneration (PLAN). (D & E) iron deposition in globus pallidus (GP) with preserved medial medullary lamina in MPAN. (F & G) Halo sign in cerebral peduncles (substantia nigra [SN]) on T1-weighted sequences (empty red arrows) and iron deposition in the corresponding T2-weighted sequences in SN (green arrow) in a Beta propeller-associated neurodegeneration (BPAN) patients. (H) deep white-matter changes suggesting leukodystrophy (green arrows) in a fatty acid hydroxylase-associated neurodegeneration (FAHN) patient. (I–K) T2-weighted hypointensity signal in GP (red arrows) and SN and small pituitary gland (K) in the Woodhouse–Sakati syndrome (WSS) patients. (L–N) Susceptibility weighted imaging (SWI) in three affected to JABELS showing mild hypointensity in GP (empty red arrows) compatible with iron deposition.

Infantile-Onset PLAN or Infantile Neuro-Axonal Dystrophy (INAD)

Early-onset form of disease emerges between the age of 6 months to 3 years (around 2 years) with growth retardation, hypotonia, and spastic tetraparesis. ^{Reference Gregory, Westaway and Holm14–Reference Guo, Tang and Guo16} Many of the affected children can never walk. Seizure and ophthalmological impairment (strabismus, nystagmus, and optic atrophy) are prominent in these patients and death occur in infancy and early childhood. ^{Reference Guo, Tang and Guo16}

Childhood-Onset PLAN or Atypical Neuro-Axonal Dystrophy

Atypical neuro-axonal dystrophy shows more phenotypic variations in comparison with INAD. The age at onset can vary from early childhood (usually after 3 years) to the end of the second decade of life. These children show ataxia, dystonia, spasticity, delay in speaking, and autistic symptoms with degrees of intellectual disability. Strabismus, nystagmus, optic atrophy, and neuropsychological disorders are also common. These patients have slower progression and longer survival times compared to INAD cases. ^{Reference Hayflick, Kurian and Hogarth4,Reference Guo, Tang and Guo16}

Adult-Onset PLAN or Dystonia–Parkinsonism

Patients affected with this form present their symptoms at various ages, but most patients present symptoms early in adulthood (20–40 years). Patients manifest dystonia and Parkinsonism from late adolescence to early twenties and it might be accompanied by mild mental impairment. Survival for several decades is not unusual. ^{Reference Hayflick, Kurian and Hogarth4,Reference Guo, Tang and Guo16,Reference Paisan-Ruiz, Bhatia and Li17}

Genetics of PLAN

Mutation in PLA2G6, encoding for phospholipase A2, within 22q13.1 chromosomal position results in PLAN. The pattern of inheritance is considered AR. ^{Reference Morgan, Westaway and Morton18} Most of the patients have missense mutations that do not disrupt the catalytic activity of the protein. ^{Reference Engel, Jing, O’Brien, Sun and Kotzbauer19} . Currently, according to the HGMD professional 2019.4, 191 mutations in PLA2G6 have been reported.

Pathophysiology of PLAN

PLA2G6 encodes for a specific type of phospholipase, which catalyzes the release of fatty acid from phospholipid. It has been proposed that this gene plays a crucial role in phospholipid homeostasis, and the impairment of this enzyme increases the level of phospholipid and acetyl-CoA. Impairment in phospholipid homeostasis may change the lipid components of membrane, vesicles, or endosomes, resulting in membrane permeability, fluidity, iron homeostasis disturbance, and disruption of the autophagy process. ^{Reference Malik, Turk and Mancuso20,Reference Shinzawa, Sumi and Ikawa21} Alpha-synuclein and neurofibrillary tangles in the pathologic study of PLAN have suggested a link to Parkinson’s disease. Although some studies have implicated that PLA2G6 mutation is not a major risk factor for Parkinson’s disease in Asian populations. ^{Reference Guo, Tang and Guo16}

Genetics of MPAN

MPAN is caused by mutations in C19orf12 on 19q12, a gene encoding a mitochondrial membrane protein. ^{Reference Hartig, Iuso and Haack22} The pattern of inheritance is AR, but there are reports of autosomal dominant in some cases. ^{Reference Hogarth, Gregory and Kruer24} Currently, according to the HGMD professional 2019.4, 51 mutations in C19orf12 have been reported. The most frequent type of mutations affecting this gene are p.Gly69Argfs*10 and p.Thr11Met. ^{Reference Schneider3} Landoure and colleagues reported c.187G>C; p.Ala63Pro mutation in two sisters affected to hereditary spastic paraplegia type-43 (HSP-SPG43) without iron accumulation in their brain MRI. This mutation was also reported in at least two other families affected by MPAN, thus the determination of genotype–phenotype correlation is challenging. ^{Reference Gregory, Hartig, Prokisch, Kmiec, Hogarth and Hayflick26}

MPAN Pathophysiology

C19orf12 encodes for a small transmembrane protein localized in the mitochondrial membrane. The precise functionality remains to be unraveled, but based on several studies, fatty acid biosynthesis and destruction of branched amino acids (valine, leucine, and isoleucine) are two prominently involved pathways. ^{Reference Hartig, Iuso and Haack22} Studies on postmortem affected samples showed increased alpha-synuclein in stained tissue and Lewy body formation in the brain. Also, it seems that spheroidal eosinophilic structures in GP are byproducts of neuronal degeneration. More investigations are needed to understand the exact pathophysiology of mutations in this gene. ^{Reference Hogarth, Gregory and Kruer24}

Genetics of BPAN

BPAN is the only form of NBIA with an X-linked dominant pattern of inheritance, resulting from mutations in the WDR45 gene on Xp11.23. This gene encodes an autophagy protein, beta-propeller. ^{Reference Haack, Hogarth and Kruer27} Up to now, most of the BPAN cases were as a result of de novo mutations, and some affected men were mosaic; probably because of the post-zygotic mutations. By the development of next generation sequencing and whole exome sequencing (WES), many cases were identified and the spectrum of clinical features has been widening. ^{Reference Hayflick, Kurian and Hogarth4} Currently, according to the HGMD professional 2019.4, 102 mutations in WDR45 have been reported.

Pathophysiology of BPAN

In this form of the disease, there are numerous findings indicating autophagy pathway impairments. WDR45 protein is a member of a protein family that facilitates the formation of the protein complex and plays an essential role in many biological processes such as signaling, apoptosis, and gene expression regulation. WDR45 protein can bind to Autophagy Related 2A (ATG2A) and Autophagy Related 2B (ATG2B) that are two other autophagy-related proteins, indicating the affiliation of this protein in the autophagy process. ^{Reference Behrends, Sowa, Gygi and Harper32,Reference Meyer, Kurian and Hayflick33} Lymphoblastic cell line in patients affected to BPAN, with severe decrease of WDR45 protein and lower autophagy function rate leads to abnormal accumulation of initial autophagy structures. ^{Reference Saitsu, Nishimura and Muramatsu34} Neuropathological investigations of BPAN affected patients showed severe neurodegeneration, axonal spheroids, siderophages, and reactive astrocytes. Abnormality in the autophagy process leads to the accumulation of degraded components in the cell, toxic conditions, and cell stress resulting in neuronal degeneration. ^{Reference Hayflick35} Iron accumulation in BPAN patients in substantia nigra (SN) resembles a halo sign pattern on T1-weighted sequences of MRI ^{Reference Kruer, Hiken and Gregory36} (Figure 2F).

Genetics of CoPAN

Mutations in COASY, showing an AR pattern of inheritance, are the genetic causes of CoPAN. COASY is located on 17q21, which encodes the CoA-synthase enzyme. ^{Reference Dusi, Valletta and Haack37} Currently, according to the HGMD professional 2019.4, five mutations in COASY have been reported.

Pathophysiology of CoPAN

COASY is a critical enzyme in CoA biosynthesis. Defects in this gene lead to a reduction of CoA and acetyl-CoA levels. Acetyl-CoA is involved in the regulation of the autophagy process. Investigation of CoPAN patients shows strong links between lipid metabolism, CoA synthesis, and autophagy in NBIA pathology. In patients’ fibroblasts culture, it has been shown that COASY protein and acetyl-CoA levels are lower than usual significantly. ^{Reference Meyer, Kurian and Hayflick33,Reference Dusi, Valletta and Haack37,Reference Martinez, Tsuchiya and Gout40,Reference Mariño, Pietrocola and Eisenberg41}

Genetics of FAHN

Defects in this gene were described in HSP (HSP-SPG35) and since then, mutations in this gene were introduced in movement disorders with brain iron accumulation. ^{Reference Kruer, Paisán-Ruiz and Boddaert42} Nowadays, HSP-SPG35 is regarded as a type of FAHN disease because of the pattern of iron accumulation in basal ganglia in many reported cases. ^{Reference Hayflick, Kurian and Hogarth4} FAHN is due to AR mutations in FA2H on 16q23.1, ^{Reference Di Meo and Tiranti2} also there are some reports of uniparental disomy. ^{Reference Gregory, Venkateswaran, Hayflick, Adam, Ardinger and Pagon45} Currently, according to the HGMD professional 2019.4, 65 mutations in FA2H have been reported.

Pathophysiology of FAHN

FA2H enzyme is a 2-hydroxylase enzyme expressed in Schwann cells and oligodendrocytes. ^{Reference Alderson, Rembiesa, Walla, Bielawska, Bielawski and Hama46} 2-hydroxy sphingolipids produced by FA2H are the most abundant lipids in the myelin sheath. Mutations in this gene lead to a decreased level of enzyme activity and, subsequently, myelin sheath degeneration. ^{Reference Meyer, Kurian and Hayflick33,Reference Zöller, Meixner and Hartmann47} FA2H is also a lipid raft movement regulator. Lipid rafts are cholesterol and sphingolipid structures of membrane involved in membrane transporting and signaling. Guo and colleagues showed that FA2H-dependent hydroxylation affects membrane protein transport due to lysosomal degradation. Based on these findings, defects in FA2H lead to changes in the 2-hydroxy sphingolipid profile that can affect membrane lipids. ^{Reference Meyer, Kurian and Hayflick33,Reference Guo, Zhou, Pryse, Okunade and Su48}

Genetics of Neuroferritinopathy

Neuroferritinopathy is due to mutations in the FTL gene, which encodes the light chain of ferritin. Ferritin is regarded as the main iron storage protein. FTL is located on 19q13.33. Mutations in this gene are transmitted by an autosomal-dominant pattern of inheritance with complete penetrance. Most of these mutations cluster in the fourth exon. Cataract hyperferritinemia syndrome is due to a mutation in the 5′-untranslated region (5′UTR) in FTL with no neurological symptoms. ^{Reference Di Meo and Tiranti2,Reference Hayflick, Kurian and Hogarth4} Currently, according to the HGMD professional 2019.4, 66 mutations in FTL have been reported, and interestingly, 36 mutations are in regulatory regions of the gene.

Pathophysiology of Neuroferritinopathy

The affected individuals’ fibroblast culture revealed the accumulation of ferritin in the cytoplasm and nucleus, the lower expression of transferrin receptor, and the higher expression of divalent metal transporter 1 and ROS formation. Defect in ferritin leads to a higher level of unstable iron in cells, therefore increases the ROS production and oxidized protein, decreasing the proteasome proficiency. ^{Reference Cozzi, Rovelli and Frizzale60} Pathologic investigations showed ferritin inclusion body in the cytoplasm and nucleus of glia and CNS neurons and also in other organs related to iron.

NBIA7 (RalA Binding Protein 1 (RALBP1) Associated Eps Domain Containing 1 (REPS1); OMIM*614825)

Drecourt and colleagues reported two sisters with unrelated healthy parents who originated from France. The clinical features in the first sister were speech and motor delay, hypotonia, progressive cerebellar ataxia, losing the ability to walk at the age of nine, and nystagmus. Dysarthria, dysmetria, spasticity of the lower limbs, and pes cavus were also reported. Cerebellar and cerebral atrophy and iron accumulation in the GP and cerebral peduncles showed on the brain MRI. She eventually died at the age of 20 years. The second sister had the same but milder features. She can walk yet but with help at the age of 14 years. WES identified a compound heterozygous variant, c.232G>C and c.338C>A, in the REPS1 gene. Both variants were in an epsin-15 homology domain, a conserved region that interacts with RALBP1 to form the endosome recycling compartment. REPS1 is located on 6q24.1 and encodes RALBP1-associated Eps domain-containing protein-1. This protein is affiliated to autophagy because of its involvement in endocytosis, vesicle transport, localization in the cytoplasm, and endosome. This type of disease is known as NBIA7. ^{Reference Drecourt, Babdor and Dussiot82,Reference Levi and Tiranti83}

NBIA8 (Carnitine O-Acetyltransferase (CRAT); OMIM*600184)

Drecourt et al. also reported another subtype of NBIA in a Turkish girl with first cousin parents. Clinical manifestations include hypotonia, brisk deep tendon reflexes, sensory neuropathy, tremor, dysmetria with evidence of cerebellar atrophy, posterior leukoencephalopathy, hyperintensity of basal ganglia, hypointensity of GP, and SN which is suggestive for iron accumulation. Single-nucleotide polymorphism (SNP) genotyping and WES revealed a homozygous variant c.962G>A in the CRAT gene. This gene is located on 9q34.11 and encodes carnitine O-acetyltransferase. The encoded protein is localized in mitochondria and catalyzes the reversible transfer of acyl groups from carnitine to CoA and regulates the acyl-CoA/CoA ratio. It also plays a crucial role in the transport of fatty acids for beta-oxidation. This type of disease is known as NBIA8. ^{Reference Drecourt, Babdor and Dussiot82,Reference Levi and Tiranti83}

Hereditary Spastic Paraplegia-50; HSP-SPG50 (Adaptor Related Protein Complex 4 Subunit Mu 1 (AP4M1); OMIM*602296) or AP4M1-Associated NBIA

This gene encodes a subunit of the heterotetrameric AP-4 complex. AP4 is composed of four chains: beta-4 and epsilon-4 (AP4B1 and AP4E1 large chains), mu-4 (AP4M1 medium-chain), and sigma-4 (AP4S1 small chain). AP4M1 gene is located on 7q22.1 and encodes the mu subunit of the adaptor protein complex-4. This gene is ubiquitously expressed in the CNS and plays a role in vesicle formation, post-Golgi protein trafficking, and sorting processes. ^{Reference Levi and Tiranti83,Reference Tüysüz, Bilguvar and Koçer84} Although mutations in genes encoding the different subunits of adaptor protein complex-4 have been reported in patients affected to different types of HSP, Roubertie and colleagues in 2018 reported the clinical features of three patients originating from a large consanguineous Moroccan family who exhibited early-onset developmental delay, juvenile motor function deterioration, spasticity, and pyramidal tract signs and intellectual deficiency in which brain MRI findings were compatible with disorders in the NBIA spectrum. Their MRI showed the isointense pattern of the GP and hypointensity of the GP on T1 and T2 sequences, respectively. WES was performed and revealed a homozygous mutation c.916C>T;p.Arg306* in the AP4M1 gene. These data show the overlap between NBIA and hereditary spastic paraplegia. ^{Reference Roubertie, Hieu and Roux85} Previously, Najmabadi and colleagues reported a mutation in AP4M1 in a patient with microcephaly and paraplegia. ^{Reference Najmabadi, Hu and Garshasbi86}

PARK15 (FBXO7; OMIM*605648) or FBXO7-Associated NBIA

The F-box only protein 7 (FBXO7) gene is located on 22q12.3 and encodes a protein involved in the ubiquitin–proteasome protein-degradation pathway and mitophagy. In 2008, Shojaee and colleagues performed a linkage analysis in a large Iranian family with Parkinsonian-pyramidal syndrome and found a region on chromosome 22 linked to disease. Sequencing was done for candidate genes in that region which revealed a disease-associated variant in the FBXO7 gene, PARK15. Recently Correa-Vela and colleagues reported a novel homozygous mutation, c.368C>G; p.S123*, in FBXO7 in a child with spastic paraplegia, epilepsy, cognitive decline, cerebellar degeneration, levodopa nonresponsive Parkinsonism. Brain MRI revealed iron deposition in the GP and SN appeared as a hypointense signal on iron-sensitive sequences which is compatible with NBIA. All these findings suggest that FBXO7-associated NBIA can be a novel subtype of the NBIAs and there are clinically and genetically overlap between NBIA and other neurodegenerative disorders. ^{Reference Correa-Vela, Lupo and Montpeyó87,Reference Shojaee, Sina and Banihosseini88}