Neurodegeneration With Brain Iron Accumulation 4

A number sign (#) is used with this entry because of evidence that neurodegeneration with brain iron accumulation-4 (NBIA4) is caused by homozygous, compound heterozygous, or heterozygous mutation in the C19ORF12 gene (614297) on chromosome 19q12. All pathogenic heterozygous mutations have occurred in exon 3, the last exon of C19ORF12, and have resulted in premature termination.

Description

Neurodegeneration with brain iron accumulation-4 (NBIA4) is a neurodegenerative disorder characterized by progressive spastic paraplegia, parkinsonism unresponsive to L-DOPA treatment, and psychiatric or behavioral symptoms. Other neurologic features, including optic atrophy, eye movement abnormalities, dystonia, dysphagia, dysarthria, and motor axonal neuropathy, may occur. Brain MRI shows T2-weighted hypointensities in the globus pallidus and substantia nigra. Onset is usually in the first 2 decades, but later onset has been reported (summary by Dogu et al., 2013). There is phenotypic variation: some patients may not have extrapyramidal signs and may have muscle weakness and atrophy as well as cognitive impairment or developmental delay (Deschauer et al., 2012). Both autosomal recessive and autosomal dominant inheritance have been reported (see INHERITANCE and MOLECULAR GENETICS).

For a general phenotypic description and a discussion of genetic heterogeneity of NBIA, see NBIA1 (234200).

Clinical Features

Hartig et al. (2011) reported 24 Polish patients with neurodegeneration with brain iron accumulation, including 9 patients from 4 families with 2 or 3 affected sibs. The diagnosis was based on hypointensity in the globus pallidus documented by T2-weighted cranial MRI. Eighteen of the patients carried the same deletion mutation (614297.0001) and had a relatively homogeneous phenotype. The mean age at onset was 9.2 years (range 4 to 20), but most presented before age 11. The most common presenting symptom was gait or speech difficulty, followed by extrapyramidal signs, oromandibular and generalized dystonia, and parkinsonism. Most had progressive involvement of the corticospinal tract, with spasticity, hyperreflexia, and extensor plantar responses. Most retained the ability to walk, but 5 had loss of ambulation requiring a wheelchair. Eight of 18 (44%) developed a motor axonal neuropathy, and all had variable optic atrophy. Six of 24 patients had psychiatric signs, such as impulsive or compulsive behavior, depression, and emotional lability. None had seizures. Neuropathologic studies from 1 patient showed intracellular iron-containing deposits, axonal spheroids, numerous Lewy bodies and Lewy body-like inclusions, sparse Lewy neurites, and hyperphosphorylated MAPT (157140)-containing neuronal inclusions in various regions. Iron-containing deposits were concentrated in the globus pallidus and the substantia nigra. There was loss of myelin in the pyramidal tracts and optic tract.

Horvath et al. (2012) reported 2 brothers, born of consanguineous Turkish parents, with NBIA4. After early normal development, each presented at age 7 years with subtly progressive neurologic symptoms, including intention tremor, dysarthria, cognitive decline, progressive visual impairment due to optic atrophy, and spasticity with hyperreflexia and extensor plantar responses. They both remained ambulatory. Other features included facial hypomimia, orofacial dystonia, weakness and atrophy of the distal muscles, and axonal peripheral motor neuropathy. Laboratory studies showed increased serum creatine kinase; EMG showed neurogenic changes. Brain MRI in the teenage years showed low-signal intensities in the globus pallidus and substantia nigra ('eye-of-the-tiger' sign). L-DOPA therapy was ineffective.

Deschauer et al. (2012) reported 3 patients from 2 unrelated families with NBIA4. Two sibs were 27 and 17 years of age at the time of the report. Both had upper and lower motor neuron signs with pes cavus, winged scapula, and calf atrophy, and some difficulty walking. One had hyporeflexia with extensor plantar responses and atrophy of the small muscles of the hand, whereas the other had hyperreflexia with clonus. Nerve studies showed reduced amplitudes with normal conduction times consistent with axonal neuropathy. Both patients also had cognitive impairment with disinhibited and impulsive behavior; the younger sib had a history of global developmental delay since age 3 years. The third patient had a similar disorder with onset of gait difficulties at age 9 years, distal muscle weakness, hyperreflexia, pes cavus, atrophy of the thenar muscles, learning difficulties, visual impairment due to optic atrophy, and emotional lability. Extrapyramidal signs, such as parkinsonism, were not present in these patients. Serum creatine kinase was mildly increased consistent with neurogenic atrophy. Brain MRI of all 3 patients showed T2-weighted hypointensities in the globus pallidus with some hypointensities also in the substantia nigra and cerebral peduncles, consistent with iron deposition. Deschauer et al. (2012) noted that the phenotype was reminiscent of juvenile-onset amyotrophic lateral sclerosis (ALS).

Hogarth et al. (2013) reported 23 patients with MPAN confirmed by genetic analysis. The mean age at onset was 11 years (range 4 to 30 years). Patients most often presented with gait abnormalities and less commonly with visual impairment due to optic atrophy. Gait abnormalities were followed by weakness and upper motor neuron signs, including hyperreflexia affecting the lower limbs more than the upper limbs, and extensor plantar responses. Later in the disease, lower motor neuron dysfunction became prominent, with a distal-to-proximal loss of reflexes and progressive muscle atrophy. Other features included optic atrophy (in 74% of patients), dysarthria, dysphagia, dystonia, incontinence, and parkinsonism with a variable response to levodopa treatment. All patients developed cognitive decline progressing to dementia, and most had neuropsychiatric changes, such as inattention, hyperactivity, emotional lability, and depression, as well as stereotypic hand movements. EMG confirmed the presence of a motor axonopathy in 9 individuals. Two patients who underwent skin or nerve biopsy showed axonal spheroids. Radiographic findings included increased iron deposition in the globus pallidus and substantia nigra. Seven patients had cortical atrophy and 3 had cerebellar atrophy. Neuropathologic studies of 1 patient affected with dementia and parkinsonism showed neuronal loss, gliosis, large eosinophilic axonal spheroids, and widespread deposition of Lewy bodies that stained with alpha-synuclein (SNCA; 163890). Lewy bodies were present in the cortical, subcortical, and brainstem regions.

Dogu et al. (2013) reported 3 affected individuals from 2 unrelated consanguineous Turkish families with NBIA4. All patients had a relatively late onset of the disorder, between 25 and 29 years of age, and 2 showed very rapid progression leading to death 12 and 36 months after admission, respectively. The features were typical of the disorder, with progressive parkinsonism unresponsive to L-DOPA therapy, pyramidal signs with hyperreflexia, and tremor. More variable features included dysarthria, dysphagia, slowed saccadic eye movements, and dystonia. Brain MRI showed hypointensity in globus pallidus and substantia nigra. Two of the 3 patients had cognitive impairment with behavioral abnormalities. Dogu et al. (2013) suggested that the phenotypic variation in these patients compared to previously reported NBIA4 patients may be due to the involvement of other genetic, epigenetic, or environmental factors.

Landoure et al. (2013) reported 2 sibs from a consanguineous Brazilian family with NBIA4. They had onset of walking difficulties due to spastic paraplegia in the second decade. Later features included distal muscle wasting and weakness, axonal sensorimotor neuropathy, and visual loss with optic atrophy. Both became wheelchair-bound in their thirties. One of the sibs had memory loss and depression. An unrelated boy with the disorder was also reported. He presented at age 4 years with speech difficulty followed by progressive spasticity and impaired walking. Other features included psychomotor slowness, weakness and atrophy of the distal extremities, and small, pale optic discs. EMG showed denervation consistent with a motor neuropathy, and brain MRI showed iron deposition in the globus pallidus. Genetic analysis in this boy identified a homozygous mutation in the C19ORF12 gene (614297.0007).

Kleffner et al. (2015) reported 2 sisters, born of consanguineous Turkish parents, with NBIA4 confirmed by genetic analysis (P83L; 614297.0008). The patients presented with progressive optic atrophy beginning at 13 and 8 years of age, respectively. Around age 16 years, both developed spastic gait. The older sister had cerebellar signs, including ataxia, dysdiadochokinesis, dysmetria, tremor, and dysarthria, but brain imaging at that time was normal. At age 21, the older sister showed progressive cognitive decline and her symptoms worsened significantly during her twenties. She developed dysphagia and bladder incontinence, and lost the ability to walk or stand. Brain imaging showed bilateral hypointense signals in the globus pallidus, the putamen, and the substantia nigra, as well as cerebellar atrophy. Brain imaging of the younger sister showed bilateral hypointensities in the basal ganglia and cerebellar atrophy. Functional studies of the C19ORF12 mutation and studies of patient cells were not performed, but Kleffner et al. (2015) noted that the same C19ORF12 variant had previously been identified in 2 other families with the disorder by Hogarth et al. (2013). The phenotype in the sisters was reminiscent of Behr syndrome (210000).

Kasapkara et al. (2018) reported a Turkish girl, born of consanguineous parents, with genetically confirmed NBIA4. She presented at age 14 with a 10-year history of frequent falls and recent onset of intellectual decline with psychiatric disturbances. A brain MRI showed moderate hypointense signal in the globus pallidus and substantia nigra on T2, which appeared mildly hyperintense on T1, suggestive of pathologic iron accumulation. At age 19, she presented with progressive cognitive deficits, spastic tetraparesis, and severe bilateral optic atrophy. She was ataxic, dysarthric, and unable to walk, with bladder and bowel incontinence. She also had levodopa-resistant orolingual and limb dystonia.

Monfrini et al. (2018) reported a girl, born of unrelated Italian parents, with features consistent with NBIA4. Language delay was noted at the age of 3 years. At age 5 years, she developed a progressive gait imbalance associated with lower limb rigidity and she later had onset of right hand dystonia. She also had mild hirsutism and precocious puberty. Eye involvement included low vision, hypermetropia, and astigmatism; fundus examination showed optic atrophy. Neurologic findings included moderate dysarthria, cervical dystonia, dysdiadochokinesia, mild intentional and postural upper limb tremor, spastic hypertonia of the lower limbs associated with movement-exacerbated dystonic postures of feet, patellar hyperreflexia, and bilateral Babinski sign. MRI showed hypointensity at the globus pallidus and substantia nigra. EMG showed a diffuse axonal motor neuropathy.

Gregory et al. (2019) reported 2 large multigeneration families with NBIA4, which they referred to as mitochondrial membrane protein associated neurodegeneration (MPAN), segregating in an autosomal dominant manner. Family 18 had 9 clinically diagnosed individuals in 3 generations. The proband (18-307) was a 39-year-old woman with an 18-month history of neurologic symptoms. On exam she had hypomimia, hypophonia, and left-sided predominant parkinsonism with rigidity and bradykinesia. Brain MRI showed markedly decreased signal in the globus pallidus and substantia nigra on T2-weighted signals consistent with brain iron accumulation. She died at age 42. Her brother (18-304) had been described by Morphy et al. (1989), who suggested that the family's disorder, then called Hallervorden-Spatz disease, could be the first autosomal dominant instance. Age of onset for this family ranged from 29 to 55 years, and death occurred within 2 to 9 years after diagnosis. Family 748 had 5 individuals in 3 generations with clinically diagnosed MPAN. The youngest known age of onset in this family was 29, with age at death ranging from 59 to 70s. Brain autopsy findings obtained from 1 affected member in each family were consistent with MPAN, including iron accumulation in the globus pallidus with profound neuronal loss, gliosis, axonal spheroids, and degenerating neurons. Alpha-synuclein (163890) staining was positive throughout with midbrain, limbic, and neocortical Lewy bodies and neurites. Tauopathy was limited. Beta-amyloid (see 104760) plaques and TDP43 (605078)-positive inclusions were not found. Gregory et al. (2019) reported 12 additional individuals from 11 families with autosomal dominant MPAN. Age of onset range from 18 months to 55 years. There were no phenotypic differences between the patients with dominantly inherited disease and those previously described, or 22 newly reported by Gregory et al. (2019), with recessively inherited disease.

Inheritance

Although an autosomal recessive pattern of inheritance had been demonstrated in well-characterized cases of NBIA4 (Hartig et al., 2011; Hogarth et al., 2013), Gregory et al. (2019) reported 18 patients from 13 families demonstrating autosomal dominant inheritance, including 2 large multigeneration families described in detail. Additionally, they reported 22 individuals from 19 families showing autosomal recessive inheritance. They proposed a model whereby 2 alleles carrying mutations that undergo nonsense-mediated decay (NMD) or result in loss of function are required to manifest disease, but only 1 allele carrying a nonsense or frameshift mutation that escapes NMD is required to manifest disease through a dominant-negative mechanism.

Molecular Genetics

Autosomal Recessive Inheritance

In 24 Polish patients with NBIA4, Hartig et al. (2011) identified homozygous or compound heterozygous mutations in the C19ORF12 gene (see, e.g., 614297.0001-614297.0004). Eighteen patients carried the same 11-bp deletion (614297.0001), and haplotype analysis indicated a founder effect. The mutation was initially found after genomewide linkage analysis of an affected family. Two unrelated patients who were compound heterozygous for 2 missense mutations (614297.0003 and 614297.0004) had a milder phenotype. One had only impairment of fine motor skills at age 19 years, and the other presented in his forties with parkinsonism and dystonia. Hartig et al. (2011) suggested the designation 'mitochondrial membrane protein associated neurodegeneration (MPAN)' for this disorder.

In 2 brothers, born of consanguineous Turkish parents, with NBIA4, Horvath et al. (2012) identified a homozygous mutation in the C19ORF12 gene (L121Q; 614297.0005). The mutation was found by homozygosity mapping followed by exome sequencing.

Deschauer et al. (2012) identified 3 different mutations in the C19ORF12 gene (614297.0001; 614297.0002; 614297.0007) in 3 patients from 2 unrelated families with NBIA4. All mutations occurred in compound heterozygosity.

In 23 of 161 individuals with NBIA, Hogarth et al. (2013) identified pathogenic mutations in the C19ORF12 gene. Seventeen patients from 16 families had biallelic mutations, whereas 6 patients from 4 families had heterozygous mutations; however, the authors considered a second occult deleterious mutation likely to be present in these patients. The 11-bp deletion (614297.0001) identified by Hartig et al. (2011) was recurrent in individuals of Eastern European origin.

In 3 affected individuals from 2 unrelated consanguineous Turkish families with NBIA4, Dogu et al. (2013) identified a homozygous mutation in the C19ORF12 gene (T11M; 614297.0002). The mutation was found by homozygosity mapping and candidate gene sequencing.

In 2 Brazilian sibs with NBIA4, Landoure et al. (2013) identified a homozygous missense mutation in the C19ORF12 gene (A63P; 614297.0006). The same homozygous A63P mutation was also found in 2 Malian sisters with autosomal recessive spastic paraplegia-43 (SPG43; 615043) without brain iron deposition or cognitive impairment. Haplotype analysis indicated a founder effect between the Malian and Brazilian families. Landoure et al. (2013) suggested that the phenotypic differences between the 2 families may be due to other genetic or environmental factors or stochastic effects, and concluded that C19ORF12 mutations cause a spectrum of neurologic disease that may or may not include brain iron deposition.

In a 19-year-old Turkish girl with NBIA4, Kasapkara et al. (2018) identified a novel homozygous 11-bp deletion (c.171_181del) in the C19ORF12 gene. Her consanguineous parents were heterozygous for the mutation.

Gregory et al. (2019) reported 22 individuals from 19 families with homozygous or compound heterozygous mutation in the C19ORF12 gene.

Autosomal Dominant Inheritance

In a girl with NBIA4, born of unrelated Italian parents, Monfrini et al. (2018) identified a de novo heterozygous mutation in the C19ORF12 gene (c.265_266delAT; 614297.0009). Sequencing of all exons and intron-exon boundaries of other known NBIA-associated genes did not detect any pathogenic variants. Quantitative reverse-transcription PCR in lymphocytes showed that mRNA quantity in the proband was not reduced compared to that of the parents and controls, suggesting absence of degradation through nonsense-mediated decay. Sequence analysis of the C19ORF12 transcripts revealed presence of the mutation in the proband and ruled out aberrant splicing. Several other tissues (hair, saliva, and urine) in addition to lymphocytes were examined and no evidence of mosaicism was seen. The authors hypothesized that the heterozygous variant is pathogenic, but noted that the patient may have a mutation in an unidentified causative gene.

Gregory et al. (2019) identified 18 patients with MPAN (NBIA4) from 13 families who carried heterozygous mutations in the C19ORF12 gene, all in exon 3 (e.g., 614297.0010-614297.0012). The authors proposed a mechanism to explain how MPAN can display both autosomal recessive and autosomal dominant inheritance. In autosomal recessive MPAN, a mutation resulting in premature termination of the protein occurring in the first or second exon would be predicted to undergo nonsense-mediated decay (NMD). From parental studies, partial C19ORF12 loss of function from one such allele causes no neurologic problems. Therefore, 2 such mutant alleles are required to manifest disease. The homozygous and compound heterozygous cases presented by Gregory et al. (2019) comprised a mix of frameshift mutations predicted to undergo NMD, and missense and splicing mutations throughout the gene that were predicted to result in loss of function. On the other hand, truncating mutations occurring in the final exon of a gene or near the 3-prime end of the penultimate exon typically escape NMD; all heterozygous MPAN mutations reported by Gregory et al. (2019) were frameshift or nonsense mutations in the third and final exon of C19ORF12. Gregory et al. (2019) proposed a dominant-negative mechanism for these heterozygous mutant alleles based on homomultimerization of the C19ORF12 protein. Additionally, Gregory et al. (2019) identified individuals from 5 families who carried an 11-bp deletion in exon 3 (614297.0001), in homozygosity or compound heterozygosity with another pathogenic mutation. No heterozygous parent showed features of MPAN. Both this mutation and another found in a large family showing autosomal dominant inheritance (614297.0010) truncate the protein at the same codon (79); the difference in the mutant proteins is the identity of the 9 terminal amino acids. Gregory et al. (2019) hypothesized that a normal sequence of residues 69-76 may be necessary and sufficient for the heterozygous mutant protein to homomultimerize with the wildtype protein from the other allele, which could damage the function of the protein complex or induce its degradation. Gregory et al. (2019) also considered haploinsufficiency as an alternate disease mechanism for heterozygous mutations.

Pathogenesis

Drecourt et al. (2018) found that cells derived from patients with C19ORF12 mutations showed a significant increase (10- to 30-fold change) in cellular iron content when incubated with iron compared to controls. In response to high iron, patient cells showed a normal and appropriate decrease in transferrin receptor (TFRC; 190010) mRNA levels, but the amount of TFRC did not decrease in patient cells, suggesting impaired posttranslational lysosomal-based degradation of TFRC. Patient cells showed impaired transferrin (190000) and TFRC trafficking and recycling compared to controls, with clustering at the surface and in the perinuclear region, as well as abnormally enlarged lysosomes. Patient cells also showed decreased palmitoylation of TFRC, which is necessary for regulating TFRC endocytosis. Addition of the antimalarial agent artesunate rescued abnormal TFRC palmitoylation and decreased iron content in cultured patient fibroblasts. Similar findings were observed in studies of cells from NBIA patients due to mutations in other NBIA-associated genes. Drecourt et al. (2018) concluded that NBIA results from defective endosomal recycling and should be regarded as a disorder of cellular trafficking, whatever the original genetic defect.