PLP1 disorders of central nervous system myelin formation include a range of phenotypes from Pelizaeus-Merzbacher disease (PMD) to spastic paraplegia 2 (SPG2). PMD typically manifests in infancy or early childhood with nystagmus, hypotonia, and cognitive impairment; the findings progress to severe spasticity and ataxia. Life span is shortened. SPG2 manifests as spastic paraparesis with or without CNS involvement and usually normal life span. Intrafamilial variation of phenotypes can be observed, but the signs are usually fairly consistent within families. Heterozygous females may manifest mild-to-moderate signs of the disease.

Niemann-Pick disease type C (NPC) is a slowly progressive lysosomal disorder whose principal manifestations are age dependent. The manifestations in the perinatal period and infancy are predominantly visceral, with hepatosplenomegaly, jaundice, and (in some instances) pulmonary infiltrates. From late infancy onward, the presentation is dominated by neurologic manifestations. The youngest children may present with hypotonia and developmental delay, with the subsequent emergence of ataxia, dysarthria, dysphagia, and, in some individuals, epileptic seizures, dystonia, and gelastic cataplexy. Although cognitive impairment may be subtle at first, it eventually becomes apparent that affected individuals have a progressive dementia. Older teenagers and young adults may present predominantly with apparent early-onset dementia or psychiatric manifestations; however, careful examination usually identifies typical neurologic signs.

Spastic paraplegia 7 (SPG7) is characterized by insidiously progressive bilateral leg weakness and spasticity. Most affected individuals have decreased vibration sense and cerebellar signs. Onset is mostly in adulthood, although symptoms may start as early as age 11 years and as late as age 72 years. Additional features including ataxia (gait and limbs), spastic dysarthria, dysphagia, pale optic disks, ataxia, nystagmus, strabismus, ptosis, hearing loss, motor and sensory neuropathy, amyotrophy, scoliosis, pes cavus, and urinary sphincter disturbances may be observed.

The spectrum of DCTN1-related neurodegeneration includes Perry syndrome, distal hereditary motor neuronopathy type 7B (dHMN7B), frontotemporal dementia (FTD), motor neuron disease / amyotrophic lateral sclerosis (ALS), and progressive supranuclear palsy. Some individuals present with overlapping phenotypes (e.g., FTD-ALS, Perry syndrome-dHMN7B). Perry syndrome (the most common of the phenotypes associated with DCTN1) is characterized by parkinsonism, neuropsychiatric symptoms, hypoventilation, and weight loss. The mean age of onset in those with Perry syndrome is 49 years (range: 35-70 years), and the mean disease duration is five years (range: 2-14 years). In most affected persons, the reported cause/circumstance of death relates to sudden death/hypoventilation or suicide.

POLR3-related leukodystrophy, a hypomyelinating leukodystrophy with specific features on brain MRI, is characterized by varying combinations of four major clinical findings: Neurologic dysfunction, typically predominated by motor dysfunction (progressive cerebellar dysfunction, and to a lesser extent extrapyramidal [i.e., dystonia], pyramidal [i.e., spasticity] and cognitive dysfunctions). Abnormal dentition (delayed dentition, hypodontia, oligodontia, and abnormally placed or shaped teeth). Endocrine abnormalities such as short stature (in ~50% of individuals) with or without growth hormone deficiency, and more commonly, hypogonadotropic hypogonadism manifesting as delayed, arrested, or absent puberty. Ocular abnormality in the form of myopia, typically progressing over several years and becoming severe. POLR3-related leukodystrophy and 4H leukodystrophy are the two recognized terms for five previously described overlapping clinical phenotypes (initially described as distinct entities before their molecular basis was known). These include: Hypomyelination, hypodontia, hypogonadotropic hypogonadism (4H syndrome); Ataxia, delayed dentition, and hypomyelination (ADDH); Tremor-ataxia with central hypomyelination (TACH); Leukodystrophy with oligodontia (LO); Hypomyelination with cerebellar atrophy and hypoplasia of the corpus callosum (HCAHC). Age of onset is typically in early childhood but later-onset cases have also been reported. An infant with Wiedemann-Rautenstrauch syndrome (neonatal progeroid syndrome) was recently reported to have pathogenic variants in POLR3A on exome sequencing. Confirmation of this as a very severe form of POLR3-related leukodystrophy awaits replication in other individuals with a clinical diagnosis of Wiedemann-Rautenstrauch syndrome.

Generally, Parkinson's disease that begins after age 50 is called late-onset disease. The condition is described as early-onset disease if signs and symptoms begin before age 50. Early-onset cases that begin before age 20 are sometimes referred to as juvenile-onset Parkinson's disease.\n\nParkinson's disease can also affect emotions and thinking ability (cognition). Some affected individuals develop psychiatric conditions such as depression and visual hallucinations. People with Parkinson's disease also have an increased risk of developing dementia, which is a decline in intellectual functions including judgment and memory.\n\nOften the first symptom of Parkinson's disease is trembling or shaking (tremor) of a limb, especially when the body is at rest. Typically, the tremor begins on one side of the body, usually in one hand. Tremors can also affect the arms, legs, feet, and face. Other characteristic symptoms of Parkinson's disease include rigidity or stiffness of the limbs and torso, slow movement (bradykinesia) or an inability to move (akinesia), and impaired balance and coordination (postural instability). These symptoms worsen slowly over time.\n\nParkinson's disease is a progressive disorder of the nervous system. The disorder affects several regions of the brain, especially an area called the substantia nigra that controls balance and movement.

Niemann-Pick disease type C (NPC) is a slowly progressive lysosomal disorder whose principal manifestations are age dependent. The manifestations in the perinatal period and infancy are predominantly visceral, with hepatosplenomegaly, jaundice, and (in some instances) pulmonary infiltrates. From late infancy onward, the presentation is dominated by neurologic manifestations. The youngest children may present with hypotonia and developmental delay, with the subsequent emergence of ataxia, dysarthria, dysphagia, and, in some individuals, epileptic seizures, dystonia, and gelastic cataplexy. Although cognitive impairment may be subtle at first, it eventually becomes apparent that affected individuals have a progressive dementia. Older teenagers and young adults may present predominantly with apparent early-onset dementia or psychiatric manifestations; however, careful examination usually identifies typical neurologic signs.

Spinocerebellar ataxia-36 (SCA36) is a slowly progressive neurodegenerative disorder characterized by adult-onset gait ataxia, eye movement abnormalities, tongue fasciculations, and variable upper motor neuron signs. Some affected individuals may develop hearing loss (summary by Garcia-Murias et al., 2012). For a general discussion of autosomal dominant spinocerebellar ataxia, see SCA1 (164400).

Congenital bile acid synthesis defect-6 (CBAS6) is characterized by persistent hypertransaminasemia and accumulation of C27 bile acids (summary by Alonso-Pena et al., 2022). For a general phenotypic description and a discussion of genetic heterogeneity of congenital bile acid synthesis defects, see CBAS1 (607765).

Early-onset progressive encephalopathy with brain edema and/or leukoencephalopathy-1 (PEBEL1) is an autosomal recessive severe neurometabolic disorder characterized by rapidly progressive neurologic deterioration that is usually associated with a febrile illness. Affected infants tend to show normal early development followed by acute psychomotor regression with ataxia, hypotonia, respiratory insufficiency, and seizures, resulting in coma and death in the first years of life. Brain imaging shows multiple abnormalities, including brain edema and signal abnormalities in the cortical and subcortical regions (summary by Kremer et al., 2016). Genetic Heterogeneity of PEBEL See also PEBEL2 (618321), caused by mutation in the NAXD gene (615910) on chromosome 13q34.

Childhood-onset neurodegeneration with ataxia, dystonia, and gaze palsy (NADGP) is an autosomal recessive progressive disorder characterized by onset of gait ataxia, cognitive decline, and gaze palsy in the first or second decades. Additional features include dysarthria, dystonia, and athetoid movements. Some patients may become wheelchair-bound as young adults (summary by Haack et al., 2016).

Dystonia-33 (DYT33) is a neurologic disorder characterized by onset of focal or generalized dystonia in the first decades of life (from early childhood to adolescence). The disorder is slowly progressive and may result in ambulation difficulties, dysarthria, or dysphagia. There is variable expressivity even with a family, as well as incomplete penetrance of the phenotype. Most mutations are in the heterozygous state, but a homozygous mutation with autosomal recessive inheritance has been reported, indicating variable patterns of transmission of DYT33. Some patients may have a more complex neurologic disorder with motor delay, lower limb spasticity, mild developmental delay with cognitive impairments, and nonspecific brain imaging abnormalities. There may be an exacerbation of the symptoms coinciding with viral infection or stress. Deep brain stimulation (DBS) may be therapeutic (summary by Kuipers et al., 2021).