Adenylosuccinase deficiency is an autosomal recessive inborn error of metabolism caused by an enzymatic defect in de novo purine synthesis (DNPS) pathway. ADSL deficiency leads to the accumulation of toxic intermediates, including succinyladenosine (S-Ado) and succinylaminoimidazole carboxamide riboside (SAICAr) in body fluids. There are 3 major phenotypic forms of the disorder that correlate with different values of the S-Ado and SAICAr concentration ratios (S-Ado/SAICAr) in the cerebrospinal fluid. These include the most severe fatal neonatal encephalopathy (S-Ado/SAICAr ratio less than 1); childhood form (type I) with severe psychomotor retardation (S-Ado/SAICAr ratio close to 1), and a milder form (type II) with psychomotor retardation or hypotonia (S-Ado/SAICAr ratio greater than 2) (summary by Baresova et al., 2012).

Pelizaeus-Merzbacher-like disease 1 (PMLD1) is a slowly progressive leukodystrophy that typically presents during the neonatal or early-infantile period with nystagmus, commonly associated with hypotonia, delayed acquisition of motor milestones, speech delay, and dysarthria. Over time the hypotonia typically evolves into spasticity that affects the ability to walk and communicate. Cerebellar signs (gait ataxia, dysmetria, intention tremor, head titubation, and dysdiadochokinesia) frequently manifest during childhood. Some individuals develop extrapyramidal movement abnormalities (choreoathetosis and dystonia). Hearing loss and optic atrophy are observed in rare cases. Motor impairments can lead to swallowing difficulty and orthopedic complications, including hip dislocation and scoliosis. Most individuals have normal cognitive skills or mild intellectual disability – which, however, can be difficult to evaluate in the context of profound motor impairment.

TUBB4A-related leukodystrophy comprises a phenotypic spectrum in which the MRI findings range from hypomyelination with atrophy of the basal ganglia and cerebellum (H-ABC) at the severe end to isolated hypomyelination at the mild end. Progressive neurologic findings reflect involvement of the pyramidal tracts (spasticity, brisk deep tendon reflexes, and Babinski sign), extrapyramidal system (rigidity, dystonia, choreoathetosis, oculogyric crisis, and perioral dyskinesia), cerebellum (ataxia, intention tremor, dysmetria), and bulbar function (dysarthria, dysphonia, and swallowing). Cognition is variably affected, usually less severely than motor function. Typically, those with H-ABC present in early childhood (ages 1-3 years) and those with isolated hypomyelination in later childhood or adulthood. The rate of progression varies with disease severity.

STXBP1 encephalopathy with epilepsy is characterized by early-onset encephalopathy with epilepsy (i.e., moderate-to-severe intellectual disability, refractory seizures, and ongoing epileptiform activity). The median age of onset of seizures is six weeks (range 1 day to 13 years). Seizure types can include infantile spasms; generalized tonic-clonic, clonic, or tonic seizures; and myoclonic, focal, atonic, and absence seizures. Epilepsy syndromes can include Ohtahara syndrome, West syndrome, Lennox-Gaustaut syndrome, and Dravet syndrome (not SCN1A-related), classic Rett syndrome (not MECP2-related), and atypical Rett syndrome (not CDKL5-related). The EEG is characterized by focal epileptic activity, burst suppression, hypsarrhythmia, or generalized spike-and-slow waves. Other findings can include abnormal tone, movement disorders (especially ataxia and dystonia), and behavior disorders (including autism spectrum disorder). Feeding difficulties are common.

Waardenburg syndrome type 2 (WS2) is an auditory-pigmentary syndrome characterized by pigmentary abnormalities of the hair, skin, and eyes; congenital sensorineural hearing loss; and the absence of 'dystopia canthorum,' the lateral displacement of the inner canthus of each eye, which is seen in some other forms of WS (review by Read and Newton, 1997). Individuals with WS type 2E (WS2E) may have neurologic abnormalities, including mental impairment, myelination defects, and ataxia. Waardenburg syndrome type 2 is genetically heterogeneous (see WS2A, 193510). For a description of other clinical variants of Waardenburg syndrome, see WS1 (193500), WS3 (148820), and WS4 (277580).

Developmental and epileptic encephalopathy-39 with leukodystrophy (DEE39) is an autosomal recessive neurologic syndrome characterized clinically by global developmental delay apparent in early infancy, early-onset seizures, hypotonia with poor motor function, and hypomyelination on brain imaging. Other features include absent speech and inability to walk; spasticity and hyperreflexia has also been reported. Although there is significant hypomyelination on brain imaging, the disorder was not classified as a primary leukodystrophy. The myelination defect was thought to stem from primary neuronal dysfunction due to impaired mitochondrial transport activity (summary by Wibom et al., 2009 and Falk et al., 2014). However, serial brain imaging in a patient with DEE39 by Kavanaugh et al. (2019) suggested that the mechanism of disease is consistent with a leukoaxonopathy type of leukodystrophy. For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

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.

Deafness, dystonia, and cerebral hypomyelination is an X-linked recessive mental retardation syndrome characterized by almost no psychomotor development, dysmorphic facial features, sensorineural deafness, dystonia, pyramidal signs, and hypomyelination on brain imaging (summary by Cacciagli et al., 2013).

Hypomyelinating leukodystrophy-12 (HLD12) is an autosomal recessive neurologic disorder characterized by severely delayed or even lack of psychomotor development that becomes apparent in the first months of life. Patients are markedly disabled, with acquired microcephaly, lack of speech, and often lack of spontaneous movement due to hypotonia and spasticity. Brain imaging shows delayed myelination (summary by Edvardson et al., 2015). For a general phenotypic description and a discussion of genetic heterogeneity of HLD, see 312080. In a review of the pathogenesis of disorders with prominent dystonia or opisthotonic posturing as a feature, Monfrini et al. (2021) classified HLD12 as belonging to a group of neurologic disorders termed 'HOPS-associated neurologic disorders (HOPSANDs), which are caused by mutations in genes encoding various components of the autophagic/endolysosomal system, including VPS11.

Lissencephaly-8 (LIS8) is an autosomal recessive neurologic disorder characterized by delayed psychomotor development, intellectual disability with poor or absent speech, early-onset refractory seizures, and hypotonia. Brain imaging shows variable features, including cortical gyral abnormalities and hypoplasia of the corpus callosum, brainstem, and cerebellum (Jerber et al., 2016). For a general description and a discussion of genetic heterogeneity lissencephaly, see LIS1 (607432).

Neurodevelopmental disorder with microcephaly, epilepsy, and hypomyelination (NEDMEHM) is an autosomal recessive neurometabolic disorder characterized by these cardinal features. Patients also show an exaggerated startle reflex in early infancy (Rodan et al., 2018).

Any leukoencephalopathy with vanishing white matter in which the cause of the disease is a variation in the EIF2B1 gene.