TBC1D24-related disorders comprise a continuum of features that were originally described as distinct, recognized phenotypes: DOORS syndrome (deafness, onychodystrophy, osteodystrophy, mental retardation, and seizures). Profound sensorineural hearing loss, onychodystrophy, osteodystrophy, intellectual disability / developmental delay, and seizures. Familial infantile myoclonic epilepsy (FIME). Early-onset myoclonic seizures, focal epilepsy, dysarthria, and mild-to-moderate intellectual disability. Progressive myoclonus epilepsy (PME). Action myoclonus, tonic-clonic seizures, progressive neurologic decline, and ataxia. Early-infantile epileptic encephalopathy 16 (EIEE16). Epileptiform EEG abnormalities which themselves are believed to contribute to progressive disturbance in cerebral function. Autosomal recessive nonsyndromic hearing loss, DFNB86. Profound prelingual deafness. Autosomal dominant nonsyndromic hearing loss, DFNA65. Slowly progressive deafness with onset in the third decade, initially affecting the high frequencies.

Multiple mitochondrial dysfunctions syndrome is a severe autosomal recessive disorder of systemic energy metabolism, resulting in weakness, respiratory failure, lack of neurologic development, lactic acidosis, and early death (summary by Seyda et al., 2001). Genetic Heterogeneity of Multiple Mitochondrial Dysfunctions Syndrome See also MMDS2 (614299), caused by mutation in the BOLA3 gene (613183) on chromosome 2p13; MMDS3 (615330), caused by mutation in the IBA57 gene (615316) on chromosome 1q42; MMDS4 (616370), caused by mutation in the ISCA2 gene (615317) on chromosome 14q24; MMDS5 (617613), caused by mutation in the ISCA1 gene (611006) on chromosome 9q21; MMDS6 (617954), caused by mutation in the PMPCB gene (603131) on chromosome 7q22; and MMDS7 (620423), caused by mutation in the GCSH gene (238330) on chromosome 16q23.

Thiamine metabolism dysfunction syndrome-5 (THMD5) is an autosomal recessive metabolic disorder due to an inborn error of thiamine metabolism. The phenotype is highly variable, but in general, affected individuals have onset in early childhood of acute encephalopathic episodes associated with increased serum and CSF lactate. These episodes result in progressive neurologic dysfunction manifest as gait disturbances, ataxia, dystonia, and spasticity, which in some cases may result in loss of ability to walk. Cognitive function is usually preserved, although mildly delayed development has been reported. These episodes are usually associated with infection and metabolic decompensation. Some patients may have recovery of some neurologic deficits (Mayr et al., 2011). For a discussion of genetic heterogeneity of disorders due to thiamine metabolism dysfunction, see THMD1 (249270).

Acute reversible leukoencephalopathy with increased urinary alpha-ketoglutarate (ARLIAK) is an autosomal recessive disorder characterized by acute reversible neurologic deterioration in the context of a febrile illness. The disorder is associated with transient leukoencephalopathy on brain imaging concurrent with the acute episode, as well as persistently increased excretion of dicarboxylic acids, particularly alpha-ketoglutarate (summary by Dewulf et al., 2019).

Developmental and epileptic encephalopathy-80 (DEE80) is an autosomal recessive neurologic disorder characterized by the onset of refractory seizures in the first year of life. Patients have severe global developmental delay and may have additional variable features, including dysmorphic or coarse facial features, distal skeletal abnormalities, and impaired hearing or vision. At the cellular level, the disorder is caused by a defect in the synthesis of glycosylphosphatidylinositol (GPI), and thus affects the expression of GPI-anchored proteins at the cell surface (summary by Murakami et al., 2019). For a discussion of genetic heterogeneity of DEE, see 308350. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Mitochondrial complex II deficiency nuclear type 4 (MC2DN4) is a severe autosomal recessive disorder characterized by early-onset progressive neurodegeneration with leukoencephalopathy. Acute episodes of neurodegeneration are often triggered by catabolic stress such as infection or fasting.

Mitochondrial complex IV deficiency nuclear type 22 (MC4DN22) is an autosomal recessive metabolic disorder characterized by neonatal hypertrophic cardiomyopathy, encephalopathy, and severe lactic acidosis with fatal outcome (Wintjes et al., 2021). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Combined D-2- and L-2-hydroxyglutaric aciduria (D-2-HG and L-2-HG) is an autosomal recessive neurometabolic disorder characterized by neonatal-onset encephalopathy with severe muscular weakness, intractable seizures, respiratory distress, and lack of psychomotor development resulting in early death. Brain imaging shows abnormalities including enlarged ventricles, delayed myelination, and germinal layer cysts (summary by Muntau et al., 2000). See also isolated L-2-hydroxyglutaric aciduria (236792) and isolated D-2-hydroxyglutaric aciduria (see 600721).