Multiple mitochondrial dysfunctions syndrome-1 (MMDS1) is a severe autosomal recessive disorder of systemic energy metabolism, resulting in weakness, respiratory failure, lack of neurologic development, lactic acidosis, and early death (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; MMDS7 (620423), caused by mutation in the GCSH gene (238330) on chromosome 16q23; MMDS8 (251900), caused by mutation in the FDX2 gene (614585) on chromosome 19p13; MMDS9A (617717) and MMDS9B (620887), both caused by mutation in the FDXR gene (103270) on chromosome 17q25.

Multiple mitochondrial dysfunctions syndrome-2 (MMDS2) with hyperglycinemia is a severe autosomal recessive disorder characterized by developmental regression in infancy. Affected children have an encephalopathic disease course with seizures, spasticity, loss of head control, and abnormal movement. Additional more variable features include optic atrophy, cardiomyopathy, and leukodystrophy. Laboratory studies show increased serum glycine and lactate. Most patients die in childhood. The disorder represents a form of 'variant' nonketotic hyperglycinemia and is distinct from classic nonketotic hyperglycinemia (NKH, or GCE; 605899), which is characterized by significantly increased CSF glycine. Several forms of 'variant' NKH, including MMDS2, appear to result from defects of mitochondrial lipoate biosynthesis (summary by Baker et al., 2014). For a general description and a discussion of genetic heterogeneity of multiple mitochondrial dysfunctions syndrome, see MMDS1 (605711).

Multiple mitochondrial dysfunctions syndrome-3 (MMDS3) is an autosomal recessive severe neurodegenerative disorder characterized by loss of previously acquired developmental milestones in the first months or years of life. Some affected patients have normal development in early infancy before the onset of symptoms, whereas others show delays from birth. Features included loss of motor function, spasticity, pyramidal signs, loss of speech, and cognitive impairment. The disease course is highly variable: some patients die of respiratory failure early in childhood, whereas some survive but may be bedridden with a feeding tube. Less commonly, some patients may survive and have a stable course with motor deficits and mild or even absent cognitive impairment, although there may be fluctuating symptoms, often in response to infection. Other variable features include visual problems and seizures. Brain imaging shows diffuse leukodystrophy in the subcortical region, brainstem, cerebellum, and spinal cord. Laboratory studies tend to show increased lactate and CSF glycine, and decreased activity of mitochondrial complexes I and II, although these findings are also variable. There may be additional biochemical evidence of mitochondrial dysfunction (summary by Liu et al., 2018). For a general description and a discussion of genetic heterogeneity of multiple mitochondrial dysfunctions syndrome, see MMDS1 (605711).

Childhood-onset spasticity with hyperglycinemia is an autosomal recessive disorder characterized by onset of slowly progressive spasticity that results in impaired gait in the first decade of life. Imaging of the central nervous system shows leukodystrophy and/or lesions in the upper spinal cord. More variable features include visual defects and mild learning disabilities. Serum glycine is increased, but CSF glycine is only mildly increased or normal; serum lactate is normal. The disorder represents a form of 'variant' nonketotic hyperglycinemia and is distinct from classic nonketotic hyperglycinemia (NKH, or GCE; 605899), which is characterized by significantly increased CSF glycine. Several forms of 'variant' NKH, including SPAHGC, appear to result from defects of mitochondrial lipoate biosynthesis (summary by Baker et al., 2014).

Glycine encephalopathy (GCE), also called nonketotic hyperglycinemia (NKH), is an inborn error of metabolism characterized by accumulation of a large amount of glycine in body fluids. Typical cases have severe neurologic features, including seizures, lethargy, and muscular hypotonia soon after birth, and most die with the neonatal period; atypical cases have later onset and less severe psychomotor development (summary by Nanao et al., 1994). For a general description and a discussion of genetic heterogeneity of glycine encephalopathy, see GCE1 (605899).

Mitochondrial dysfunctions syndrome-7 (MMDS7) is an autosomal recessive disorder characterized by a clinical spectrum ranging from neonatal fatal glycine encephalopathy to an attenuated phenotype of developmental delay, behavioral problems, limited epilepsy, and variable movement problems (Arribas-Carreira et al., 2023). For a general description and a discussion of genetic heterogeneity of multiple mitochondrial dysfunctions syndrome, see MMDS1 (605711).