Multiple symmetric lipomatosis (MSL) is an autosomal recessive metabolic disorder characterized by the growth of unencapsulated masses of adipose tissue with predilection for the cervical and thoracic regions. The lipoma growth is striking and disfiguring, and growth around the neck may cause difficulty swallowing or breathing. The age at onset ranges from childhood to young adulthood. Most, but not all, patients develop axonal peripheral neuropathy, which can appear at any age and varies in severity. Laboratory studies in MSL show low leptin (164160), low adiponectin (605441), variably increased lactate, and increased FGF21 (609436). Some patients may have insulin resistance. The disorder is exclusively associated with a particular MFN2 mutation (R707W; 608507.0013), usually in the homozygous state, but sometimes in the compound heterozygous state (Rocha et al., 2017; Capel et al., 2018).

Marinesco-Sjögren syndrome (MSS) is characterized by cerebellar ataxia with cerebellar atrophy, dysarthria, nystagmus, early-onset (not necessarily congenital) cataracts, myopathy, muscle weakness, and hypotonia. Additional features may include psychomotor delay, hypergonadotropic hypogonadism, short stature, and various skeletal abnormalities. Children with MSS usually present with muscular hypotonia in early infancy; distal and proximal muscular weakness is noticed during the first decade of life. Later, cerebellar findings of truncal ataxia, dysdiadochokinesia, nystagmus, and dysarthria become apparent. Motor function worsens progressively for some years, then stabilizes at an unpredictable age and degree of severity. Cataracts can develop rapidly and typically require lens extraction in the first decade of life. Although many adults have severe disabilities, life span in MSS appears to be near normal.

Phosphoribosylpyrophosphate synthetase (PRS) deficiency, an X-linked disorder, is a phenotypic continuum comprising three disorders previously thought to be clinically distinct: Arts syndrome, Charcot-Marie-Tooth neuropathy X type 5 (CMTX5), and X-linked nonsyndromic sensorineural hearing loss (DFNX1). In affected males, the PRS deficiency phenotypic spectrum ranges from severe congenital profound sensorineural hearing loss, intellectual disability, delayed motor development, and progressive ophthalmologic involvement (retinal dystrophy and optic atrophy) to normal cognitive abilities and relatively later-onset, somewhat milder manifestations, such as mild sensorineural hearing loss, peripheral neuropathy, and gait ataxia. Heterozygous females can show isolated and/or milder manifestations in the PRS deficiency spectrum. To date, 40 families with PRS deficiency have been reported.

POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined before the molecular basis was known. POLG-related disorders can therefore be considered an overlapping spectrum of disease presenting from early childhood to late adulthood. The age of onset broadly correlates with the clinical phenotype. In individuals with early-onset disease (prior to age 12 years), liver involvement, feeding difficulties, seizures, hypotonia, and muscle weakness are the most common clinical features. This group has the worst prognosis. In the juvenile/adult-onset form (age 12-40 years), disease is typically characterized by peripheral neuropathy, ataxia, seizures, stroke-like episodes, and, in individuals with longer survival, progressive external ophthalmoplegia (PEO). This group generally has a better prognosis than the early-onset group. Late-onset disease (after age 40 years) is characterized by ptosis and PEO, with additional features such as peripheral neuropathy, ataxia, and muscle weakness. This group overall has the best prognosis.

Progressive external ophthalmoplegia is characterized by multiple mitochondrial DNA deletions in skeletal muscle. The most common clinical features include adult onset of weakness of the external eye muscles and exercise intolerance. Patients with C10ORF2-linked adPEO may have other clinical features including proximal muscle weakness, ataxia, peripheral neuropathy, cardiomyopathy, cataracts, depression, and endocrine abnormalities (summary by Fratter et al., 2010). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal dominant progressive external ophthalmoplegia, see PEOA1 (157640). PEO caused by mutations in the POLG gene (174763) is associated with more complicated phenotypes than PEO caused by mutations in the SLC25A4 (103220) or C10ORF2 genes (Lamantea et al., 2002).

A neurodegenerative disease with characteristics of progressive muscular paralysis reflecting degeneration of motor neurons in the primary motor cortex, corticospinal tracts, brainstem and spinal cord. Caused by heterozygous mutation in the VAPB gene on chromosome 20q13.

MPV17-related mitochondrial DNA (mtDNA) maintenance defect presents in the vast majority of affected individuals as an early-onset encephalohepatopathic (hepatocerebral) disease that is typically associated with mtDNA depletion, particularly in the liver. A later-onset neuromyopathic disease characterized by myopathy and neuropathy, and associated with multiple mtDNA deletions in muscle, has also rarely been described. MPV17-related mtDNA maintenance defect, encephalohepatopathic form is characterized by: Hepatic manifestations (liver dysfunction that typically progresses to liver failure, cholestasis, hepatomegaly, and steatosis); Neurologic involvement (developmental delay, hypotonia, microcephaly, and motor and sensory peripheral neuropathy); Gastrointestinal manifestations (gastrointestinal dysmotility, feeding difficulties, and failure to thrive); and Metabolic derangements (lactic acidosis and hypoglycemia). Less frequent manifestations include renal tubulopathy, nephrocalcinosis, and hypoparathyroidism. Progressive liver disease often leads to death in infancy or early childhood. Hepatocellular carcinoma has been reported.

Autosomal recessive severe infantile nemaline myopathy-5A (NEM5A) is a skeletal muscle disorder characterized by symptom onset soon after birth or in early infancy. Affected infants show axial hypotonia, stiffness, rigid spine with progressive kyphosis, pectus deformities, and contractures or limited movement of the large joints. Some patients show transient tremors. There is muscle atrophy and poor gross motor development. Respiratory insufficiency develops in the first years of life, often leading to death. Muscle biopsy shows nemaline rods (Johnston et al., 2000; Geraud et al., 2021). For a discussion of genetic heterogeneity of nemaline myopathy, see NEM2 (256030).

Triosephosphate isomerase deficiency (TPID) is an autosomal recessive multisystem disorder characterized by congenital hemolytic anemia, and progressive neuromuscular dysfunction beginning in early childhood. Many patients die from respiratory failure in childhood. The neurologic syndrome is variable, but usually includes lower motor neuron dysfunction with hypotonia, muscle weakness and atrophy, and hyporeflexia. Some patients may show additional signs such as dystonic posturing and/or spasticity. Laboratory studies show intracellular accumulation of dihydroxyacetone phosphate (DHAP), particularly in red blood cells (summary by Fermo et al., 2010).

Progressive external ophthalmoplegia-4 (PEOA4) is an autosomal dominant form of mitochondrial disease that variably affects skeletal muscle, the nervous system, the liver, and the gastrointestinal tract. Age at onset ranges from infancy to adulthood. The phenotype ranges from relatively mild, with adult-onset skeletal muscle weakness and weakness of the external eye muscles, to severe, with a multisystem disorder characterized by delayed psychomotor development, lactic acidosis, constipation, and liver involvement (summary by Young et al., 2011). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal dominant progressive external ophthalmoplegia, see PEOA1 (157640).

MDDGC4 is an autosomal recessive muscular dystrophy with onset in infancy or early childhood. Cognition and brain structure are usually normal (Godfrey et al., 2006). It is part of a group of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as 'dystroglycanopathies' (Mercuri et al., 2009).

Congenital myopathy-10B (CMYO10B) is an autosomal recessive skeletal muscle disorder characterized by infantile- or childhood-onset myopathy, areflexia, dysphagia, and respiratory distress that usually requires nocturnal ventilation. Other common features include facial and neck muscle weakness, feeding difficulties, contractures, scoliosis, high-arched palate, hyporeflexia, and difficulties walking. The disorder is slowly progressive and most patients follow a chronic course. Muscle biopsy shows variable findings, including type 1 fiber predominance, minicore lesions, and myofibrillar disorganization (Boyden et al., 2012; Harris et al., 2018). Patients with missense mutations affecting conserved cysteine residues in the EGF-like domain show the mild variant phenotype (CMYO10B) with later onset of respiratory failure and minicores on muscle biopsy, whereas patients with more damaging mutations, including nonsense or frameshift null mutations, show the severe variant phenotype (CMYO10A) (Croci et al., 2022). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Primary CoQ10 deficiency is a rare, clinically heterogeneous autosomal recessive disorder caused by mutation in any of the genes encoding proteins directly involved in the synthesis of coenzyme Q (review by Quinzii and Hirano, 2011). Coenzyme Q10 (CoQ10), or ubiquinone, is a mobile lipophilic electron carrier critical for electron transfer by the mitochondrial inner membrane respiratory chain (Duncan et al., 2009). The disorder has been associated with 4 major phenotypes, but the molecular basis has not been determined in most patients with the disorder and there are no clear genotype/phenotype correlations. The phenotypes include an encephalomyopathic form with seizures and ataxia (Ogasahara et al., 1989); a multisystem infantile form with encephalopathy, cardiomyopathy and renal failure (Rotig et al., 2000); a predominantly cerebellar form with ataxia and cerebellar atrophy (Lamperti et al., 2003); and Leigh syndrome with growth retardation (van Maldergem et al., 2002). The correct diagnosis is important because some patients may show a favorable response to CoQ10 treatment. Genetic Heterogeneity of Primary Coenzyme Q10 Deficiency See also COQ10D2 (614651), caused by mutation in the PDSS1 gene (607429) on chromosome 10p12; COQ10D3 (614652), caused by mutation in the PDSS2 gene (610564) on chromosome 6q21; COQ10D4 (612016), caused by mutation in the COQ8 gene (ADCK3; 606980) on chromosome 1q42; COQ10D5 (614654), caused by mutation in the COQ9 gene (612837) on chromosome 16q21; COQ10D6 (614650), caused by mutation in the COQ6 gene (614647) on chromosome 14q24; COQ10D7 (616276), caused by mutation in the COQ4 gene (612898) on chromosome 9q34; COQ10D8 (616733), caused by mutation in the COQ7 gene (601683) on chromosome 16p13; and COQ10D9 (619028), caused by mutation in the COQ5 gene (616359) on chromosome 12q24. Secondary CoQ10 deficiency has been reported in association with glutaric aciduria type IIC (MADD; 231680), caused by mutation in the ETFDH gene (231675) on chromosome 4q, and with ataxia-oculomotor apraxia syndrome-1 (AOA1; 208920), caused by mutation in the APTX gene (606350) on chromosome 9p13.

The overlapping phenotypes of neonatal adrenoleukodystrophy (NALD) and infantile Refsum disease (IRD) represent the milder manifestations of the Zellweger syndrome spectrum (ZSS) of peroxisome biogenesis disorders. The clinical course of patients with the NALD and IRD presentation is variable and may include developmental delay, hypotonia, liver dysfunction, sensorineural hearing loss, retinal dystrophy, and visual impairment. Children with the NALD presentation may reach their teens, and those with the IRD presentation may reach adulthood (summary by Waterham and Ebberink, 2012). For a complete phenotypic description and a discussion of genetic heterogeneity of PBD(NALD/IRD), see 601539. Individuals with mutations in the PEX13 gene have cells of complementation group 13 (CG13, equivalent to CGH). For information on the history of PBD complementation groups, see 214100.

Ataxia-telangiectasia-like disorder-2 (ATLD2) is an autosomal recessive syndrome resulting from defects in DNA excision repair. Affected individuals have a neurodegenerative phenotype characterized by developmental delay, ataxia, and sensorineural hearing loss. Other features include short stature, cutaneous and ocular telangiectasia, and photosensitivity (summary by Baple et al., 2014). For a discussion of genetic heterogeneity of ATLD, see ATLD1 (604391).

Scapulohumeroperoneal myopathy is an autosomal dominant muscle disorder characterized by slowly progressive muscle weakness and atrophy affecting both proximal and distal muscles of the upper and lower limbs. Onset is usually in the first decade and can be as early as infancy, although some patients do not notice symptoms until young adulthood. There is marked variability in severity (summary by Zukosky et al., 2015).

Myopathy, lactic acidosis, and sideroblastic anemia (MLASA) is a rare autosomal recessive oxidative phosphorylation disorder specific to skeletal muscle and bone marrow (Bykhovskaya et al., 2004). Genetic Heterogeneity of Myopathy, Lactic Acidosis, and Sideroblastic Anemia MLASA2 (613561) is caused by mutation in the YARS2 gene (610957) on chromosome 12p11. MLASA3 (500011) is caused by heteroplasmic mutation in the mitochondrially-encoded MTATP6 gene (516060).

Myofibrillar myopathy-4 (MFM4) is an autosomal dominant disorder characterized by adult-onset distal muscle weakness primarily affecting the lower limbs at onset. Affected individuals usually present with gait difficulties in their forties, followed by slow progression with eventual involvement of the hands and proximal muscles of the lower limbs. Rare patients may develop cardiomyopathy. Skeletal muscle biopsy shows myopathic changes with myofibrillar changes (Selcen and Engel, 2005; Griggs et al., 2007). For a phenotypic description and a discussion of genetic heterogeneity of myofibrillar myopathy, see MFM1 (601419).

Muscular dystrophy, congenital hearing loss, and ovarian insufficiency syndrome (MDHLO) is an autosomal recessive systemic disorder characterized by progressive muscle weakness, sensorineural hearing loss, and endocrine abnormalities, mainly primary amenorrhea due to ovarian insufficiency. Features of the disorder appear soon after birth, although endocrine anomalies are not noted until puberty. The severity of the phenotype is variable: some patients may lose ambulation and have significant respiratory insufficiency, whereas others retain the ability to walk (Foley et al., 2020).

Congenital muscular dystrophy with or without seizures (MYOS) is an autosomal recessive disorder characterized by severe muscle hypotonia apparent from birth, as well as developmental delay. Laboratory studies show increased serum creatine kinase and muscle biopsy shows nonspecific dystrophic features. Most patients develop seizures or have abnormal epileptiform findings on EEG studies; other variable findings may include feeding difficulties, nystagmus, myopathic facies, areflexia, and brain atrophy on MRI (summary by Larson et al., 2018 and Henige et al., 2021).

Juvenile amyotrophic lateral sclerosis-27 (ALS27) is an autosomal dominant disorder characterized by early childhood-onset lower extremity spasticity manifesting as toe walking and gait abnormalities, followed by progressive lower motor neuron-mediated weakness without sensory signs or symptoms (Mohassel et al., 2021). For a discussion of genetic heterogeneity of amyotrophic lateral sclerosis, see ALS1 (105400).