Methemoglobinemia due to NADH-cytochrome b5 reductase deficiency is an autosomal recessive disorder characterized clinically by decreased oxygen carrying capacity of the blood, with resultant cyanosis and hypoxia (review by Percy and Lappin, 2008). There are 2 types of methemoglobin reductase deficiency. In type I, the defect affects the soluble form of the enzyme, is restricted to red blood cells, and causes well-tolerated methemoglobinemia. In type II, the defect affects both the soluble and microsomal forms of the enzyme and is thus generalized, affecting red cells, leukocytes, and all body tissues. Type II methemoglobinemia is associated with mental deficiency and other neurologic symptoms. The neurologic symptoms may be related to the major role played by the cytochrome b5 system in the desaturation of fatty acids (Vives-Corrons et al., 1978; Kaplan et al., 1979).

A rare, fulminant small vessel vasculitis that affects the capillary beds of the kidneys and lungs and characterized by the presence of anti-glomerular basement membrane (GBM) and, in its full-blown form, anti-alveolar basement membrane (ABM) antibodies. Consequently, it may manifest as a rapidly progressive, isolated glomerulonephritis (anti-GBM nephritis) or as a pulmonary-renal syndrome with severe lung hemorrhage.

Emery-Dreifuss muscular dystrophy (EDMD) is characterized by the clinical triad of: joint contractures that begin in early childhood; slowly progressive muscle weakness and wasting initially in a humero-peroneal distribution that later extends to the scapular and pelvic girdle muscles; and cardiac involvement that may manifest as palpitations, presyncope and syncope, poor exercise tolerance, and congestive heart failure along with variable cardiac rhythm disturbances. Age of onset, severity, and progression of muscle and cardiac involvement demonstrate both inter- and intrafamilial variability. Clinical variability ranges from early onset with severe presentation in childhood to late onset with slow progression in adulthood. In general, joint contractures appear during the first two decades, followed by muscle weakness and wasting. Cardiac involvement usually occurs after the second decade and respiratory function may be impaired in some individuals.

Any hypertrophic cardiomyopathy in which the cause of the disease is a mutation in the MYL3 gene.

Autosomal recessive myosin storage congenital myopathy-7B (CMYP7B) is a skeletal muscle disorder characterized by the onset of scapuloperoneal muscle weakness in early childhood or young adulthood. Affected individuals have difficulty walking, steppage gait, and scapular winging due to shoulder girdle involvement. The severity and progression of the disorder is highly variable, even within families. Most patients develop respiratory insufficiency, nocturnal hypoventilation, and restrictive lung disease; some develop hypertrophic cardiomyopathy. Additional features include myopathic facies, high-arched palate, scoliosis, and muscle wasting with thin body habitus. Serum creatine kinase may be normal or elevated. Skeletal muscle biopsy shows variable findings, including myosin storage disease, type 1 fiber predominance, centralized nuclei, and multiminicore disease (Onengut et al., 2004; Tajsharghi et al., 2007; Beecroft et al., 2019). For a discussion of genetic heterogeneity of congenital myopathy, see CMYP1A (117000).

Distal arthrogryposis type 5 is distinguished from other forms of DA by the presence of ocular abnormalities, typically ptosis, ophthalmoplegia, and/or strabismus, in addition to contractures of the skeletal muscles. Some cases have been reported to have pulmonary hypertension as a result of restrictive lung disease (summary by Bamshad et al., 2009). There are 2 syndromes with features overlapping those of DA5 that are also caused by heterozygous mutation in PIEZO2: distal arthrogryposis type 3 (DA3, or Gordon syndrome; 114300) and Marden-Walker syndrome (MWKS; 248700), which are distinguished by the presence of cleft palate and mental retardation, respectively. McMillin et al. (2014) suggested that the 3 disorders might represent variable expressivity of the same condition. For a general phenotypic description and a discussion of genetic heterogeneity of distal arthrogryposis, see DA1A (108120). Genetic Heterogeneity of Distal Arthrogryposis 5 A subtype of DA5 due to mutation in the ECEL1 gene (605896) on chromosome 2q36 has been designated DA5D (615065). See NOMENCLATURE.

For a general phenotypic description and a discussion of genetic heterogeneity of pulmonary surfactant metabolism dysfunction, see SMDP1 (265120).

Any hypertrophic cardiomyopathy in which the cause of the disease is a mutation in the VCL gene.

Any hypertrophic cardiomyopathy in which the cause of the disease is a mutation in the TNNC1 gene.

Pulmonary surfactant metabolism dysfunction-5 (SMDP5) is an autosomal recessive lung disorder manifest clinically and pathologically as pulmonary alveolar proteinosis (PAP). PAP is a rare lung disease characterized by the ineffective clearance of surfactant by alveolar macrophages. This results in the accumulation of surfactant-derived lipoproteinaceous material in the alveoli and terminal bronchioles, causing respiratory failure (summary by Greenhill and Kotton, 2009). For a general phenotypic description and a discussion of genetic heterogeneity of pulmonary surfactant metabolism dysfunction, see SMDP1 (265120).

PEOA6 is characterized by muscle weakness, mainly affecting the lower limbs, external ophthalmoplegia, exercise intolerance, and mitochondrial DNA (mtDNA) deletions on muscle biopsy. Symptoms may appear in childhood or adulthood and show slow progression (summary by Ronchi et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal dominant progressive external ophthalmoplegia, see PEOA1 (157640).

Primary familial and congenital polycythemia (PFCP) is characterized by isolated erythrocytosis in an individual with a normal-sized spleen and absence of disorders causing secondary erythrocytosis. Clinical manifestations relate to the erythrocytosis and can include plethora, the hyperviscosity syndrome (headache, dizziness, fatigue, lassitude, visual and auditory disturbances, paresthesia, myalgia), altered mental status caused by hypoperfusion and local hypoxia, and arterial and/or venous thromboembolic events. Although the majority of individuals with PFCP have only mild manifestations of hyperviscosity such as dizziness or headache, some affected individuals have had severe and even fatal complications including arterial hypertension, intracerebral hemorrhage, deep vein thrombosis, coronary disease, and myocardial infarction. To date 116 affected individuals from 24 families have been reported.

Hereditary hemorrhagic telangiectasia (HHT) is characterized by the presence of multiple arteriovenous malformations (AVMs) that lack intervening capillaries and result in direct connections between arteries and veins. The most common clinical manifestation is spontaneous and recurrent nosebleeds (epistaxis) beginning on average at age 12 years. Telangiectases (small AVMs) are characteristically found on the lips, tongue, buccal and gastrointestinal (GI) mucosa, face, and fingers. The appearance of telangiectases is generally later than epistaxis but may be during childhood. Large AVMs occur most often in the lungs, liver, or brain; complications from bleeding or shunting may be sudden and catastrophic. A minority of individuals with HHT have GI bleeding, which is rarely seen before age 50 years.

Mitochondrial complex IV deficiency nuclear type 1 (MC4DN1) is an autosomal recessive metabolic disorder characterized by rapidly progressive neurodegeneration and encephalopathy with loss of motor and cognitive skills between about 5 and 18 months of age after normal early development. Affected individuals show hypotonia, failure to thrive, loss of the ability to sit or walk, poor communication, and poor eye contact. Other features may include oculomotor abnormalities, including slow saccades, strabismus, ophthalmoplegia, and nystagmus, as well as deafness, apneic episodes, ataxia, tremor, and brisk tendon reflexes. Brain imaging shows bilateral symmetric lesions in the basal ganglia, consistent with a clinical diagnosis of Leigh syndrome (see 256000). Some patients may also have abnormalities in the brainstem and cerebellum. Laboratory studies usually show increased serum and CSF lactate and decreased levels and activity of mitochondrial respiratory complex IV in patient tissues. There is phenotypic variability, but death in childhood, often due to central respiratory failure, is common (summary by Tiranti et al., 1998; Tiranti et al., 1999; Teraoka et al., 1999; Poyau et al., 2000) Genetic Heterogeneity of Mitochondrial Complex IV Deficiency Most isolated COX deficiencies are inherited as autosomal recessive disorders caused by mutations in nuclear-encoded genes; mutations in the mtDNA-encoded COX subunit genes are relatively rare (Shoubridge, 2001; Sacconi et al., 2003). Mitochondrial complex IV deficiency caused by mutation in nuclear-encoded genes, in addition to MC4DN1, include MC4DN2 (604377), caused by mutation in the SCO2 gene (604272); MC4DN3 (619046), caused by mutation in the COX10 gene (602125); MC4DN4 (619048), caused by mutation in the SCO1 gene (603664); MC4DN5 (220111), caused by mutation in the LRPPRC gene (607544); MC4DN6 (615119), caused by mutation in the COX15 gene (603646); MC4DN7 (619051), caused by mutation in the COX6B1 gene (124089); MC4DN8 (619052), caused by mutation in the TACO1 gene (612958); MC4DN9 (616500), caused by mutation in the COA5 gene (613920); MC4DN10 (619053), caused by mutation in the COX14 gene (614478); MC4DN11 (619054), caused by mutation in the COX20 gene (614698); MC4DN12 (619055), caused by mutation in the PET100 gene (614770); MC4DN13 (616501), caused by mutation in the COA6 gene (614772); MC4DN14 (619058), caused by mutation in the COA3 gene (614775); MC4DN15 (619059), caused by mutation in the COX8A gene (123870); MC4DN16 (619060), caused by mutation in the COX4I1 gene (123864); MC4DN17 (619061), caused by mutation in the APOPT1 gene (616003); MC4DN18 (619062), caused by mutation in the COX6A2 gene (602009); MC4DN19 (619063), caused by mutation in the PET117 gene (614771); MC4DN20 (619064), caused by mutation in the COX5A gene (603773); MC4DN21 (619065), caused by mutation in the COXFA4 gene (603883); MC4DN22 (619355), caused by mutation in the COX16 gene (618064); and MC4DN23 (620275), caused by mutation in the COX11 gene (603648). Mitochondrial complex IV deficiency has been associated with mutations in several mitochondrial genes, including MTCO1 (516030), MTCO2 (516040), MTCO3 (516050), MTTS1 (590080), MTTL1 (590050), and MTTN (590010).

Interstitial lung disease (ILD) comprises a heterogeneous group of rare diseases affecting the distal part of the lung and characterized by a progressive remodeling of the alveolar interstitium. The manifestations form a spectrum ranging from idiopathic interstitial pneumonia (IIP) or pneumonitis to the more severe idiopathic pulmonary fibrosis (IPF). IPF is associated with an increased risk of developing lung cancer, which occurs in a subset of patients with ILD. Clinical features of ILD include dyspnea, clubbing of the fingers, and restrictive lung capacity. Imaging typically shows ground glass opacities and inter- and intraseptal thickening, while histologic studies usually show a pattern consistent with 'usual interstitial pneumonia' (UIP) (review by Gross and Hunninghake, 2001; summary by Legendre et al., 2020). Idiopathic pulmonary fibrosis is one of a family of idiopathic pneumonias sharing clinical features of shortness of breath, radiographically evident diffuse pulmonary infiltrates, and varying degrees in inflammation, fibrosis, or both on lung biopsy. In some cases, the disorder can be rapidly progressive and characterized by sequential acute lung injury with subsequent scarring and end-stage lung disease. Although older studies included several forms of interstitial pneumonia under the term 'idiopathic pulmonary fibrosis,' the clinical label of 'idiopathic pulmonary fibrosis' should be reserved for patients with a specific form of fibrosing interstitial pneumonia referred to as usual interstitial pneumonia (Gross and Hunninghake, 2001). It is estimated that 0.5 to 2.2% of cases of idiopathic pulmonary fibrosis are familial (Marshall et al., 2000). Gross and Hunninghake (2001) reviewed idiopathic pulmonary fibrosis, emphasizing definition, pathogenesis, diagnosis, natural history, and therapy. Antoniou et al. (2004) provided a 'top ten list' of references pertaining to etiopathogenesis, prognosis, diagnosis, therapy, and other aspects of idiopathic pulmonary fibrosis. For a discussion of genetic heterogeneity of ILD, see ILD1 (619611). Pulmonary fibrosis can also be a feature in patients with mutations in the TERT (187270) or the TERC (602322) gene; see PFBMFT1 (614742) and PFBMFT2 (614743). Some patients with surfactant protein C deficiency (610913) who survive to adulthood manifest features of pulmonary fibrosis.

Peripheral neuropathy with variable spasticity, exercise intolerance, and developmental delay (PNSED) is an autosomal recessive multisystemic disorder with highly variable manifestations, even within the same family. Some patients present in infancy with hypotonia and global developmental delay with poor or absent motor skill acquisition and poor growth, whereas others present as young adults with exercise intolerance and muscle weakness. All patients have signs of a peripheral neuropathy, usually demyelinating, with distal muscle weakness and atrophy and distal sensory impairment; many become wheelchair-bound. Additional features include spasticity, extensor plantar responses, contractures, cerebellar signs, seizures, short stature, and rare involvement of other organ systems, including the heart, pancreas, and kidney. Biochemical analysis may show deficiencies in mitochondrial respiratory complex enzyme activities in patient tissue, although this is not always apparent. Lactate is frequently increased, suggesting mitochondrial dysfunction (Powell et al., 2015; Argente-Escrig et al., 2022). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Primary pulmonary hypertension-5 (PPH5) is an autosomal recessive disorder characterized by the onset of pulmonary arterial hypertension in infancy, resulting in right heart dysfunction and ultimately right heart failure. Death in early childhood is common (Machado et al., 2022). For a discussion of genetic heterogeneity of primary pulmonary hypertension, see PPH1 (178600).

Recurrent respiratory infections and failure to thrive with or without diarrhea (RIFTD) is characterized by neonatal onset of chronic cough, episodic wheezing, recurrent lower respiratory tract infections, chronic diarrhea, and failure to thrive. Despite the resemblance to cystic fibrosis (CF; 219700), these patients have normal sweat chloride and pancreatic elastase tests (Bertoli-Avella et al., 2022).