Primary myelofibrosis is a condition characterized by the buildup of scar tissue (fibrosis) in the bone marrow, the tissue that produces blood cells. Because of the fibrosis, the bone marrow is unable to make enough normal blood cells. The shortage of blood cells causes many of the signs and symptoms of primary myelofibrosis.\n\nInitially, most people with primary myelofibrosis have no signs or symptoms. Eventually, fibrosis can lead to a reduction in the number of red blood cells, white blood cells, and platelets. A shortage of red blood cells (anemia) often causes extreme tiredness (fatigue) or shortness of breath. A loss of white blood cells can lead to an increased number of infections, and a reduction of platelets can cause easy bleeding or bruising.\n\nBecause blood cell formation (hematopoiesis) in the bone marrow is disrupted, other organs such as the spleen or liver may begin to produce blood cells. This process, called extramedullary hematopoiesis, often leads to an enlarged spleen (splenomegaly) or an enlarged liver (hepatomegaly). People with splenomegaly may feel pain or fullness in the abdomen, especially below the ribs on the left side. Other common signs and symptoms of primary myelofibrosis include fever, night sweats, and bone pain.\n\nPrimary myelofibrosis is most commonly diagnosed in people aged 50 to 80 but can occur at any age.

Incontinentia pigmenti (IP) is a disorder that affects the skin, hair, teeth, nails, eyes, and central nervous system; it occurs primarily in females and on occasion in males. Characteristic skin lesions evolve through four stages: I. Blistering (birth to age ~4 months). II. Wart-like rash (for several months). III. Swirling macular hyperpigmentation (age ~6 months into adulthood). IV. Linear hypopigmentation. Alopecia, hypodontia, abnormal tooth shape, and dystrophic nails are observed. Neovascularization of the retina, present in some individuals, predisposes to retinal detachment. Neurologic findings including seizures, intellectual disability, and developmental delays are occasionally seen.

A multifocal, multisystem form of Langerhans-cell histiocytosis. There is involvement of multiple organ systems including the bones, skin, liver, spleen, and lymph nodes. Patients are usually infants presenting with fever, hepatosplenomegaly, lymphadenopathy, bone and skin lesions, and pancytopenia.

HEXA disorders are best considered as a disease continuum based on the amount of residual beta-hexosaminidase A (HEX A) enzyme activity. This, in turn, depends on the molecular characteristics and biological impact of the HEXA pathogenic variants. HEX A is necessary for degradation of GM2 ganglioside; without well-functioning enzymes, GM2 ganglioside builds up in the lysosomes of brain and nerve cells. The classic clinical phenotype is known as Tay-Sachs disease (TSD), characterized by progressive weakness, loss of motor skills beginning between ages three and six months, decreased visual attentiveness, and increased or exaggerated startle response with a cherry-red spot observable on the retina followed by developmental plateau and loss of skills after eight to ten months. Seizures are common by 12 months with further deterioration in the second year of life and death occurring between ages two and three years with some survival to five to seven years. Subacute juvenile TSD is associated with normal developmental milestones until age two years, when the emergence of abnormal gait or dysarthria is noted followed by loss of previously acquired skills and cognitive decline. Spasticity, dysphagia, and seizures are present by the end of the first decade of life, with death within the second decade of life, usually by aspiration. Late-onset TSD presents in older teens or young adults with a slowly progressive spectrum of neurologic symptoms including lower-extremity weakness with muscle atrophy, dysarthria, incoordination, tremor, mild spasticity and/or dystonia, and psychiatric manifestations including acute psychosis. Clinical variability even among affected members of the same family is observed in both the subacute juvenile and the late-onset TSD phenotypes.

A condition characterized by recurrent, self-limiting episodes of vomiting associated with intense nausea, pallor, and lethargy. It is commonly a migraine precursor.

3-Hydroxy-3-methylglutaryl-CoA lyase deficiency (HMGCLD) is a rare autosomal recessive disorder with the cardinal manifestations of metabolic acidosis without ketonuria, hypoglycemia, and a characteristic pattern of elevated urinary organic acid metabolites, including 3-hydroxy-3-methylglutaric, 3-methylglutaric, and 3-hydroxyisovaleric acids. Urinary levels of 3-methylcrotonylglycine may be increased. Dicarboxylic aciduria, hepatomegaly, and hyperammonemia may also be observed. Presenting clinical signs include irritability, lethargy, coma, and vomiting (summary by Gibson et al., 1988).

Optic atrophy type 1 (OPA1, or Kjer type optic atrophy) is characterized by bilateral and symmetric optic nerve pallor associated with insidious decrease in visual acuity (usually between ages 4 and 6 years), visual field defects, and color vision defects. Visual impairment is usually moderate (6/10 to 2/10), but ranges from mild or even insignificant to severe (legal blindness with acuity <1/20). The visual field defect is typically centrocecal, central, or paracentral; it is often large in those with severe disease. The color vision defect is often described as acquired blue-yellow loss (tritanopia). Other findings can include auditory neuropathy resulting in sensorineural hearing loss that ranges from severe and congenital to subclinical (i.e., identified by specific audiologic testing only). Visual evoked potentials are typically absent or delayed; pattern electroretinogram shows an abnormal N95:P50 ratio. Tritanopia is the classic feature of color vision defect, but more diffuse nonspecific dyschromatopsia is not uncommon. Ophthalmoscopic examination discloses temporal or diffuse pallor of the optic discs, sometimes associated with optic disc excavation. The neuroretinal rim shows some pallor in most cases, sometimes associated with a temporal pigmentary gray crescent.

Red cell pyruvate kinase deficiency, or congenital nonspherocytic hemolyic anemia-2 (CNSHA2), is the most common cause of hereditary nonspherocytic hemolytic anemia. PK deficiency is also the most frequent enzyme abnormality of the glycolytic pathway (Zanella et al., 2005).

Fumarate hydratase (FH) deficiency results in severe neonatal and early infantile encephalopathy that is characterized by poor feeding, failure to thrive, hypotonia, lethargy, and seizures. Dysmorphic facial features include frontal bossing, depressed nasal bridge, and widely spaced eyes. Many affected individuals are microcephalic. A spectrum of brain abnormalities are seen on magnetic resonance imaging, including cerebral atrophy, enlarged ventricles and generous extra-axial cerebral spinal fluid (CSF) spaces, delayed myelination for age, thinning of the corpus callosum, and an abnormally small brain stem. Brain malformations including bilateral polymicrogyria and absence of the corpus callosum can also be observed. Development is severely affected: most affected individuals are nonverbal and nonambulatory, and many die during early childhood. Less severely affected individuals with moderate cognitive impairment and long-term survival have been reported.

Mitochondrial DNA (mtDNA) deletion syndromes predominantly comprise three overlapping phenotypes that are usually simplex (i.e., a single occurrence in a family), but rarely may be observed in different members of the same family or may evolve from one clinical syndrome to another in a given individual over time. The three classic phenotypes caused by mtDNA deletions are Kearns-Sayre syndrome (KSS), Pearson syndrome, and progressive external ophthalmoplegia (PEO). KSS is a progressive multisystem disorder defined by onset before age 20 years, pigmentary retinopathy, and PEO; additional features include cerebellar ataxia, impaired intellect (intellectual disability, dementia, or both), sensorineural hearing loss, ptosis, oropharyngeal and esophageal dysfunction, exercise intolerance, muscle weakness, cardiac conduction block, and endocrinopathy. Pearson syndrome is characterized by sideroblastic anemia and exocrine pancreas dysfunction and may be fatal in infancy without appropriate hematologic management. PEO is characterized by ptosis, impaired eye movements due to paralysis of the extraocular muscles (ophthalmoplegia), oropharyngeal weakness, and variably severe proximal limb weakness with exercise intolerance. Rarely, a mtDNA deletion can manifest as Leigh syndrome.

Goodpasture syndrome, also known as anti-GBM disease, is a rare autoimmune disease consisting of alveolar hemorrhage and glomerulonephritis secondary to circulating antiglomerular basement membrane (anti-GBM) antibodies. Anti-GBM antibodies are directed against an antigen intrinsic to the alpha-3 chain of type IV collagen (COL4A3; 120070) that is expressed in the GBMs of the glomerular capillary loops and the basal membrane of the pulmonary alveoli. Goodpasture syndrome is suspected in patients with hemoptysis and hematuria and is confirmed by the presence of anti-GBM antibodies in renal biopsy specimens and serum. Patients with human leukocyte antigen HLA-DR15 and HLA-DR4 are susceptible to the development of Goodpasture syndrome. Reported cases of familial Goodpasture syndrome are extremely rare (summary by Angioi et al., 2017).

The diagnosis of severe breath-holding spells (BHS) in childhood is based on a distinctive and stereotyped sequence of clinical events beginning with a provocation resulting in crying or emotional upset that leads to a noiseless state of expiration accompanied by color change and ultimately loss of consciousness and postural tone (Lombroso and Lerman, 1967; DiMario, 1992). Two clinical types are recognized based on the child's coloration (cyanotic or pallid) during these events. Most children experience the cyanotic type, although some experience mixed types. BHS is an involuntary, nonvolitional, reflexic, nonepileptic paroxysmal phenomenon of childhood. The episodes occur during full expiration despite its misnomer. Autonomic dysregulation has been hypothesized as an underlying mechanism that results in loss of consciousness (Hunt, 1990; DiMario and Burleson, 1993; Dimario et al., 1998).

Congenital hypotrichosis with juvenile macular dystrophy (HJMD) is an autosomal recessive disorder characterized by hair loss followed by progressive macular degeneration and early blindness. Scalp hair is lost during the first months of life, with onset of retinal degeneration and vision loss a few years to 2 decades later (summary by Sprecher et al., 2001 and Indelman et al., 2002).

There are more than 30 types of cone-rod dystrophy, which are distinguished by their genetic cause and their pattern of inheritance: autosomal recessive, autosomal dominant, and X-linked. Additionally, cone-rod dystrophy can occur alone without any other signs and symptoms or it can occur as part of a syndrome that affects multiple parts of the body.\n\nThe first signs and symptoms of cone-rod dystrophy, which often occur in childhood, are usually decreased sharpness of vision (visual acuity) and increased sensitivity to light (photophobia). These features are typically followed by impaired color vision (dyschromatopsia), blind spots (scotomas) in the center of the visual field, and partial side (peripheral) vision loss. Over time, affected individuals develop night blindness and a worsening of their peripheral vision, which can limit independent mobility. Decreasing visual acuity makes reading increasingly difficult and most affected individuals are legally blind by mid-adulthood. As the condition progresses, individuals may develop involuntary eye movements (nystagmus).\n\nCone-rod dystrophy is a group of related eye disorders that causes vision loss, which becomes more severe over time. These disorders affect the retina, which is the layer of light-sensitive tissue at the back of the eye. In people with cone-rod dystrophy, vision loss occurs as the light-sensing cells of the retina gradually deteriorate.

X-linked spinocerebellar ataxia-6 with or without sideroblastic anemia (SCAX6) is an X-linked recessive disorder characterized by delayed motor development apparent in infancy with delayed walking (often by several years) due to ataxia and poor coordination. Additional features may include dysmetria, dysarthria, spasticity of the lower limbs, hyperreflexia, dysdiadochokinesis, strabismus, and nystagmus. The disorder is slowly progressive, and patients often lose ambulation. Brain imaging usually shows cerebellar atrophy. Most affected individuals have mild hypochromic, microcytic sideroblastic anemia, which may be asymptomatic. Laboratory studies show increased free erythrocyte protoporphyrin (FEP) and ringed sideroblasts on bone marrow biopsy. Female carriers do not have neurologic abnormalities, but may have subtle findings on peripheral blood smear (Pagon et al., 1985; D'Hooghe et al., 2012). For a discussion of genetic heterogeneity of X-linked spinocerebellar ataxia (SCAX), see SCAX1 (302500).

Disorders of intracellular cobalamin metabolism have a variable phenotype and age of onset that are influenced by the severity and location within the pathway of the defect. The prototype and best understood phenotype is cblC; it is also the most common of these disorders. The age of initial presentation of cblC spans a wide range: In utero with fetal presentation of nonimmune hydrops, cardiomyopathy, and intrauterine growth restriction. Newborns, who can have microcephaly, poor feeding, and encephalopathy. Infants, who can have poor feeding and slow growth, neurologic abnormality, and, rarely, hemolytic uremic syndrome (HUS). Toddlers, who can have poor growth, progressive microcephaly, cytopenias (including megaloblastic anemia), global developmental delay, encephalopathy, and neurologic signs such as hypotonia and seizures. Adolescents and adults, who can have neuropsychiatric symptoms, progressive cognitive decline, thromboembolic complications, and/or subacute combined degeneration of the spinal cord.

ALS2-related disorder involves retrograde degeneration of the upper motor neurons of the pyramidal tracts and comprises a clinical continuum of the following three phenotypes: Infantile ascending hereditary spastic paraplegia (IAHSP), characterized by onset of spasticity with increased reflexes and sustained clonus of the lower limbs within the first two years of life, progressive weakness and spasticity of the upper limbs by age seven to eight years, and wheelchair dependence in the second decade with progression toward severe spastic tetraparesis and a pseudobulbar syndrome caused by progressive cranial nerve involvement. Juvenile primary lateral sclerosis (JPLS), characterized by upper motor neuron findings of pseudobulbar palsy and spastic quadriplegia without dementia or cerebellar, extrapyramidal, or sensory signs. Juvenile amyotrophic lateral sclerosis (JALS or ALS2), characterized by onset between ages three and 20 years. All affected individuals show a spastic pseudobulbar syndrome (spasticity of speech and swallowing) together with spastic paraplegia. Some individuals are bedridden by age 12 to 50 years.

A cone-rod dystrophy that has material basis in variation in the chromosome region 1q12-q24.

Diamond-Blackfan anemia (DBA) is characterized by a profound normochromic and usually macrocytic anemia with normal leukocytes and platelets, congenital malformations in up to 50%, and growth deficiency in 30% of affected individuals. The hematologic complications occur in 90% of affected individuals during the first year of life. The phenotypic spectrum ranges from a mild form (e.g., mild anemia or no anemia with only subtle erythroid abnormalities, physical malformations without anemia) to a severe form of fetal anemia resulting in nonimmune hydrops fetalis. DBA is associated with an increased risk for acute myelogenous leukemia (AML), myelodysplastic syndrome (MDS), and solid tumors including osteogenic sarcoma.

Elliptocytosis is a hematologic disorder characterized by elliptically shaped erythrocytes and a variable degree of hemolytic anemia. Usually inherited as an autosomal dominant trait, elliptocytosis results from mutation in any one of several genes encoding proteins of the red cell membrane skeleton (summary by McGuire et al., 1988). Genetic Heterogeneity of Elliptocytosis Elliptocytosis-2 (130600) is caused by mutation in the SPTA1 gene (182860). Elliptocytosis-3 (617948) is caused by mutation in the SPTB gene (182870). Elliptocytosis-4 (166900), also known as Southeast Asian ovalocytosis, is caused by mutation in the SLC4A1 gene (109270). Also see pyropoikilocytosis (266140). See Delaunay (2007) for a discussion of the molecular basis of hereditary red cell membrane disorders.

Congenital nonspherocytic hemolytic anemia-1 (CNSHA1), caused by mutation in the G6PD gene, is the most common genetic form of chronic and drug-, food-, or infection-induced hemolytic anemia. G6PD catalyzes the first reaction in the pentose phosphate pathway, which is the only NADPH-generation process in mature red cells; therefore, defense against oxidative damage is dependent on G6PD. Most G6PD-deficient patients are asymptomatic throughout their life, but G6PD deficiency can be life-threatening. The most common clinical manifestations of G6PD deficiency are neonatal jaundice and acute hemolytic anemia, which in most patients is triggered by an exogenous agent, e.g., primaquine or fava beans. Acute hemolysis is characterized by fatigue, back pain, anemia, and jaundice. Increased unconjugated bilirubin, lactate dehydrogenase, and reticulocytosis are markers of the disorder. The striking similarity between the areas where G6PD deficiency is common and Plasmodium falciparum malaria (see 611162) is endemic provided evidence that G6PD deficiency confers resistance against malaria (summary by Cappellini and Fiorelli, 2008).

Autosomal recessive childhood-onset severe retinal dystrophy is a heterogeneous group of disorders affecting rod and cone photoreceptors simultaneously. The most severe cases are termed Leber congenital amaurosis, whereas the less aggressive forms are usually considered juvenile retinitis pigmentosa (Gu et al., 1997). For a general phenotypic description and a discussion of genetic heterogeneity of Leber congenital amaurosis, see LCA1 (204000); for retinitis pigmentosa, see 268000.

A rare, syndromic, hereditary optic neuropathy disorder characterized by early-onset, severe, progressive visual impairment, optic disc pallor and central scotoma, variably associated with dyschromatopsia, auditory neuropathy (e.g. mild progressive sensorineural hearing loss), sensorimotor axonal neuropathy and, occasionally, moderate hypertrophic cardiomyopathy.

Any retinitis pigmentosa in which the cause of the disease is a mutation in the TTC8 gene.

Myopathy, lactic acidosis, and sideroblastic anemia-2 (MLASA2) is an autosomal recessive disorder of the mitochondrial respiratory chain. The disorder shows marked phenotypic variability: some patients have a severe multisystem disorder from infancy, including cardiomyopathy and respiratory insufficiency resulting in early death, whereas others present in the second or third decade of life with sideroblastic anemia and mild muscle weakness (summary by Riley et al., 2013). For a discussion of genetic heterogeneity of MLASA, see MLASA1 (600462).

Dihydrofolate reductase deficiency is an autosomal recessive metabolic disorder characterized by the hematologic findings of megaloblastic anemia and variable neurologic symptoms, ranging from severe developmental delay and generalized seizures in infancy (Banka et al., 2011) to childhood absence epilepsy with learning difficulties to lack of symptoms (Cario et al., 2011). Treatment with folinic acid can ameliorate some of the symptoms.

An autosomal recessive muscular dystrophy caused by mutations in the POMGNT1 gene. It is associated with characteristic brain and eye malformations, profound mental retardation, and death usually in the first years of life.

Multiple congenital anomalies-hypotonia-seizures syndrome-2 (MCAHS2) is an X-linked recessive neurodevelopmental disorder characterized by dysmorphic features, neonatal hypotonia, early-onset myoclonic seizures, and variable congenital anomalies involving the central nervous, cardiac, and urinary systems. Some affected individuals die in infancy (summary by Johnston et al., 2012). The phenotype shows clinical variability with regard to severity and extraneurologic features. However, most patients present in infancy with early-onset epileptic encephalopathy associated with developmental arrest and subsequent severe neurologic disability; these features are consistent with a form of developmental and epileptic encephalopathy (DEE) (summary by Belet et al., 2014, Kato et al., 2014). The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis. For a discussion of genetic heterogeneity of MCAHS, see MCAHS1 (614080). For a discussion of nomenclature and genetic heterogeneity of DEE, see 308350. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

STEAP3/TSAP6-related sideroblastic anemia is a very rare severe non-syndromic hypochromic anemia, which is characterized by transfusion-dependent hypochromic, poorly regenerative anemia, iron overload, resembling non-syndromic sideroblastic anemia (see this term) except for increased erythrocyte protoporphyrin levels.

Congenital dyserythropoietic anemia type I (CDA I) is characterized by moderate-to-severe macrocytic anemia presenting occasionally in utero as severe anemia associated with hydrops fetalis but more commonly in neonates as hepatomegaly, early jaundice, and intrauterine growth restriction. Some individuals present in childhood or adulthood. After the neonatal period, most affected individuals have lifelong moderate anemia, usually accompanied by jaundice and splenomegaly. Secondary hemochromatosis develops with age as a result of increased iron absorption even in those who are not transfused. Distal limb anomalies occur in 4%-14% of affected individuals.

Any Senior-Loken syndrome in which the cause of the disease is a mutation in the WDR19 gene.

Any retinitis pigmentosa in which the cause of the disease is a mutation in the AGBL5 gene.

Mitochondrial myopathy and ataxia (MMYAT) is an autosomal recessive mtDNA depletion disorder characterized by cerebellar ataxia, congenital muscle involvement with histologic findings ranging from myopathic to dystrophic, and pigmentary retinopathy (summary by Donkervoort et al., 2019).

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).

Individuals with hereditary distal renal tubular acidosis (dRTA) typically present in infancy with failure to thrive, although later presentations can occur, especially in individuals with autosomal dominant SLC4A1-dRTA. Initial clinical manifestations can also include emesis, polyuria, polydipsia, constipation, diarrhea, decreased appetite, and episodes of dehydration. Electrolyte manifestations include hyperchloremic non-anion gap metabolic acidosis and hypokalemia. Renal complications of dRTA include nephrocalcinosis, nephrolithiasis, medullary cysts, and impaired renal function. Additional manifestations include bone demineralization (rickets, osteomalacia), growth deficiency, sensorineural hearing loss (in ATP6V0A4-, ATP6V1B1-, and FOXI1-dRTA), and hereditary hemolytic anemia (in some individuals with SLC4A1-dRTA).

DEGCAGS syndrome is an autosomal recessive syndromic neurodevelopmental disorder characterized by global developmental delay, coarse and dysmorphic facial features, and poor growth and feeding apparent from infancy. Affected individuals have variable systemic manifestations often with significant structural defects of the cardiovascular, genitourinary, gastrointestinal, and/or skeletal systems. Additional features may include sensorineural hearing loss, hypotonia, anemia or pancytopenia, and immunodeficiency with recurrent infections. Death in childhood may occur (summary by Bertoli-Avella et al., 2021).