Polycythemia vera (PV), the most common form of primary polycythemia, is caused by somatic mutation in a single hematopoietic stem cell leading to clonal hematopoiesis. PV is a myeloproliferative disorder characterized predominantly by erythroid hyperplasia, but also by myeloid leukocytosis, thrombocytosis, and splenomegaly. Familial cases of PV are very rare and usually manifest in elderly patients (Cario, 2005). PV is distinct from the familial erythrocytoses (see, e.g., ECYT1, 133100), which are caused by inherited mutations resulting in hypersensitivity of erythroid progenitors to hormonal influences or increased levels of circulating hormones, namely erythropoietin (EPO; 133170) (Prchal, 2005).

Complement hyperactivation, angiopathic thrombosis, and protein-losing enteropathy (CHAPLE) is characterized by abdominal pain and diarrhea, primary intestinal lymphangiectasia, hypoproteinemic edema, and malabsorption. Some patients also exhibit bowel inflammation, recurrent infections associated with hypogammaglobulinemia, and/or angiopathic thromboembolic disease. Patient T lymphocytes show increased complement activation, causing surface deposition of complement and generating soluble C5a (Ozen et al., 2017).

PMM2-CDG, the most common of a group of disorders of abnormal glycosylation of N-linked oligosaccharides, is divided into three clinical stages: infantile multisystem, late-infantile and childhood ataxia–intellectual disability, and adult stable disability. The clinical manifestations and course are highly variable, ranging from infants who die in the first year of life to mildly affected adults. Clinical findings tend to be similar in sibs. In the infantile multisystem presentation, infants show axial hypotonia, hyporeflexia, esotropia, and developmental delay. Feeding problems, vomiting, faltering growth, and developmental delay are frequently seen. Subcutaneous fat may be excessive over the buttocks and suprapubic region. Two distinct clinical courses are observed: (1) a nonfatal neurologic course with faltering growth, strabismus, developmental delay, cerebellar hypoplasia, and hepatopathy in infancy followed by neuropathy and retinitis pigmentosa in the first or second decade; and (2) a more severe neurologic-multivisceral course with approximately 20% mortality in the first year of life. The late-infantile and childhood ataxia–intellectual disability stage, which begins between ages three and ten years, is characterized by hypotonia, ataxia, severely delayed language and motor development, inability to walk, and IQ of 40 to 70; other findings include seizures, stroke-like episodes or transient unilateral loss of function, coagulopathy, retinitis pigmentosa, joint contractures, and skeletal deformities. In the adult stable disability stage, intellectual ability is stable; peripheral neuropathy is variable, progressive retinitis pigmentosa and myopia are seen, thoracic and spinal deformities with osteoporosis worsen, and premature aging is observed; females may lack secondary sexual development and males may exhibit decreased testicular volume. Hypogonadotropic hypogonadism and coagulopathy may occur. The risk for deep venous thrombosis is increased.

Isolated congenital asplenia (ICAS) is a rare cause of primary immunodeficiency. Most affected individuals die of severe bacterial infections in early childhood. Isolated asplenia is distinct from asplenia associated with other complex visceral defects, notably heterotaxy syndromes such as Ivemark syndrome (208530) (summary by Mahlaoui et al., 2011).

Heme oxygenase-1 deficiency (HMOX1D) is a rare autosomal recessive disorder with a complex clinical presentation including direct antibody negative hemolytic anemia, low bilirubin, and hyperinflammation (summary by Chau et al., 2020). Other features may include asplenia and nephritis (Radhakrishnan et al., 2011).

Pyogenic sterile arthritis, pyoderma gangrenosum, and acne (PAPA) is a rare autosomal dominant autoinflammatory disease that typically presents with recurrent sterile, erosive arthritis in childhood, occurring spontaneously or after minor trauma, occasionally resulting in significant joint destruction. By puberty, joint symptoms tend to subside and cutaneous symptoms predominate, including pathergy, frequently with abscesses at the sites of injections, severe cystic acne, and recurrent nonhealing sterile ulcers, often diagnosed as pyoderma gangrenosum (summary by Demidowich et al., 2012).

Celiac disease is a systemic autoimmune disease that can be associated with gastrointestinal findings (diarrhea, malabsorption, abdominal pain and distension, bloating, vomiting, and weight loss) and/or highly variable non-gastrointestinal findings (dermatitis herpetiformis, chronic fatigue, joint pain/inflammation, iron deficiency anemia, migraines, depression, attention-deficit disorder, epilepsy, osteoporosis/osteopenia, infertility and/or recurrent fetal loss, vitamin deficiencies, short stature, failure to thrive, delayed puberty, dental enamel defects, and autoimmune disorders). Classic celiac disease, characterized by mild to severe gastrointestinal symptoms, is less common than non-classic celiac disease, characterized by absence of gastrointestinal symptoms.

ELANE-related neutropenia includes congenital neutropenia and cyclic neutropenia, both of which are primary hematologic disorders characterized by recurrent fever, skin and oropharyngeal inflammation (i.e., mouth ulcers, gingivitis, sinusitis, and pharyngitis), and cervical adenopathy. Infectious complications are generally more severe in congenital neutropenia than in cyclic neutropenia. In congenital neutropenia, omphalitis immediately after birth may be the first sign; in untreated children diarrhea, pneumonia, and deep abscesses in the liver, lungs, and subcutaneous tissues are common in the first year of life. After 15 years with granulocyte colony-stimulating factor treatment, the risk of developing myelodysplasia (MDS) or acute myelogenous leukemia (AML) is approximately 15%-25%. Cyclic neutropenia is usually diagnosed within the first year of life based on approximately three-week intervals of fever and oral ulcerations and regular oscillations of blood cell counts. Cellulitis, especially perianal cellulitis, is common during neutropenic periods. Between neutropenic periods, affected individuals are generally healthy. Symptoms improve in adulthood. Cyclic neutropenia is not associated with risk of malignancy or conversion to leukemia.

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.

Familial thrombocytosis in which the cause of the disease is a mutation in the MPL gene.

Thrombocythemia, or thrombocytosis, is a myeloproliferative disorder characterized by excessive platelet production resulting in increased numbers of circulating platelets. Thrombocythemia can be associated with thrombotic or hemorrhagic episodes and occasional leukemic transformation (summary by Wiestner et al., 1998). Genetic Heterogeneity of Thrombocythemia THCYT2 (601977) is caused by germline or somatic mutation in the THPO receptor gene (MPL; 159530) on chromosome 1p34, and THCYT3 (614521) is caused by germline or somatic mutation in the JAK2 gene (147796) on chromosome 9p. Somatic mutations in the TET2 (612839), ASXL1 (612990), SH2B3 (605093), and SF3B1 (605590) genes have also been found in cases of essential thrombocythemia. Somatic mutation in the CALR gene (109091) occurs in approximately 70% of essential thrombocythemia patients who lack JAK2 and MPL mutations (Klampfl et al., 2013; Nangalia et al., 2013).

Thrombocythemia-3 is an autosomal dominant hematologic disorder characterized by increased platelet production resulting in increased numbers of circulating platelets. Thrombocythemia can be associated with thrombotic episodes, such as cerebrovascular events or myocardial infarction (summary by Mead et al., 2012). For a discussion of genetic heterogeneity of thrombocythemia, see THCYT1 (187950).

A rare, heterogeneous group of rheumatologic diseases characterized by arthritis which has an onset before 16 years of age, persists for more than 6 weeks, and is of unknown origin.

Adenosine deaminase 2 deficiency (DADA2) is a complex systemic autoinflammatory disorder in which vasculopathy/vasculitis, dysregulated immune function, and/or hematologic abnormalities may predominate. Inflammatory features include intermittent fevers, rash (often livedo racemosa/reticularis), and musculoskeletal involvement (myalgia/arthralgia, arthritis, myositis). Vasculitis, which usually begins before age ten years, may manifest as early-onset ischemic (lacunar) and/or hemorrhagic strokes, or as cutaneous or systemic polyarteritis nodosa. Hypertension and hepatosplenomegaly are often found. More severe involvement may lead to progressive central neurologic deficits (dysarthria, ataxia, cranial nerve palsies, cognitive impairment) or to ischemic injury to the kidney, intestine, and/or digits. Dysregulation of immune function can lead to immunodeficiency or autoimmunity of varying severity; lymphadenopathy may be present and some affected individuals have had lymphoproliferative disease. Hematologic disorders may begin early in life or in late adulthood, and can include lymphopenia, neutropenia, pure red cell aplasia, thrombocytopenia, or pancytopenia. Of note, both interfamilial and intrafamilial phenotypic variability (e.g., in age of onset, frequency and severity of manifestations) can be observed; also, individuals with biallelic ADA2 pathogenic variants may remain asymptomatic until adulthood or may never develop clinical manifestations of DADA2.

Immunodeficiency-27A (IMD27A) results from autosomal recessive (AR) IFNGR1 deficiency. Patients with complete IFNGR1 deficiency have a severe clinical phenotype characterized by early and often fatal mycobacterial infections. The disorder can thus be categorized as a form of mendelian susceptibility to mycobacterial disease (MSMD). Bacillus Calmette-Guerin (BCG) and environmental mycobacteria are the most frequent pathogens, and infection typically begins before the age of 3 years. Plasma from patients with complete AR IFNGR1 deficiency usually contains large amounts of IFNG (147570), and their cells do not respond to IFNG in vitro. In contrast, cells from patients with partial AR IFNGR1 deficiency, which is caused by a specific mutation in IFNGR1, retain residual responses to high IFNG concentrations. Patients with partial AR IFNGR1 deficiency are susceptible to BCG and environmental mycobacteria, but they have a milder clinical disease and better prognosis than patients with complete AR IFNGR1 deficiency. The clinical features of children with complete AR IFNGR1 deficiency are usually more severe than those in individuals with AD IFNGR1 deficiency (IMD27B), and mycobacterial infection often occurs earlier (mean age of 1.3 years vs 13.4 years), with patients having shorter mean disease-free survival. Salmonellosis is present in about 5% of patients with AR or AD IFNGR1 deficiency, and other infections have been reported in single patients (review by Al-Muhsen and Casanova, 2008).

STING-associated vasculopathy with onset in infancy is an autoinflammatory vasculopathy causing severe skin lesions, particularly affecting the face, ears, nose, and digits, and resulting in ulceration, eschar formation, necrosis, and, in some cases, amputation. Many patients have interstitial lung disease. Tissue biopsy and laboratory findings show a hyperinflammatory state, with evidence of increased beta-interferon (IFNB1; 147640) signaling (summary by Liu et al., 2014).

In dehydrated hereditary stomatocytosis (DHS), also known as hereditary xerocytosis, red blood cells exhibit altered intracellular cation content and cellular dehydration, resulting in increased erythrocyte mean corpuscular hemoglobin concentration (MCHC) and decreased erythrocyte osmotic fragility. Blood films show various cell shape abnormalities, the most characteristic being the stomatocyte, with a straight or crescent-shaped central pallor (summary by Rapetti-Mauss et al., 2015). For discussion of clinical and genetic heterogeneity of the stomatocytoses, see DHS1 (194380).

Interstitial lung and liver disease is an autosomal recessive disorder characterized by onset of respiratory insufficiency and progressive liver disease in infancy or early childhood. Pathologic examination of lung lavage is consistent with pulmonary alveolar proteinosis (summary by Hadchouel et al., 2015).

Combined immunodeficiency and megaloblastic anemia with or without hyperhomocysteinemia is an inborn error of folate metabolism due to deficiency of methylenetetrahydrofolate dehydrogenase-1. Manifestations may include hemolytic uremic syndrome, macrocytosis, epilepsy, hearing loss, retinopathy, mild mental retardation, lymphopenia involving all subsets, and low T-cell receptor excision circles. Folinic acid supplementation is an effective treatment (summary by Ramakrishnan et al., 2016).

Trichohepatoenteric syndrome (THES), generally considered to be a neonatal enteropathy, is characterized by intractable diarrhea (seen in almost all affected children), woolly hair (seen in all), intrauterine growth restriction, facial dysmorphism, and short stature. Additional findings include poorly characterized immunodeficiency, recurrent infections, skin abnormalities, and liver disease. Mild intellectual disability (ID) is seen in about 50% of affected individuals. Less common findings include congenital heart defects and platelet anomalies. To date 52 affected individuals have been reported.

A rare genetic disease characterized by infantile onset of severe inflammatory bowel disease manifesting with bloody diarrhea and failure to thrive, and central nervous system disease with global developmental delay and regression, impaired speech, hypotonia, hyperreflexia, and epilepsy. Brain imaging shows global cerebral atrophy, thin corpus callosum, delayed myelination, and posterior leukoencephalopathy. Cases with recurrent infections and impaired T-cell responses to stimulation, as well as decreased T-cell subsets, have been reported.

Immunodeficiency-69 (IMD69) is an autosomal recessive disorder characterized by increased susceptibility to disseminated mycobacterial infection, including after BCG (bacille Calmette-Guerin) vaccination. Affected individuals develop fever, hepatosplenomegaly, leukocytosis, and thrombocytosis during the acute infection. There appears to be normal immunologic function against other pathogens, including viruses and bacteria. Immunologic work-up shows normal parameters, but patient T and NK cells fail to produce gamma-interferon (IFNG) when stimulated in vitro (summary by Kerner et al., 2020). IMD69 is a form of mendelian susceptibility to mycobacterial disease (MSMD) (see, e.g., IMD27A; 209950).

Autosomal recessive primary immunodeficiency-14B (IMD14B) is characterized by onset of recurrent infections in early childhood. Most patients have respiratory infections, but some may develop inflammatory bowel disease or osteomyelitis. Laboratory studies tend to show hypogammaglobulinemia and decreased levels of B cells. Although NK cell and T cell numbers are normal, there may be evidence of impaired immune-mediated cytotoxicity and defective T-cell function (summary by et al., 2018 and et al., 2019).

Immunodeficiency-92 (IMD92) is an autosomal recessive primary immunodeficiency characterized by the onset of recurrent infections in infancy or early childhood. Infectious agents are broad, including bacterial, viral, fungal, and parasitic, including Cryptosporidium and Mycobacteria. Patient lymphocytes show defects in both T- and B-cell proliferation, cytokine secretion, and overall function, and there is also evidence of dysfunction of NK, certain antigen-presenting cells, and myeloid subsets. Hematopoietic stem cell transplantation may be curative (summary by Beaussant-Cohen et al., 2019 and Levy et al., 2021).

X-linked familial Behcet-like autoinflammatory syndrome-2 (AIFBL2) is an X-linked recessive disorder characterized by the onset of inflammatory symptoms in the first decade of life in male patients. Affected males often present with oral mucosal ulceration and skin inflammation. More variable features may include gastrointestinal ulceration, arthritis, recurrent fevers, and iron deficiency anemia. Laboratory studies are consistent with immune dysregulation manifest as increased inflammatory markers and variable immune cell abnormalities, such as decreased NK cells and low memory B cells. One patient presented with recurrent infections and immunodeficiency in addition to autoinflammation. The disorder results from a defect in ELF4, which normally acts as a negative regulator of inflammatory disease. Symptoms may respond to blockade of IL1 (see 147760) or TNFA (191160) (summary by Tyler et al., 2021 and Sun et al., 2022). For a discussion of genetic heterogeneity of AIFBL, see AIFBL1 (616744).

Gastrointestinal defects and immunodeficiency syndrome-1 (GIDID1) is characterized by multiple intestinal atresia, in which atresia occurs at various levels throughout the small and large intestines. Surgical outcomes are poor, and the condition is usually fatal within the first month of life. Some patients exhibit inflammatory bowel disease (IBD), with or without intestinal atresia, and in some cases, the intestinal features are associated with either mild or severe combined immunodeficiency (Samuels et al., 2013; Avitzur et al., 2014; Lemoine et al., 2014). Genetic Heterogeneity of GIDID See also GIDID2 (619708), caused by mutation in the PI4KA gene (600286) on chromosome 22q11.