Alpha-thalassemia (a-thalassemia) has two clinically significant forms: hemoglobin Bart hydrops fetalis (Hb Bart) syndrome (caused by deletion/inactivation of all four a-globin genes; --/--), and hemoglobin H (HbH) disease (most frequently caused by deletion/inactivation of three a-globin genes; --/-a). Hb Bart syndrome, the more severe form, is characterized by prenatal onset of generalized edema and pleural and pericardial effusions as a result of congestive heart failure induced by severe anemia. Extramedullary erythropoiesis, marked hepatosplenomegaly, and a massive placenta are common. Death usually occurs in the neonatal period. HbH disease has a broad phenotypic spectrum: although clinical features usually develop in the first years of life, HbH disease may not present until adulthood or may be diagnosed only during routine hematologic analysis in an asymptomatic individual. The majority of individuals have enlargement of the spleen (and less commonly of the liver), mild jaundice, and sometimes thalassemia-like bone changes. Individuals with HbH disease may develop gallstones and experience acute episodes of hemolysis in response to infections or exposure to oxidant drugs.

Beta-thalassemia (ß-thalassemia) is characterized by reduced synthesis of the hemoglobin subunit beta (hemoglobin beta chain) that results in microcytic hypochromic anemia, an abnormal peripheral blood smear with nucleated red blood cells, and reduced amounts of hemoglobin A (HbA) on hemoglobin analysis. Individuals with thalassemia major have severe anemia and hepatosplenomegaly; they usually come to medical attention within the first two years of life. Without treatment, affected children have severe failure to thrive and shortened life expectancy. Treatment with a regular transfusion program and chelation therapy, aimed at reducing transfusion iron overload, allows for normal growth and development and may improve the overall prognosis. Individuals with thalassemia intermedia present later and have milder anemia that does not require regular treatment with blood transfusion. These individuals are at risk for iron overload secondary to increased intestinal absorption of iron as a result of ineffective erythropoiesis.

Finberg et al. (2008) referred to this phenotype as iron-refractory iron deficiency anemia (IRIDA) and reviewed the key features: a congenital hypochromic, microcytic anemia; a very low mean corpuscular erythrocyte volume; a low transferrin saturation; abnormal iron absorption characterized by no hematologic improvement following treatment with oral iron; and abnormal iron utilization characterized by a sluggish, incomplete response to parenteral iron. The authors noted that although urinary levels of hepcidin (606464) are typically undetectable in individuals with iron deficiency, in 5 individuals with IRIDA urinary hepcidin/creatinine ratios were within or above the normal range.

Barth syndrome is characterized in affected males by cardiomyopathy, neutropenia, skeletal myopathy, prepubertal growth delay, and distinctive facial gestalt (most evident in infancy); not all features may be present in a given affected male. Cardiomyopathy, which is almost always present before age five years, is typically dilated cardiomyopathy with or without endocardial fibroelastosis or left ventricular noncompaction; hypertrophic cardiomyopathy can also occur. Heart failure is a significant cause of morbidity and mortality; risk of arrhythmia and sudden death is increased. Neutropenia is most often associated with mouth ulcers, pneumonia, and sepsis. The nonprogressive myopathy predominantly affects the proximal muscles, and results in early motor delays. Prepubertal growth delay is followed by a postpubertal growth spurt with remarkable "catch-up" growth. Heterozygous females who have a normal karyotype are asymptomatic and have normal biochemical studies.

An acquired form of alpha-thalassemia characterized by a myelodysplastic syndrome (MDS) or more rarely a myeloproliferative disease (MPD) associated with hemoglobin H disease (HbH).

Alpha-thalassemia X-linked intellectual disability (ATR-X) syndrome is characterized by distinctive craniofacial features, genital anomalies, hypotonia, and mild-to-profound developmental delay / intellectual disability (DD/ID). Craniofacial abnormalities include small head circumference, telecanthus or widely spaced eyes, short triangular nose, tented upper lip, and thick or everted lower lip with coarsening of the facial features over time. While all affected individuals have a normal 46,XY karyotype, genital anomalies comprise a range from hypospadias and undescended testicles, to severe hypospadias and ambiguous genitalia, to normal-appearing female external genitalia. Alpha-thalassemia, observed in about 75% of affected individuals, is mild and typically does not require treatment. Osteosarcoma has been reported in a few males with germline pathogenic variants.

Autosomal recessive osteopetrosis-5 is a form of infantile malignant osteopetrosis, characterized by defective osteoclast function resulting in decreased bone resorption and generalized osteosclerosis. Defective resorption causes development of densely sclerotic fragile bones and progressive obliteration of the marrow spaces and cranial foramina. Marrow obliteration is associated with extramedullary hematopoiesis and hepatosplenomegaly, and results in anemia and thrombocytopenia, whereas nerve entrapment accounts for progressive blindness and hearing loss. Other major manifestations include failure to thrive, pathologic fractures, and increased infection rate. Most affected children succumb to severe bone marrow failure and overwhelming infection in the first few years of life (Quarello et al., 2004).

Sideroblastic anemia with B-cell immunodeficiency, periodic fevers, and developmental delay (SIFD) is an autosomal recessive syndromic disorder characterized by onset of severe sideroblastic anemia in the neonatal period or infancy. Affected individuals show delayed psychomotor development with variable neurodegeneration. Recurrent periodic fevers without an infectious etiology occur throughout infancy and childhood; immunologic work-up shows B-cell lymphopenia and hypogammaglobulinemia. Other more variable features include sensorineural hearing loss, retinitis pigmentosa, nephrocalcinosis, and cardiomyopathy. Death in the first decade may occur (summary by Wiseman et al., 2013).

X-linked sideroblastic anemia is an inherited disorder that prevents developing red blood cells (erythroblasts) from making enough hemoglobin, which is the protein that carries oxygen in the blood. People with X-linked sideroblastic anemia have mature red blood cells that are smaller than normal (microcytic) and appear pale (hypochromic) because of the shortage of hemoglobin. This disorder also leads to an abnormal accumulation of iron in red blood cells. The iron-loaded erythroblasts, which are present in bone marrow, are called ring sideroblasts. These abnormal cells give the condition its name.\n\nThe signs and symptoms of X-linked sideroblastic anemia result from a combination of reduced hemoglobin and an overload of iron. They range from mild to severe and most often appear in young adulthood. Common features include fatigue, dizziness, a rapid heartbeat, pale skin, and an enlarged liver and spleen (hepatosplenomegaly). Over time, severe medical problems such as heart disease and liver damage (cirrhosis) can result from the buildup of excess iron in these organs.

Infantile-onset progressive leukoencephalopathy with or without deafness (LEPID) is an autosomal recessive complex neurodegenerative disorder with onset of symptoms in infancy or early childhood. Most patients present with sensorineural deafness or hypoacousia and global developmental delay. Affected individuals show episodic regression with progressive motor deterioration resulting in spastic tetraplegia and loss of ambulation, as well as impaired intellectual development with poor or absent speech. Additional more variable features may include poor overall growth with microcephaly, seizures, visual loss, microcytic anemia, and hepatic enlargement or abnormal liver enzymes. Brain imaging shows deep white matter abnormalities consistent with a progressive leukoencephalopathy. The brain and spinal cord are usually both involved; calcifications of these regions are often observed. Laboratory studies show increased serum lactate and deficiencies of mitochondrial respiratory chain complexes, consistent with global mitochondrial dysfunction. Early death often occurs (summary by Itoh et al., 2019).

Combined oxidative phosphorylation deficiency-53 (COXPD53) is an autosomal recessive disorder characterized by hypomyelination, microcephaly, liver dysfunction, and recurrent autoinflammation (summary by Lausberg et al., 2021). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Sideroblastic anemia-5 (SIDBA5) is an autosomal recessive hematologic disorder characterized by abnormal iron accumulation in the mitochondria or erythroid cells. The pathologic iron deposits appear to ring the nucleus, resulting in a 'ringed sideroblast' on pathologic examination. Affected individuals have congenital hypochromic microcytic anemia apparent in childhood; they may also develop thrombocytopenia or pancytopenia (summary by Crispin et al., 2020). For a discussion of genetic heterogeneity of sideroblastic anemia, see SIDBA1 (300751).

Immunodeficiency-89 and autoimmunity (IMD89) is an autosomal recessive immune disorder characterized by adult onset of recurrent infections, allergies, microcytic anemia, and Crohn disease (see 266600) (Yang et al., 2020).