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Holocarboxylase synthetase deficiency
Holocarboxylase synthetase (HCS) deficiency is a life-threatening early-onset form of multiple carboxylase deficiency (see this term), an inborn error of biotin metabolism, that, if untreated, is characterized by vomiting, tachypnea, irritability, lethargy, exfoliative dermatitis, and seizures that can worsen to coma.
The exact prevalence of HCSD is unknown, but the condition is one of the rarest inborn errors of metabolism. Annual incidence is estimated to be less than 1/200,000 live births.
Clinical onset is usually within hours, days or weeks of birth. Individuals with the disorder usually exhibit poor appetite, vomiting, lethargy, irritability, hypotonia and exfoliative dermatitis. Metabolically, they have ketolactic acidosis, organic acidemia (-uria) and hyperammonemia. Without treatment, affected infants may progress to intractable seizures, cerebral edema and coma. These children often develop growth and developmental delays.
Holocarboxylase synthetase deficiency is caused by mutations in the HLCS gene (21q22.1) resulting in reduced HCS activity. This enzyme is important in covalent binding of biotin to the various biotin-dependent carboxylases that require the vitamin for activity. Failure to attach the biotin results in multiple carboxylase deficiency and accumulation of various, specific abnormal organic acids.
Some affected individuals are identified through newborn screening by demonstration of abnormal organic acids, consistent with multiple carboxyalse deficiency. Diagnosis is based on clinical signs and typical organic acid abnormalities. Confirmational testing can be performed by demonstrating deficient HCS activity in leukocytes or fibroblast extracts or by mutation analysis.
Conditions to be considered in the differential diagnosis based on organic acids include biotinidase deficiency (see this term) and isolated carboxyalse deficiencies; based on hyperammonemia, include urea cycle defects (see this term); and based on neurological compromise and seizures in the neonatal period, include sepsis and other inborn errors of metabolism.
Prenatal diagnosis can be performed by organic acid analysis by stable isotope dilution techniques in amniotic fluid, enzymatic determination of HCS activity in amniocytes, or mutation analysis on DNA from chorionic villus biopsy or amniocentesis.
HCS deficiency is inherited as an autosomal recessive trait. Genetic counseling is available to families who have children with the disorder. Siblings of affected children are unlikely to have the disorder or they would have developed symptoms, but they may be carriers.
Management and treatment
The primary treatment for HCS deficiency is free biotin supplementation which can improve the clinical status of symptomatic individuals with the enzyme deficiency and prevent some or all symptoms from developing in asymptomatic individuals with the disorder. Treatment should be started as soon as possible after diagnosis and must be continued lifelong. Affected individuals should be monitored for later-onset complications and for compliance with therapy. Timely and ongoing treatment makes it possible to reduce symptoms considerably, although some patients develop complications despite appropriate treatment often requiring higher doses of biotin.
In the absence of early diagnosis and treatment, mortality is high. Morbidity in surviving affected individuals depends on the time of diagnosis and on the degree of damage related to metabolic crises.
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