Holocarboxylase Synthetase (HCS) Deficiency

A type of multiple carboxylase deficiency, HCS deficiency is a recessive genetic disorder that typically appears up until 10 years of age. The most common symptom at onset is respiratory problems, although diabetic ketoacidosis was reported as an initial finding in one patient. Age of onset seems to be partially dependent upon ethnicity, with non-Japanese patients first presenting symptoms later in age than Japanese infants.⁵⁵

Defects in the holocarboxylase synthetase enzyme block biotin from binding to carboxylase enzymes. HCS deficiency may also impact DNA functions, since holocarboxylase synthetase mediates the biotinylation of histones (DNA-binding proteins). In turn this reduces the biotin-dependent carboxylase enzyme metabolic activities, and without treatment can lead to permanent neurological damage, coma, and even death.^{7, 56}

Even though there are normal levels of biotin in the blood, patients with this condition require high doses of biotin to compensate for this lack of enzyme.⁵⁶