Holocarboxylase Synthase Deficiency · HLCS

Description

Holocarboxylase synthetase deficiency, HLCS deficiency, is an autosomal recessive disorder of biotin metabolism. Holocarboxylase synthetase, encoded by HLCS on chromosome 21, attaches biotin to four mitochondrial and cytosolic carboxylases: pyruvate carboxylase, propionyl-CoA carboxylase, 3-methylcrotonyl-CoA carboxylase, and acetyl-CoA carboxylase. Pathogenic variants in HLCS reduce or abolish biotin attachment, all four carboxylases lose activity, and the result is multiple carboxylase deficiency in its neonatal or early-infantile form. Lactic acidosis, organic aciduria, and ketoacidosis develop because the affected enzymes participate in gluconeogenesis, fatty acid synthesis, and the catabolism of leucine, isoleucine, and valine.

The classical presentation is in the first weeks to months of life, with poor feeding, vomiting, lethargy, hypotonia, tachypnea from metabolic acidosis, and frequently a characteristic skin rash and partial alopecia. Untreated, the course progresses to coma. A milder later-onset form is described and presents with episodic decompensation. The presentation overlaps with biotinidase deficiency, which has a different molecular cause but the same multiple-carboxylase biochemical signature; the two are distinguished by enzyme assay and gene sequencing.

Detection is by newborn screening on the dried blood spot, using tandem mass spectrometry to identify elevated 3-hydroxyisovalerylcarnitine, C5-OH, sometimes accompanied by elevated propionylcarnitine, C3. C5-OH is shared with isovaleric acidemia, 3-MCC, biotinidase deficiency, HMG-CoA lyase deficiency, and beta-ketothiolase deficiency. Confirmation uses urine organic acids showing elevations of 3-hydroxyisovaleric acid, 3-methylcrotonylglycine, methylcitrate, and lactate, biotinidase activity assay to exclude biotinidase deficiency, and HLCS sequencing. HLCS deficiency is rare. Reported incidence is well below 1 in 200,000 live births and varies by population and ascertainment.

Treatments to date

Standard of care is lifelong oral biotin supplementation. Dosing typically begins at 10 to 40 mg per day and is titrated to clinical and biochemical response; some HLCS variants respond at lower doses and a subset require higher doses. Most affected children respond completely to biotin started early, with normalization of organic acid excretion, resolution of skin findings, and normal growth and development. There is no FDA-approved disease-specific drug therapy beyond biotin itself, which is available as a generic supplement.

A subset of HLCS variants, including some involving the biotin-binding region, respond incompletely to biotin alone. In those cases, additional measures include modest protein restriction, L-carnitine supplementation, sick-day protocols with intravenous dextrose during illness, and careful management of acute decompensations. Acute crises are managed with fluid resuscitation, correction of acidosis, intravenous dextrose, and ammonia scavengers when hyperammonemia is present. Outcomes for children identified by newborn screening and started on biotin in the first weeks of life are generally good, with the responsiveness of the underlying HLCS variant the principal determinant of long-term course.