Holocarboxylase Synthetase Deficiency: A Second Case Report With Neonatal Cholestatic Liver Disease

Holocarboxylase synthetase deficiency is an autosomal recessive inborn error of metabolism characterised by life‐threatening metabolic acidosis, ketoacidosis and hyperammonaemia through reduced biotin‐dependent carboxylase activity. We report the presentation of a Polynesian neonate with severe metabolic acidosis secondary to holocarboxylase synthetase deficiency with the development of cholestatic liver disease thought to be secondary to holocarboxylase synthetase deficiency. This is only the second reported case of holocarboxylase synthetase deficiency associated with cholestatic liver disease. Both of these cases were a result of the same homozygous c.647T>G L216R pathogenic variants in the HLCS gene suggesting a possible genotype–phenotype correlation and broadening the phenotypic understanding of this disease.