A case of hyperlysinemia identified by urine newborn screening

Hyperlysinemia is a rare autosomal recessive deficiency of 2‐aminoadipic semialdehyde synthase (AASS) affecting the initial step in lysine degradation. It is thought to be a benign biochemical abnormality, but reports on cases remain scarce. The description of additional cases, in particular, those identified without ascertainment bias, may help counseling of new cases in the future. It may also help to establish the risks associated with pharmacological inhibition of AASS, a potential therapeutic strategy that is under investigation for other inborn errors of lysine degradation. We describe the identification of a hyperlysinemia case identified in the Provincial Neonatal Urine Screening Program in Sherbrooke, Quebec. This case presented with a profile of cystinuria but with a very high increase in urinary lysine. A diagnosis of hyperlysinemia was confirmed through biochemical testing and the identification of biallelic variants in AASS. The p.R146W and p.T371I variants are novel and affect the folding of the lysine‐2‐oxoglutarate domain of AASS. The 11‐month‐old boy is currently doing well without any therapeutic interventions. The identification of this case through newborn urine screening further establishes that hyperlysinemia is a biochemical abnormality with limited clinical consequences and may not require any intervention.