Hyperlysinemia is an inherited condition that is characterized by elevated blood levels of lysine, which is one of the building blocks (amino acids) of proteins. Amino acids are not stored in the body and must be broken down when they are no longer needed. When the body's ability to break down lysine is impaired, lysine can build up and cause hyperlysinemia. Hyperlysinemia is classified as either type I or type II, depending on which steps of lysine breakdown are interrupted.

Variants (also called mutations) in the AASS gene cause both types of hyperlysinemia. The AASS gene provides instructions for making an enzyme called alpha-aminoadipic semialdehyde synthase. This enzyme performs two functions during the breakdown of lysine. First, the enzyme converts lysine to a molecule called saccharopine. This same enzyme also converts saccharopine to a molecule called alpha-aminoadipic semialdehyde.

The exact incidence of hyperlysinemia is unknown. Researchers have suggested that hyperlysinemia type I affects approximately 1 in 411,000 newborns. Hyperlysinemia type II is less common than type I.

Hyperlysinemia is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell must have a variant to cause the disorder. The parents of an individual with an autosomal recessive condition each carry one copy of the altered gene, but they typically do not show signs and symptoms of the condition.

Published Date: June 11, 2025
Published By: National Institutes of Health