Meyahealth
HomeGeneticsConditionHyperlysinemia
Back to Condition
Hyperlysinemia
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Condition

Hyperlysinemia

Hyperlysinemia is an inherited metabolic disorder characterized by elevated levels of the amino acid lysine in the blood due to impaired breakdown processes.

Overview

Hyperlysinemia is an inherited condition characterized by elevated blood levels of lysine, one of the building blocks 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. Hyperlysinemia is classified as either type I or type II, depending on which steps of lysine breakdown are interrupted.

The signs and symptoms of hyperlysinemia can vary widely, even among members of the same family. Reported features include:

  • Short stature
  • Speech and language delays
  • Intellectual disabilities
  • Behavioral abnormalities
  • Abnormal muscle stiffness (spasticity)
  • Seizures

However, as many as half of all people with hyperlysinemia are asymptomatic and may not be aware they have the condition. Additionally, people with a condition called 2,4-dienoyl-CoA reductase (DECR) deficiency may also have hyperlysinemia, which can present with failure to thrive, developmental delays, encephalopathy, and optic atrophy.

Frequency

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.

Causes

Variants 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: converting lysine to saccharopine, and then converting saccharopine to alpha-aminoadipic semialdehyde.

  • Type I: Variants that impair both the breakdown of lysine and the breakdown of saccharopine.
  • Type II: Variants that mostly impair the breakdown of saccharopine.

Both types result in elevated levels of lysine in the blood; type II may also result in elevated levels of saccharopine in the urine.

Inheritance

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.

Frequently Asked Questions

DECR (2,4‑dienoyl‑CoA reductase) deficiency is a separate mitochondrial disorder that can also cause elevated lysine. It presents with additional features such as failure to thrive, optic atrophy, and encephalopathy. When hyperlysinemia is accompanied by these signs, clinicians should test for DECR deficiency (e.g., enzyme assay or genetic panel). Management then includes broader mitochondrial support measures, such as co‑factor supplementation (riboflavin, carnitine) and more aggressive nutritional monitoring, beyond lysine restriction alone.
symptomatic individuals often have a normal life expectancy and may never require treatment. Symptomatic patients—especially those with seizures, severe spasticity, or developmental delay—may have a more variable prognosis, but early seizure control, physiotherapy, and educational support can improve functional outcomes. Regular neurodevelopmental assessments help track progress and adjust therapies.
Most newborn screening programs do not include lysine levels, so hyperlysinemia is often missed initially. If an elevated lysine is found incidentally or through targeted metabolic testing, confirmatory plasma amino‑acid analysis and molecular testing of the AASS gene are required. A metabolic specialist should review the results and determine whether monitoring or intervention is indicated.
Dietary restriction is only recommended for symptomatic patients (e.g., those with seizures, spasticity, or growth failure). A low‑lysine diet limits high‑lysine foods such as red meat, cheese, soy, and certain legumes, while ensuring adequate protein from lysine‑restricted formulas. Regular monitoring of plasma lysine levels guides dietary adjustments. Asymptomatic individuals are usually observed without restriction.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

Meyahealth

Ⓒ 2026. All rights reserved by MEYA

About UsTerms & PolicyFAQ