Carnosinemia Cell Models for Research

Carnosinemia is an extremely rare autosomal recessive metabolic disorder characterized by elevated levels of carnosine in the blood and urine due to deficiency of serum carnosinase. The exact prevalence is unknown, but fewer than 50 cases have been reported worldwide. Clinical manifestations are highly variable, ranging from asymptomatic to severe neurological symptoms including intellectual disability, myoclonus, and sensory neuropathy. The condition is not associated with increased mortality, but quality of life can be significantly affected. Due to its rarity, epidemiological data are limited, and most knowledge comes from case reports and small series. The NCI does not track this non-malignant condition, but the National Organization for Rare Disorders (NORD) provides resources. Research significance lies in understanding carnosine metabolism and its role in neurological function.

Carnosinemia serves as a valuable model for studying carnosine metabolism, dipeptide transport, and the physiological roles of carnosine in the brain and muscle. The disease is caused by mutations in the CNDP1 gene, which encodes serum carnosinase, and possibly CNDP2, encoding tissue carnosinase. Gene-edited cell models with CNDP1 or CNDP2 knockouts allow researchers to dissect the enzymatic pathways and investigate the downstream effects of carnosine accumulation. Open questions include the precise pathophysiology of neurological symptoms and the potential therapeutic benefits of carnosine supplementation. These models enable mechanistic studies and drug screening for potential treatments.

Carnosinemia is primarily caused by deficiency of carnosinase, leading to accumulation of carnosine (β-alanyl-L-histidine) and related dipeptides. The major pathways involved are:

• • Carnosine metabolism: Carnosine is synthesized by carnosine synthase (CARNS1) and hydrolyzed by carnosinase (CNDP1 in serum, CNDP2 in tissues). Deficiency of CNDP1 leads to elevated serum carnosine.
• • Dipeptide transport: Carnosine is transported by peptide transporters (PEPT1, PEPT2). Accumulation may affect transport dynamics.
• • Neurological impact: Elevated carnosine may interfere with neurotransmitter systems, particularly histaminergic and GABAergic pathways, contributing to neurological symptoms.
• • Oxidative stress: Carnosine has antioxidant properties; its accumulation may paradoxically cause oxidative stress in certain tissues.

Data from ClinVar and case reports. The most common mutation is a trinucleotide repeat expansion in the CNDP1 gene (CTG repeat), which is associated with reduced enzyme activity.

Carnosine accumulation may affect multiple signaling pathways:

• • Histaminergic signaling: Carnosine is a precursor for histidine, which is decarboxylated to histamine. Elevated carnosine may alter histamine levels.
• • GABAergic signaling: Carnosine may act as a neuromodulator, affecting GABA receptors.
• • Antioxidant defense: Carnosine scavenges reactive oxygen species, but excessive levels may disrupt redox balance.
• • Protein glycation: Carnosine inhibits protein glycation, but accumulation may alter this process.
• • Cellular stress responses: Carnosine can induce heat shock proteins and affect cellular stress pathways.

Organoids derived from patient-derived induced pluripotent stem cells (iPSCs) can recapitulate tissue-specific carnosine metabolism. However, due to the rarity of the disease, patient-derived organoids are scarce. Gene-edited organoids with CNDP1 or CNDP2 knockouts can be generated for mechanistic studies.

• • CNDP1 knockout mouse: Generated by homologous recombination, exhibits elevated serum carnosine and neurological deficits.
• • CNDP2 knockout mouse: Shows altered tissue carnosine levels and metabolic changes.
• • Zebrafish models: Used for developmental studies due to easy genetic manipulation.
• • No PDX models are applicable as carnosinemia is not a cancer.

CRISPR-Cas9 gene editing enables the creation of isogenic cell lines with precise mutations in CNDP1 or CNDP2. For example:

• • CNDP1 knockout HEK293 cells: Complete loss of serum carnosinase activity, mimicking the enzymatic deficiency.
• • CNDP2 knockout HepG2 cells: Reduced tissue carnosinase activity.
• • CNDP1 point mutation knock-in cells: Introduction of a specific patient mutation (e.g., a missense variant) to study its functional impact.

These models are sequence-verified and can be generated with or without reporter tags. Commercially available gene-edited cell lines accelerate research by providing consistent, validated tools. They are essential for drug screening and functional studies.

Gene-edited cell lines are used to validate the function of CNDP1 and CNDP2 in carnosine metabolism. For example:

• • Knockout of CNDP1 in HEK293 cells confirms its role in serum carnosinase activity.
• • Knock-in of a pathogenic mutation allows assessment of enzyme activity and protein stability.
• • CRISPR screens can identify modifier genes that influence carnosine toxicity.

Isogenic pairs (wild-type vs. knockout) are used to screen for compounds that can compensate for carnosinase deficiency. For instance:

• • High-throughput screening for small molecules that increase CNDP1 expression or activity.
• • Testing of carnosine analogs or enzyme replacement therapies.
• • Assessing the effect of potential drugs on carnosine accumulation and downstream signaling.

CRISPR-based synthetic lethality screens can identify genes whose loss is lethal in CNDP1-deficient cells, revealing potential therapeutic targets. Additionally, gene-edited cells can be used to discover biomarkers for disease progression and response to therapy.