rs201065226 — CPT2 p.Arg124Ter (R124X)

When Your Mitochondria Can't Accept Fat Fuel

The human body runs on fat during sustained exercise and fasting — but getting fatty acids into mitochondria requires a molecular ferry service. CPT2 (carnitine palmitoyltransferase II) is the enzyme that completes this ferry crossing, releasing long-chain acyl groups into the mitochondrial matrix for beta-oxidation. When CPT2 fails, fatty acids accumulate outside mitochondria, cells starve of energy, and skeletal muscle breaks down¹¹ skeletal muscle breaks down
rhabdomyolysis: the dissolution of muscle fiber contents including myoglobin, potassium, and creatine kinase into the bloodstream.

rs201065226 is an ultra-rare stop-gain variant in CPT2: the T allele substitutes a premature stop codon (p.Arg124Ter) at amino acid 124, truncating the 658-amino-acid protein before it can fold into its active form. With no functional CPT2 protein produced from this allele, carriers depend entirely on the one working copy; individuals inheriting two T alleles have no enzyme activity at all.

The Mechanism

CPT2 sits on the inner mitochondrial membrane and works in tandem with CPT1 on the outer membrane. CPT1 attaches a carnitine group to long-chain acyl-CoA (forming acylcarnitine), which crosses into the matrix; CPT2 then reverses this reaction, regenerating acyl-CoA for oxidation and releasing free carnitine to be recycled. The p.Arg124Ter truncation eliminates all of CPT2's catalytic domain. Without CPT2, acylcarnitines accumulate in plasma, free carnitine falls, and muscles must shift entirely to glucose and short-chain fuels — a supply that is rapidly exhausted during prolonged exercise or caloric restriction.

CPT2 is also extremely thermoliable²² extremely thermoliable
the enzyme loses activity faster than normal at elevated body temperatures, which is why fever and vigorous exercise in heat disproportionately trigger crises even in heterozygous carriers.

The Evidence

ClinVar classifies the T allele of rs201065226 as Pathogenic across multiple submissions (RCV000185829 and related records), associated with CPT2 deficiency in its severe infantile, lethal neonatal, and myopathic forms, as well as susceptibility to infection-induced acute encephalopathy. The T allele frequency is approximately 1 in 148,768 chromosomes in gnomAD genomes v4 — one observed heterozygote across the entire database — confirming extreme rarity and high penetrance.

Anichini et al. 2011³³ Anichini et al. 2011
Neurol Res cohort of 25 biochemically confirmed CPT2-deficient patients documented that individuals with null mutations (stop codons, frameshifts) or homozygous missense mutations showed more pronounced reductions in enzyme activity and more severe phenotypes. Critically, the study identified symptomatic obligate heterozygotes, confirming that a single loss-of-function allele can manifest clinically when combined with environmental stressors or additional modifying factors.

Vladutiu et al. 2000⁴⁴ Vladutiu et al. 2000
Mol Genet Metab documented CPT2 enzyme activity reduced to 13–47% of normal in heterozygous carriers across different tissues. Triggers for symptomatic episodes included prolonged exercise, fasting, viral infection, anesthesia, and temperature extremes — the same constellation that characterizes the adult myopathic form.

For treatment, Bonnefont et al. 2010⁵⁵ Bonnefont et al. 2010
Clin Pharmacol Ther, n=6 demonstrated that bezafibrate, a peroxisome proliferator-activated receptor alpha (PPARα) agonist, increased palmitoyl-CoA oxidation rates by 39–206% (P=0.028) in myopathic CPT2 patients and significantly improved physical activity capacity and quality of life over a 3-year follow-up. Djouadi et al. 2006⁶⁶ Djouadi et al. 2006 showed the mechanism: fibrates upregulate residual CPT2 enzyme activity in patient-derived fibroblasts. Medium-chain triglycerides (MCT oil) bypass the CPT1/CPT2 transport system entirely, providing an alternative mitochondrial fuel during exercise.

Practical Actions

Heterozygous T carriers should structure exercise to avoid prolonged aerobic work that drains glycogen stores and forces heavy reliance on long-chain fat oxidation. Avoid exercising in the fasted state. Keep a fast-acting carbohydrate source available during endurance activity and shift toward carbohydrate-dominant fueling strategies for sessions exceeding 60–90 minutes. Fasting beyond 12–14 hours should be avoided or carefully supervised.

Any illness with fever should prompt reduced physical activity — the combination of CPT2's thermoliability and illness-driven metabolic stress is a recognized rhabdomyolysis trigger. Dark urine (myoglobinuria) after intense exercise or illness is an emergency requiring immediate hospital evaluation and aggressive IV hydration.

Adding MCT oil to the diet provides medium-chain fatty acids (8–12 carbons) that reach mitochondria without CPT2, supplementing fuel availability during fat-burning demand. L-carnitine supplementation may help maintain free carnitine levels, which fall when acylcarnitines accumulate. Bezafibrate is the best-evidenced pharmacological option for the myopathic form, though it requires a clinician's prescription.

Interactions

The most common pathogenic CPT2 variant worldwide is p.Ser113Leu (rs74315294). Individuals who are compound heterozygous for one p.Ser113Leu allele and one p.Arg124Ter allele (like rs201065226) carry zero functional CPT2 copies and typically present with the severe myopathic or infantile form, with near-zero residual enzyme activity. This compound heterozygous combination is the most clinically relevant scenario for rs201065226 T carriers, since inheriting two T alleles of this ultra-rare variant is essentially impossible in the general population. Genetic testing for the full CPT2 coding region — especially p.Ser113Leu — should accompany any rs201065226 T result to assess compound heterozygosity.