rs121918391 — APOB APOB Tyr1200Ter

APOB Tyr1200Ter — When Apolipoprotein B is Cut Short

Apolipoprotein B-100 (apoB-100) is the structural backbone of every LDL particle in your blood. It is one of the largest proteins in the human body — 4,536 amino acids — and its sheer size is what allows it to scaffold a lipoprotein shell large enough to carry thousands of cholesterol and triglyceride molecules through the bloodstream. The rs121918391 variant introduces a premature stop codon at position 1,200, producing a protein only 26.5% of normal length. This truncated fragment is so short that it falls below the minimum length needed to assemble a functional lipoprotein particle¹¹ A threshold of approximately apoB-28 to apoB-29 is required for lipoprotein particle formation; truncations shorter than this are not secreted as lipoproteins, and as a result, the truncated protein is not detectable in plasma.

The Mechanism

The variant arises from a T-to-A transversion on the APOB coding strand (c.3600T>A; NM_000384.3), which on the genomic plus strand corresponds to an A→T change at GRCh38 chr2:21,015,169. Because APOB is transcribed from the minus strand, plasma genotype files report the A allele (reference) and T allele (risk) at this position.

The resulting stop codon at position 1,200 (p.Tyr1200Ter) produces a truncated protein designated apoB-26.6²² apoB-26.6
Named by convention as a percentage of full-length apoB-100; apoB-26.6 = 1,200/4,536 × 100 ≈ 26.5%. This truncation removes the C-terminal lipid-binding domain necessary for VLDL assembly in the liver and chylomicron assembly in the intestine. The liver continues producing the wild-type apoB-100 from the intact allele (in heterozygotes), but the truncated product is degraded intracellularly. Net result: approximately half the normal number of LDL particles in the bloodstream, reflected as dramatically low LDL cholesterol and apoB concentrations.

The Evidence

The variant was first documented by Homer et al. 2005 (Ann Neurology)³³ Homer et al. 2005 (Ann Neurology)
Mental retardation and ataxia due to normotriglyceridemic hypobetalipoproteinemia. Ann Neurol, 2005 in a patient with compound heterozygous APOB mutations (including Tyr1200Ter) presenting with normotriglyceridemic hypobetalipoproteinemia, cognitive impairment, and ataxia — a severe biallelic phenotype that does not represent the heterozygous carrier state.

At the population level, the clinical significance of APOB protein-truncating variants (PTVs) was quantified by Peloso et al. 2019⁴⁴ Peloso et al. 2019
Rare Protein-Truncating Variants in APOB, Lower LDL-C, and Protection Against Coronary Heart Disease. Circ Genom Precis Med, 2019 in a study of 57,973 individuals across 12 CHD case-control studies. Heterozygous APOB PTV carriers had on average 43 mg/dL lower LDL-C and 32% lower triglycerides compared to non-carriers. In the case-control analysis (18,442 CHD cases, 39,531 controls), APOB PTV carriers showed 72% lower coronary heart disease risk (OR 0.28; 95% CI 0.12–0.64; P=0.002). The cardiovascular protection from low LDL comes with a cost, however — the same impairment in apoB secretion that reduces atherogenic lipoproteins also reduces delivery of fat-soluble vitamins (A, D, E, K) to peripheral tissues.

Cefalù et al. 2013⁵⁵ Cefalù et al. 2013
A novel APOB mutation identified by exome sequencing cosegregates with steatosis, liver cancer and hypocholesterolemia. Arterioscler Thromb Vasc Biol identified a family where a nonsense APOB mutation cosegregated with fatty liver in 7 of 10 carriers and with hepatocarcinoma in 4 family members. This case established that impaired hepatic apoB secretion causes intracellular lipid accumulation — the liver cannot export its fat load because the lipoprotein assembly mechanism is compromised. Heterozygous APOB-FHBL carriers have an estimated three- to five-fold increase in hepatic fat content and a 6-fold higher odds of steatosis on imaging compared to the general population.

Tocopherol (vitamin E) metabolism is specifically impaired in FHBL heterozygotes: Clarke et al. 2006⁶⁶ Clarke et al. 2006
Assessment of tocopherol metabolism and oxidative stress in familial hypobetalipoproteinemia. Clin Chem found plasma alpha-tocopherol concentrations of 13.6 µmol/L in heterozygotes versus 28.7 µmol/L in controls (P<0.03). Despite this reduction in absolute circulating tocopherol, the study concluded that heterozygotes do not require routine vitamin E supplementation when lipid-adjusted values are considered. Supplementation is recommended for biallelic cases, where deficiency is severe and progressive.

Practical Actions

Heterozygous carriers (AT genotype) typically have LDL-C well below 70 mg/dL and plasma apoB below 50 mg/dL. Most are asymptomatic throughout life and derive substantial cardiovascular protection from chronically low LDL. The primary clinical concern is hepatic steatosis — fatty liver occurs in approximately 50–54% of heterozygotes and progresses to nonalcoholic steatohepatitis (NASH) in 5–10%.

Annual liver function tests (ALT, AST) and lipid panels should be performed. Hepatic ultrasound is recommended every three years from age 10 in carriers with elevated transaminases, even without symptoms.

Dietary saturated fat restriction below 30% of total calories is advised to reduce the additional hepatic fat-loading that worsens APOB-related steatosis. Unlike APOE E4 (where the risk is cardiovascular), the dietary fat concern here is hepatic — reducing saturated fat limits the substrate for triglyceride accumulation in an already-compromised liver.

Fat-soluble vitamins (A, D, E, K) are transported primarily in apoB-containing lipoproteins. Heterozygotes have roughly half the normal LDL particle number, which translates to modestly reduced fat-soluble vitamin delivery. Checking serum levels of 25(OH)D and alpha-tocopherol at initial diagnosis establishes a personal baseline. Routine high-dose supplementation is not indicated for heterozygotes based on current evidence, but borderline-low levels warrant correction with moderate supplementation.

Interactions

The most important interaction is with other lipid-regulating variants. Carriers of APOE E4 (rs429358) normally face elevated cardiovascular risk from high LDL — but APOB Tyr1200Ter dramatically counteracts this by reducing LDL particle number. Whether an APOB PTV fully offsets APOE4 cardiovascular risk has not been studied in a combined cohort, but the LDL-lowering magnitude (43 mg/dL) exceeds what statins typically achieve at moderate doses.

The PNPLA3 I148M variant (rs738409) and TM6SF2 E167K (rs58542926) both independently increase hepatic steatosis risk. In heterozygous APOB-FHBL carriers who co-inherit PNPLA3 or TM6SF2 risk alleles, hepatic fat accumulation and progression to NASH or cirrhosis is substantially accelerated — a clinically important interaction documented in Chouik et al. 2024 (Liver International). Regular hepatic ultrasound is especially warranted if any of these co-variants are present.