Familial Defective Apolipoprotein B-100

Mendelian MONDO:0007751 Pathograph 14 Show in embeddings browser Familial Hypercholesterolemia

Familial defective apolipoprotein B-100 (FDB; hypercholesterolemia, autosomal dominant, type B) is a monogenic hypercholesterolemia caused by missense variants in the LDL-receptor-binding region of APOB. It is the mechanistic mirror image of LDLR-mediated familial hypercholesterolemia: the LDL receptor itself is structurally and functionally normal, but its ligand is not. The classic allele is p.Arg3527Gln in current HGVS numbering, the same substitution historically written R3500Q under mature-protein numbering. It destabilizes the apoB-100 conformation that presents the receptor-binding site, so LDL particles carrying mutant apoB-100 bind the normal LDL receptor poorly, are cleared inefficiently from plasma, and accumulate as elevated LDL cholesterol. The result is lifelong LDL elevation and premature atherosclerotic cardiovascular disease. Because the receptor pathway is intact and can still be upregulated, FDB heterozygotes generally respond well to LDL-receptor-inducing therapy such as statins. Whether the FDB phenotype is milder than LDLR-mediated FH is not fully settled and is curated here as an open controversy.

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1
Inheritance
7
Pathophys.
10
Phenotypes
2
Gaps
14
Pathograph
3
Genes
3
Variants
4
Medical Actions
1
References
1
Deep Research
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Classifications

Harrison's Part
ENDOCRINOLOGY METABOLISM GENETICS ENVIRONMENT DISEASE
ICIMD (Inherited Metabolic Disorders)
hypercholesterolemias
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Inheritance

1
Autosomal Dominant HP:0000006
A single mutant APOB allele is sufficient. Heterozygotes carry a mixed LDL pool of normal and receptor-binding-defective particles, and the defective particles accumulate preferentially because only they escape efficient receptor-mediated clearance. Each child of a heterozygous carrier has a 50% chance of inheriting the allele. Penetrance is recorded as INCOMPLETE and expressivity as VARIABLE, and the two qualifiers refer to different things. The biochemical phenotype emerges with age - unselected carriers identified in their late teens need not be hypercholesterolemic and rise into the elevated range in their early twenties - so a normal cholesterol in a young carrier is not evidence against the genotype. Clinical penetrance measured against formal FH criteria is genuinely incomplete: only a subset of molecularly confirmed FDB carriers meet them, which is the mechanism by which FDB is underdiagnosed. Expressivity is variable in the same cohorts, with intraindividual cholesterol fluctuation markedly larger than in LDLR-mediated FH. Neither qualifier is a quantitative penetrance estimate; none is available for genotype-first FDB, so penetrance_percentage is deliberately omitted.
Autosomal dominant inheritance Penetrance: INCOMPLETE Expressivity: VARIABLE
Show evidence (5 references)
PMID:24404629 SUPPORT Other
"Each child of an individual with a heterozygous pathogenic variant in APOB, LDLR, or PCSK9 has a 50% chance of inheriting the pathogenic variant and having FH."
GeneReviews states the 50% transmission risk for APOB-related FH, the genetic-counseling consequence of autosomal dominant inheritance here.
PMID:7583549 SUPPORT Human Clinical
"only a part of the subjects with FDB fulfill the established criteria for identifying FH"
Supports INCOMPLETE penetrance of the clinical FH phenotype among molecularly confirmed FDB carriers.
PMID:8006512 SUPPORT Human Clinical
"The three volunteers with the point mutation demonstrated an increase in total cholesterol concentrations by 1.30 mmol/l or by 25% within 2 years, suggesting that, in the early twenties, cholesterol concentrations increase markedly from normal to elevated levels."
Supports age-dependent emergence of the biochemical phenotype in unselected carriers, the basis for the age qualifier on penetrance.
+ 2 more references
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Discussions and Knowledge Gaps

2
Is the FDB phenotype genuinely milder than LDLR-mediated familial hypercholesterolemia, or does it only appear milder because the two disorders have been compared in cohorts ascertained through clinical FH criteria that FDB carriers with lower LDL cholesterol fail to meet?
CONTROVERSY OPEN controversy_fdb_severity_versus_ldlr_fh
Attached to
pathophysiology#Elevated Plasma LDL Cholesterol phenotypes#Hypercholesterolemia
Two directly relevant studies disagree. A 1993 Dutch/Canadian series of 18 molecularly confirmed FDB patients concluded the disorder was clinically indistinguishable from FH in physical characteristics and lipoprotein measures - but every one of those 18 had been ascertained through a clinical FH diagnosis, which selects for the high-cholesterol tail of FDB. A 1995 multivariate comparison of 28 FDB against 129 FH heterozygotes found significantly lower total cholesterol, LDL cholesterol and triglycerides and higher HDL cholesterol in FDB, and explicitly noted that only some FDB subjects meet FH criteria. The 2016 review sides with the milder reading and draws the practical consequence, that FDB is underdiagnosed by standard FH criteria. The disagreement is not resolvable from these data because the ascertainment differs, so the entry curates the milder reading as the prevailing view while retaining the contradicting evidence rather than quietly dropping it.
Proposed experiments
Genotype-first comparison of FDB and LDLR-FH lipid phenotypes in an unselected biobank
exp_fdb_genotype_first_severity_comparison
Identify APOB p.Arg3527Gln and pathogenic LDLR carriers by sequence alone in a population biobank with linked lipid measurements, without any clinical FH ascertainment, and compare LDL cholesterol distributions and incident ASCVD. A genotype-first design removes the criteria-based selection that confounds every clinic-derived comparison to date.
Show evidence (3 references)
PMID:8215738 REFUTE Human Clinical
"The disorder was clinically indistinguishable from familial hypercholesterolemia in terms of physical characteristics and lipoprotein measures."
Refutes the milder-phenotype claim, in a cohort ascertained via clinical FH diagnosis.
PMID:7583549 SUPPORT Human Clinical
"these results demonstrate that FDB subjects tend to have a milder form of hyperlipoproteinemia than FH subjects and that only a part of the subjects with FDB fulfill the established criteria for identifying FH"
Supports the milder-phenotype claim and names the ascertainment bias that would hide it.
PMID:27919345 SUPPORT Human Clinical
"the elevation of plasma low-density lipoprotein cholesterol observed in FDB is frequently milder than that of FH due to mutations in LDLR, and FDB is subsequently underdiagnosed according to standard FH diagnostic criteria"
Contemporary review statement of the milder phenotype and its diagnostic consequence.
Does FDB alter apoB-100 lipoprotein production and the VLDL-to-IDL-to-LDL delipidation cascade, or is it purely a clearance defect at the LDL step?
KNOWLEDGE GAP OPEN gap_vldl_kinetic_arm_of_fdb
Attached to
pathophysiology#Reduced Receptor-Mediated Hepatic LDL Clearance
The disorder is modeled here as a clearance defect, which the receptor-binding data strongly support. But a stable-isotope kinetic study found VLDL apoB production rates in FDB intermediate between FH and controls with large interindividual variability, and IDL apoB production reduced relative to controls - so the precursor arm is not simply normal. With only six FDB subjects studied, the finding cannot yet support a curated production-side mechanism node, and none is asserted.
Proposed experiments
Adequately powered stable-isotope apoB kinetic study in genotyped FDB
exp_fdb_apob_kinetics_powered
Repeat the primed-constant [1-13C]leucine infusion design in a larger, genotype-confirmed FDB cohort with matched LDLR-FH and normolipidemic controls, powered to resolve VLDL and IDL apoB production and fractional catabolic rates. This would settle whether a production-side node belongs in the FDB mechanism graph.
Show evidence (2 references)
PMID:9507998 SUPPORT Human Clinical
"VLDL APR in FDB were between those of FH and controls (24.3+/-4.8 mg/kg/day), and demonstrated a relatively large inter-individual variability."
Shows an unresolved production-side signal in FDB, underdetermined at this sample size.
PMID:9507998 SUPPORT Human Clinical
"VLDL and IDL kinetics differ when LDL concentrations are elevated either due to a LDL receptor defect or due to defective apolipoprotein B-100."
States that the precursor kinetics genuinely differ between the receptor-side and ligand-side disorders.
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Pathophysiology

7
APOB Receptor-Binding-Region Missense Variant
The initiating lesion is a missense substitution in the LDL-receptor-binding region of APOB, most commonly at the arginine residue numbered 3500 in the mature apoB-100 protein (p.Arg3527Gln in current HGVS numbering; c.10580G>A). The codon sits at a CpG mutational hot spot, which explains its recurrence. Critically, the variant is in the ligand, not the receptor: LDLR, PCSK9, and LDLRAP1 are wild type.
APOB hgnc:603 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves APOB (hgnc:603). hgnc:603 is a gene from the HUGO Gene Nomenclature Committee.
Genetic context APOB hgnc:603 HUGO Gene Nomenclature Committee (hgnc) Relation: this genetic context concerns this gene This genetic context concerns APOB (hgnc:603). hgnc:603 is a gene from the HUGO Gene Nomenclature Committee. variant_origin: GERMLINE zygosity: HETEROZYGOUS functional_impact_category: PARTIAL_LOSS_OF_FUNCTION
Scored as PARTIAL_LOSS_OF_FUNCTION rather than LOSS_OF_FUNCTION or DOMINANT_NEGATIVE. The protein is made and assembled normally into an LDL particle of normal chemical and physical composition; only the receptor-binding activity of that particle is reduced, and it is reduced partially rather than abolished - LDL from an FDB heterozygote retained about 32% of normal receptor binding activity in the original fibroblast binding study. DOMINANT_NEGATIVE would be the wrong call: the mutant apoB-100 does not interfere with the wild-type allele's product. Each LDL particle carries exactly one apoB-100 molecule, so mutant and wild-type particles are separate species that behave independently, and dominance is explained by preferential plasma accumulation of the poorly cleared mutant particles rather than by any trans effect on the normal protein. Supporting evidence for this call is carried on this node's evidence list, since GeneticContext has no evidence slot.
Downstream
Defective ApoB-100 Binding to a Structurally Normal LDL Receptor The substitution directly produces the ligand-side binding defect.
Show evidence (5 references)
PMID:2563166 SUPPORT Human Clinical
"Extensive sequence analysis of the two alleles of one subject heterozygous for the disorder has revealed a previously unreported mutation in the codon for amino acid 3500 that results in the substitution of glutamine for arginine."
The original identification of the causal APOB missense variant.
PMID:3477815 SUPPORT In Vitro
"The G.R. LDL possessed 32% of normal receptor binding activity"
Quantifies residual (partial, not absent) receptor-binding activity, supporting the PARTIAL_LOSS_OF_FUNCTION call on this node's genetic_context.
PMID:3477815 SUPPORT In Vitro
"the defect in receptor binding does not appear to be associated with an abnormal lipid composition or structure of the LDL: the chemical and physical properties of the particles were normal"
Shows the particle is otherwise normally assembled, arguing against a global loss of apoB-100 function.
+ 2 more references
Defective ApoB-100 Binding to a Structurally Normal LDL Receptor
The defining mechanistic step, and the point at which FDB diverges from LDLR-mediated familial hypercholesterolemia. The LDL receptor is normal; the ligand is defective. Arg3527 is not itself a contact residue of the receptor binding site - site B (residues 3359-3369) makes the contact. Instead Arg3527 interacts with the carboxyl terminus of apoB-100 (including Trp4396) to hold the belt of apoB-100 around the LDL particle in the conformation that exposes and stabilizes the basic-residue clusters of the binding site. Losing that arginine destabilizes the interaction and misconfigures the binding site, so the receptor cannot recognize the particle. Removing the carboxyl terminus of FDB LDL restores normal receptor binding, which shows the defect is conformational rather than a direct loss of a contact residue.
APOB hgnc:603 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves APOB (hgnc:603). hgnc:603 is a gene from the HUGO Gene Nomenclature Committee.
low-density lipoprotein particle receptor binding GO:0050750 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased low-density lipoprotein particle receptor binding (GO:0050750). GO:0050750 is a molecular function from the Gene Ontology. ↓ DECREASED
low-density lipoprotein particle GO:0034362 Gene Ontology (GO) Relation: this pathophysiological event involves this cellular component This pathophysiological event involves low-density lipoprotein particle (GO:0034362). GO:0034362 is a cellular component from the Gene Ontology.
Downstream
Reduced Receptor-Mediated Hepatic LDL Clearance A particle the LDL receptor cannot bind cannot be internalized, so receptor-mediated catabolism of the mutant LDL pool fails.
Small Dense LDL Particle Formation Prolonged plasma residence of poorly cleared LDL exposes the particle to lipase and lipid-transfer remodeling, shifting the LDL population smaller.
Show evidence (6 references)
PMID:9486979 SUPPORT Model Organism
"Site-directed mutagenesis and other evidence indicated that Site B (amino acids 3,359-3,369) binds to the LDL receptor and that arginine-3,500 is not directly involved in receptor binding."
Establishes that the mutated arginine is not itself a receptor contact residue, so the defect must be conformational.
PMID:9486979 SUPPORT Model Organism
"The carboxyl-terminal 20% of apo-B100 is necessary for the R3500Q mutation to disrupt receptor binding, since removal of the carboxyl terminus in FDB LDL results in normal receptor-binding activity."
Rescue of binding by carboxyl-terminal removal shows the mechanism is a conformational misconfiguration of the binding site.
PMID:9486979 SUPPORT Model Organism
"the loss of arginine at this site destabilizes this interaction, resulting in receptor-binding defective LDL"
States the proposed destabilization mechanism producing binding-defective LDL.
+ 3 more references
Small Dense LDL Particle Formation
LDL isolated from p.Arg3527Gln heterozygotes is measurably smaller than wild-type LDL and shows an altered apoB-100 secondary structure, with reduced beta-strand content. This is a consequence of the prolonged plasma residence time and of the conformational change itself, and is a plausible atherogenic amplifier rather than the primary disease step. Curated as a supporting node, not as the driver of the cardiovascular risk.
low-density lipoprotein particle GO:0034362 Gene Ontology (GO) Relation: this pathophysiological event involves this cellular component This pathophysiological event involves low-density lipoprotein particle (GO:0034362). GO:0034362 is a cellular component from the Gene Ontology.
Downstream
Subendothelial Retention of ApoB-Containing Lipoproteins Smaller, denser LDL particles are described as more atherogenic; treated here as a contributory rather than established route.
Show evidence (2 references)
PMID:26643808 SUPPORT In Vitro
"LDL particles obtained from heterozygous patients carrying p.(Arg3527Gln) or p.(Arg1164Thr) variants show a main population of particles of ~27 nm, significantly smaller than the ones carrying wt ApoB100 (~29 nm)"
Direct measurement of reduced LDL particle size in Arg3527Gln heterozygotes.
PMID:26643808 SUPPORT In Vitro
"ApoB100 p.(Arg3527Gln) variant showed a reduced content in β-strands"
Reports the altered apoB-100 secondary structure in the variant particle.
Reduced Receptor-Mediated Hepatic LDL Clearance
Because the mutant LDL cannot engage the hepatic LDL receptor, it is not internalized or delivered to the lysosome, and its fractional catabolic rate falls. Uptake of the wild-type LDL pool and of apoE-containing precursors is unaffected, which is why hepatic receptor abundance remains a therapeutic lever. Downstream sterol sensing in the hepatocyte is also blunted, since mutant LDL is less effective at stimulating intracellular cholesteryl ester synthesis.
hepatocyte CL:0000182 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves hepatocyte (CL:0000182). CL:0000182 is a cell type from the Cell Ontology.
low-density lipoprotein particle clearance GO:0034383 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased low-density lipoprotein particle clearance (GO:0034383). GO:0034383 is a biological process from the Gene Ontology. ↓ DECREASED receptor-mediated endocytosis GO:0006898 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased receptor-mediated endocytosis (GO:0006898). GO:0006898 is a biological process from the Gene Ontology. ↓ DECREASED cholesterol homeostasis GO:0042632 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves dysregulated cholesterol homeostasis (GO:0042632). GO:0042632 is a biological process from the Gene Ontology. ↕ DYSREGULATED
liver UBERON:0002107 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in liver (UBERON:0002107). UBERON:0002107 is an anatomical location from the Uberon multi-species anatomy ontology.
Downstream
Elevated Plasma LDL Cholesterol Impaired clearance is the direct cause of plasma LDL accumulation.
Show evidence (2 references)
PMID:3477815 SUPPORT In Vitro
"the G.R. LDL were much less effective than normal LDL in competing with 125I-labeled normal LDL for cellular uptake and degradation and in stimulating intracellular cholesteryl ester synthesis"
Demonstrates reduced cellular uptake, degradation, and downstream sterol signaling of mutant LDL.
PMID:2280177 SUPPORT Human Clinical
"A single amino acid mutation in apolipoprotein B diminishes the ability of low density lipoproteins to bind to the low density lipoprotein receptor."
Restates the ligand-side clearance defect at the level of the disorder.
Elevated Plasma LDL Cholesterol
The clinically measured consequence: lifelong elevation of plasma total and LDL cholesterol. In Danish general-population carriers of Arg3500Gln, total cholesterol was about 100 mg/dL higher than in noncarriers. Levels tend to fluctuate more within individuals than in LDLR-mediated FH and may not be clearly elevated in young carriers, which contributes to underdiagnosis.
cholesterol homeostasis GO:0042632 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves dysregulated cholesterol homeostasis (GO:0042632). GO:0042632 is a biological process from the Gene Ontology. ↕ DYSREGULATED
Downstream
Subendothelial Retention of ApoB-Containing Lipoproteins Sustained elevation of circulating apoB-containing LDL drives the initiating step of atherogenesis.
Show evidence (3 references)
PMID:9603795 SUPPORT Human Clinical
"Among carriers of the Arg3500Gln mutation, cholesterol levels were significantly higher than among noncarriers in the general population - by 100 mg per deciliter (2.6 mmol per liter) among carriers in the general population"
Quantifies the cholesterol elevation attributable to the FDB allele in an unselected general population.
PMID:8215738 SUPPORT Human Clinical
"This amino acid substitution diminishes the binding capacity of the low-density lipoprotein particle for the low-density lipoprotein receptor, which in turn leads to an increase in levels of plasma total and low-density lipoprotein cholesterol."
States the causal link from diminished binding capacity to raised plasma total and LDL cholesterol.
PMID:8006512 SUPPORT Human Clinical
"the presence of the mutation was not necessarily associated with an elevation of serum cholesterol levels, particularly in young individuals"
Qualifies the phenotype - cholesterol elevation is age-dependent and may be absent in young carriers, so support is PARTIAL.
Subendothelial Retention of ApoB-Containing Lipoproteins
Chronically raised circulating LDL drives infiltration and retention of apoB-containing lipoproteins in the arterial intima, the shared initiating event of atherogenesis. FDB substitutes an elevated ligand-defective LDL burden at this trigger; the downstream plaque biology is the conserved pattern captured by the atherogenesis module and is not re-derived here.
endothelial cell of vascular tree CL:0002139 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves endothelial cell of vascular tree (CL:0002139). CL:0002139 is a cell type from the Cell Ontology.
cholesterol homeostasis GO:0042632 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves dysregulated cholesterol homeostasis (GO:0042632). GO:0042632 is a biological process from the Gene Ontology. ↕ DYSREGULATED
artery UBERON:0001637 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in artery (UBERON:0001637). UBERON:0001637 is an anatomical location from the Uberon multi-species anatomy ontology.
Downstream
Premature Atherosclerotic Cardiovascular Disease Cumulative arterial lipoprotein retention over decades produces clinically manifest premature ischemic heart disease in FDB carriers.
Show evidence (1 reference)
PMID:26844337 SUPPORT Other
"infiltration and retention of apoB containing lipoproteins in the artery wall is a critical initiating event that sparks an inflammatory response and promotes the development of atherosclerosis."
Establishes subendothelial apoB-lipoprotein retention as the initiating event of atherosclerosis, the module node this entry conforms to. Evidence source is OTHER because this is an authoritative review chapter.
Premature Atherosclerotic Cardiovascular Disease
The terminal clinical consequence: accelerated coronary atherosclerosis with premature ischemic heart disease. In the Danish general-population study the Arg3500Gln allele conferred a sevenfold odds of ischemic heart disease, and the effect was allele-specific - the neighbouring Arg3531Cys substitution raised neither cholesterol nor ischemic heart disease risk, an internal negative control for the mechanism.
coronary artery UBERON:0001621 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in coronary artery (UBERON:0001621). UBERON:0001621 is an anatomical location from the Uberon multi-species anatomy ontology.
Show evidence (3 references)
PMID:9603795 SUPPORT Human Clinical
"The Arg3500Gln mutation in the apolipoprotein B gene, which is responsible for familial defective apolipoprotein B-100 and is present in approximately 1 in 1000 persons in Denmark, causes severe hypercholesterolemia and increases the risk of ischemic heart disease."
States the population-level conclusion that the FDB allele causes hypercholesterolemia and raises ischemic heart disease risk.
PMID:9603795 SUPPORT Human Clinical
"Heterzygous carriers of the Arg3531Cys mutation in the general population did not have higher-than-normal plasma cholesterol levels or an increased risk of ischemic heart disease"
Allele-specific negative control - a different APOB substitution produced neither phenotype, supporting specificity of the binding-region lesion.
PMID:8141833 SUPPORT Human Clinical
"The presence of mutant apo B-100 in low-density lipoproteins (LDL) markedly reduces their affinity for the LDL receptor, leading to hypercholesterolaemia and increased proneness to coronary artery disease."
Summarizes the whole chain from reduced LDL receptor affinity to coronary artery disease.
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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Familial Defective Apolipoprotein B-100 Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
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Phenotypes

10
Cardiovascular 3
Premature Coronary Artery Atherosclerosis HP:0005181 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Premature coronary artery atherosclerosis (HP:0005181). HP:0005181 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:8141833 SUPPORT Human Clinical
"The presence of mutant apo B-100 in low-density lipoproteins (LDL) markedly reduces their affinity for the LDL receptor, leading to hypercholesterolaemia and increased proneness to coronary artery disease."
Links the FDB mechanism to coronary artery disease proneness.
PMID:9603795 SUPPORT Human Clinical
"Heterozygous carriers of the Arg3500Gln mutation were significantly more common among patients with ischemic heart disease (odds ratio, 7.0; 95 percent confidence interval, 2.2 to 22; P=0.003)"
Quantifies the excess ischemic heart disease risk in FDB carriers.
Angina Pectoris FREQUENT HP:0001681 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Angina pectoris (HP:0001681). HP:0001681 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:8215738 SUPPORT Human Clinical
"eight of 18 patients had angina or other evidence of coronary artery disease"
8/18 = 44% supports the FREQUENT band (30-79%). The count is for angina or other coronary artery disease evidence combined, so the band is an upper bound for isolated angina.
Myocardial Infarction HP:0001658 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Myocardial infarction (HP:0001658). HP:0001658 is a phenotype from the Human Phenotype Ontology.
Frequency deliberately omitted. The 5.6% figure is for coronary heart disease as a whole rather than myocardial infarction specifically, and the two compared cohorts differ in age distribution and ascertainment, so it cannot be resolved into a frequency band for infarction.
Show evidence (3 references)
PMID:38393015 SUPPORT Human Clinical
"Familial defective apolipoprotein B100 has been linked to increased rates of both mild and severe coronary artery calcification, and patients often experience early cardiovascular events, including coronary artery disease, ischemic heart disease, and myocardial infarction"
FDB-specific statement that carriers experience early myocardial infarction.
PMID:38393015 SUPPORT Human Clinical
"Almost 40% of FH patients had coronary heart disease (CHD) at a mean age of 41 years, while CHD was present in 5.6% of FDB subjects with a median age of 52 years."
Quantifies the FDB coronary event burden against LDLR-mediated FH. PARTIAL because the endpoint is coronary heart disease rather than infarction specifically.
PMID:24404629 SUPPORT Other
"increases the risk of premature cardiovascular events such as angina and myocardial infarction"
GeneReviews names myocardial infarction as a core FH manifestation. PARTIAL because the chapter covers FH across LDLR, APOB and PCSK9 rather than FDB specifically.
Eye 1
Corneal Arcus HP:0001084 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Corneal arcus (HP:0001084). HP:0001084 is a phenotype from the Human Phenotype Ontology.
Frequency deliberately omitted for the same reason as tendon xanthomatosis - the source count pools the two signs.
Show evidence (1 reference)
PMID:8215738 SUPPORT Human Clinical
"Ten of 18 patients had tendon xanthomas or an arcus cornealis or both"
Documents corneal arcus in FDB, but pooled with tendon xanthomas, hence PARTIAL.
Head and Neck 1
Xanthelasma HP:0001114 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Xanthelasma (HP:0001114). HP:0001114 is a phenotype from the Human Phenotype Ontology.
Frequency deliberately omitted - the only source is class-level FH literature, and the visible-xanthoma literature is drawn from broader FH cohorts in which FDB carriers are a minority.
Show evidence (1 reference)
PMID:24404629 SUPPORT Other
"Xanthelasmas (yellowish, waxy deposits) can occur around the eyelids."
GeneReviews names xanthelasma as an FH manifestation. PARTIAL because the chapter describes FH across LDLR, APOB and PCSK9 rather than FDB alone.
Integument 1
Tendon Xanthomatosis HP:0010874 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Tendon xanthomatosis (HP:0010874). HP:0010874 is a phenotype from the Human Phenotype Ontology.
Frequency deliberately omitted. The available count (10 of 18) pools tendon xanthomas and corneal arcus, so it cannot be resolved into a frequency band for either sign alone.
Show evidence (1 reference)
PMID:8215738 SUPPORT Human Clinical
"Ten of 18 patients had tendon xanthomas or an arcus cornealis or both"
Documents tendon xanthomas in FDB, but pooled with corneal arcus, hence PARTIAL.
Metabolism 2
Increased LDL Cholesterol Concentration HP:0003141 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Increased LDL cholesterol concentration (HP:0003141). HP:0003141 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:8215738 SUPPORT Human Clinical
"This amino acid substitution diminishes the binding capacity of the low-density lipoprotein particle for the low-density lipoprotein receptor, which in turn leads to an increase in levels of plasma total and low-density lipoprotein cholesterol."
States that the FDB substitution raises plasma LDL cholesterol.
Hypercholesterolemia HP:0003124 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypercholesterolemia (HP:0003124). HP:0003124 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:2280177 SUPPORT Human Clinical
"Familial defective apolipoprotein B-100 is a genetic disorder of apolipoprotein B-100 that causes moderate to severe hypercholesterolemia."
States the total-cholesterol phenotype and its severity range.
PMID:7583549 SUPPORT Human Clinical
"FDB subjects showed much larger total cholesterol fluctuations than FH subjects (median of intraindividual coefficients of variation: FDB, 14.5%; FH, 5.3%; P < .001)"
Quantifies the greater intraindividual variability of total cholesterol in FDB.
Other 2
Coronary Artery Calcification HP:0001717 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Coronary artery calcification (HP:0001717). HP:0001717 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38393015 SUPPORT Human Clinical
"Familial defective apolipoprotein B100 has been linked to increased rates of both mild and severe coronary artery calcification"
FDB-specific statement of increased coronary artery calcification.
Carotid Artery Stenosis HP:0100546 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Carotid artery stenosis (HP:0100546). HP:0100546 is a phenotype from the Human Phenotype Ontology.
Frequency deliberately omitted. 4% would fall in the VERY_RARE band (<5%), but it is a single clinic-derived comparison whose FDB and FH arms differ in age and ascertainment, so the band is not supportable on this source alone.
Show evidence (1 reference)
PMID:38393015 SUPPORT Human Clinical
"internal carotid artery stenoses were more prevalent in FH (15% versus 4% in the FDB group)"
Reports carotid artery stenosis in FDB with a directly compared FH group.
🧬

Genetic Associations

3
APOB (Causal)
Gene: APOB hgnc:603 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is APOB (hgnc:603). hgnc:603 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Autosomal Dominant
Show evidence (2 references)
PMID:27919345 SUPPORT Human Clinical
"FDB is caused by mutations in APOB reducing the binding affinity between apolipoprotein B-100 and the low-density lipoprotein receptor."
Establishes APOB as the causal gene and the binding-affinity mechanism.
PMID:2563166 SUPPORT Human Clinical
"This same mutant allele occurs in six other, unrelated subjects and in eight affected relatives in two of these families."
Original demonstration of segregation of the APOB allele with the FDB phenotype.
Variants (3)
p.Arg3527Gln (R3500Q; c.10580G>A) Pathogenic
missense
The classic and by far the most common FDB allele. Numbering differs by convention: 3500 counts from the mature protein, 3527 includes the signal peptide and is the current HGVS form. Arises at a CpG mutational hot spot. Carriers of Western European descent share a common haplotype, consistent with a founder allele.
Show evidence (2 references)
PMID:8215738 SUPPORT Human Clinical
"The mutation was associated with a similar haplotype, which was also reported in other patients of Western European descent with familial defective apolipoprotein B100. This strongly suggests that the mutation has a common chromosomal background that originated in Western Europe."
Establishes the shared founder haplotype of the p.Arg3527Gln allele in Western Europe.
PMID:32591292 SUPPORT Human Clinical
"The known p.(Arg3527Gln) variant in the APOB gene was identified in one Japanese family."
Confirms the c.10580G>A / p.(Arg3527Gln) designation and documents its rarity outside European-descent populations.
p.Arg3527Trp (R3500W) Pathogenic
missense
A distinct substitution at the same codon, reported in hyperlipidemic East Asian populations on its own founder haplotype. It is the APOB allele of note for FDB outside European-descent populations.
Show evidence (3 references)
PMID:9702952 SUPPORT Human Clinical
"A total of 373 hyperlipidemic patients and 309 controls were screened for R3500W. Nine unrelated subjects were shown to be heterozygous for the mutation, and no R3500W carriers were found in the control group (P = 0.004)."
Demonstrates enrichment of R3500W in hyperlipidemic Chinese subjects versus controls.
PMID:9702952 SUPPORT Human Clinical
"The fact that the same mutant allele was identified in other Asians with FDB indicates a common Asian origin for the R3500W mutations."
Establishes a separate Asian founder origin for this allele.
PMID:27919345 SUPPORT Human Clinical
"the APOB R3500 W variant is known to make a significant contribution to familial hypercholesterolemia (FH) among East Asians"
Review confirms the population-specific contribution of R3500W.
p.Arg3558Cys (R3531C) Uncertain Significance
missense
A neighbouring APOB substitution that was screened alongside the FDB alleles but did not raise cholesterol or ischemic heart disease risk in the general population. Recorded here as an explicit negative, since it delimits which APOB substitutions produce the FDB phenotype.
Show evidence (1 reference)
PMID:9603795 REFUTE Human Clinical
"Heterzygous carriers of the Arg3531Cys mutation in the general population did not have higher-than-normal plasma cholesterol levels or an increased risk of ischemic heart disease"
Refutes a causal FDB role for this substitution in an unselected population.
LDLR (Modifier (digenic second locus))
Gene: LDLR hgnc:6547 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is LDLR (hgnc:6547). hgnc:6547 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: MODIFIER variant_origin: GERMLINE
Show evidence (1 reference)
PMID:33519890 SUPPORT Human Clinical
"double-heterozygotes carrying LDLR and p.(R3500Q) APOB mutations have more severe phenotypes when compared to the heterozygote FH cases carrying only one mutation in any of the mentioned genes"
Names the APOB p.Arg3527Gln plus LDLR double-heterozygous state as more severe than either single heterozygote, the basis for the MODIFIER call.
PCSK9 (Modifier (LDL receptor abundance))
Gene: PCSK9 hgnc:20001 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is PCSK9 (hgnc:20001). hgnc:20001 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: MODIFIER variant_origin: GERMLINE
Show evidence (2 references)
PMID:33519890 SUPPORT Human Clinical
"PCSK9 has been identified as an FH modifier gene as it generates significant variable phenotypes even in patients having the same mutation in LDLR"
Identifies PCSK9 as a modifier gene producing variable phenotypes at a fixed primary lesion.
PMID:33519890 SUPPORT Human Clinical
"recently several reports have identified digenic mutations in familial cases that do not necessarily reflect a much severe phenotype"
Retained as the counterweight: digenic second hits do not uniformly worsen the phenotype, so modifier status is recorded without a claimed effect size. PARTIAL because it qualifies rather than supports the severity direction.
💊

Medical Actions

4
Statin (HMG-CoA Reductase Inhibitor) Therapy
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: statin CHEBI:87631 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses statin (CHEBI:87631). CHEBI:87631 is a therapeutic agent from Chemical Entities of Biological Interest.
First-line LDL-lowering therapy. The mechanistic rationale is specific to FDB: because the LDL receptor is normal, statin-induced hepatic LDL receptor upregulation still works. The extra receptor capacity clears the wild-type LDL pool and apoE-containing LDL precursors more efficiently even though the mutant LDL particles remain poor ligands, so LDL cholesterol falls.
Mechanism Target:
ACTIVATES Reduced Receptor-Mediated Hepatic LDL Clearance — Statins raise hepatic LDL receptor abundance, partially compensating for the ligand defect by increasing clearance of the receptor-competent lipoprotein pool.
Show evidence (1 reference)
PMID:8141833 SUPPORT Human Clinical
"This may be due partly to increased receptor-mediated hepatic removal of mutant and normal precursors of LDL, using apo E as recognition element."
States the proposed receptor-mediated mechanism by which receptor-inducing drugs work in FDB.
Show evidence (4 references)
PMID:8141833 SUPPORT Human Clinical
"Most FDB heterozygotes respond well to drugs that lower plasma LDL levels by inducing receptor activity."
States preserved responsiveness to LDL-receptor-inducing therapy in FDB.
PMID:8215738 SUPPORT Human Clinical
"Response to lipid-lowering therapy with beta-hydroxy-beta-methylglutaryl coenzyme A reductase inhibitors was similar to that reported in patients with familial hypercholesterolemia."
Direct clinical observation that statin response in FDB matches that in FH.
PMID:24404629 SUPPORT Other
"statins can be used in children starting around age eight years"
GeneReviews sets the paediatric initiation age for statins in FH. PARTIAL because the recommendation is made for FH as a class, not for APOB-related FH specifically.
+ 1 more reference
PCSK9 Monoclonal Antibody Therapy
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: alirocumab NCIT:C174849 NCI Thesaurus (NCIT) Relation: this treatment uses this therapeutic agent This treatment uses alirocumab (NCIT:C174849). NCIT:C174849 is a therapeutic agent from the NCI Thesaurus. evolocumab NCIT:C174672 NCI Thesaurus (NCIT) Relation: this treatment uses this therapeutic agent This treatment uses evolocumab (NCIT:C174672). NCIT:C174672 is a therapeutic agent from the NCI Thesaurus.
Anti-PCSK9 monoclonal antibodies (alirocumab, evolocumab) block PCSK9-mediated degradation of the hepatic LDL receptor, increasing receptor recycling and surface abundance. The mechanistic rationale in FDB is the same one that makes statins work and is arguably stronger: the receptor is structurally normal, so every additional receptor molecule is fully functional. In a pooled genotyped analysis of six alirocumab trials, the 46 heterozygous APOB-defective patients lowered LDL cholesterol comparably to LDLR-mutation carriers.
Mechanism Target:
ACTIVATES Reduced Receptor-Mediated Hepatic LDL Clearance — Raising hepatic LDL receptor abundance partially compensates for the ligand defect by increasing clearance of the receptor-competent lipoprotein pool.
Show evidence (1 reference)
PMID:28964736 SUPPORT Human Clinical
"54.1% (n = 20) and 50.1% (n = 6) in APOB-defective heterozygotes"
Quantifies LDL cholesterol lowering specifically in APOB-defective (FDB) heterozygotes.
Show evidence (2 references)
PMID:28964736 SUPPORT Human Clinical
"In this large patient cohort, individuals with a wide spectrum of mutations in genes underlying familial hypercholesterolemia responded substantially and similarly to alirocumab treatment."
Supports comparable alirocumab efficacy across FH genotypes including APOB-defective carriers.
PMID:28964736 SUPPORT Human Clinical
"46 had a heterozygous APOB-defective mutation"
Documents the size of the FDB-relevant genotype stratum, the basis for the precision caveat.
Second-Line LDL-Lowering Therapy (Ezetimibe and Add-On Agents)
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: ezetimibe CHEBI:49040 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses ezetimibe (CHEBI:49040). CHEBI:49040 is a therapeutic agent from Chemical Entities of Biological Interest.
Where a statin alone does not bring LDL cholesterol to target, guidelines add further LDL-lowering agents, ezetimibe first among them. Ezetimibe inhibits intestinal NPC1L1-mediated cholesterol absorption, which lowers the hepatic cholesterol pool and, like a statin, drives compensatory hepatic LDL receptor upregulation - so its mechanistic rationale in FDB is the same receptor-side one, and it is likewise unaffected by the ligand defect.
Mechanism Target:
ACTIVATES Reduced Receptor-Mediated Hepatic LDL Clearance — Lowering the hepatic cholesterol pool upregulates LDL receptor expression, increasing clearance of the receptor-competent lipoprotein pool despite the persisting ligand defect. The mechanism is the same receptor-abundance lever exploited by statins and anti-PCSK9 antibodies.
Show evidence (1 reference)
PMID:38393015 SUPPORT Human Clinical
"certain lipid-modifying agents may act via the LDL-R, which is typically normal in subjects with FDB"
Supports the receptor-side rationale that makes add-on LDL-lowering agents mechanistically applicable in FDB. PARTIAL because it states the principle without naming ezetimibe or giving an FDB effect size.
Show evidence (1 reference)
PMID:24404629 SUPPORT Other
"Adults: pharmacotherapy (statins with additional medications as needed) to reduce lipid levels"
GeneReviews establishes add-on pharmacotherapy beyond statins as standard FH management. PARTIAL because it is class-level FH guidance, not FDB-specific evidence.
Cardiovascular Risk Factor Modification and Lipid Surveillance
Action: behavioral counselingNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is behavioral counseling (NCIT:C181743). NCIT:C181743 is a clinical intervention from the NCI Thesaurus. Ontology label: Behavioral Counseling NCIT:C181743
Because FDB is a lifelong LDL-exposure disorder whose clinical endpoint is atherosclerosis, non-pharmacological management targets the modifiable determinants of that endpoint - smoking, saturated and trans fat intake, physical inactivity, obesity, hypertension and diabetes - alongside lipid monitoring from early childhood. Cascade evaluation of at-risk relatives belongs to the same arm and is especially consequential here, since half of a carrier's children inherit the allele and clinical criteria calibrated on LDLR-mediated FH will miss part of the FDB distribution.
Show evidence (3 references)
PMID:24404629 SUPPORT Other
"Agents/circumstances to avoid: Smoking, high intake of saturated and trans unsaturated fat, sedentary lifestyle, obesity, hypertension, and diabetes mellitus."
GeneReviews enumerates the modifiable exposures to avoid in FH. PARTIAL because the guidance is class-level rather than FDB-specific.
PMID:24404629 SUPPORT Other
"Monitor lipid levels from age two years"
GeneReviews sets the surveillance schedule. PARTIAL for the same class-level reason; it is nonetheless pointed for FDB, where carriers can be normocholesterolemic when first tested.
PMID:24404629 SUPPORT Other
"Early diagnosis and treatment of first-degree and second-degree relatives at risk for FH can reduce morbidity and mortality."
GeneReviews supports cascade evaluation of at-risk relatives. PARTIAL because the benefit is established for FH as a class.
🔬

Biochemical Markers

3
LDL Cholesterol (Increased)
Context: Plasma
Show evidence (1 reference)
PMID:7583549 SUPPORT Human Clinical
"FDB subjects demonstrated significantly lower concentrations of total cholesterol (8.1 versus 10.2 mmol/L, P < .001), LDL cholesterol (6.3 versus 8.2 mmol/L, P < .001), and triglycerides (1.3 versus 1.8 mmol/L, P = .025) and higher concentrations of HDL cholesterol (1.4 versus 1.2 mmol/L, P =..."
Reports FDB LDL cholesterol concentrations against a directly compared FH group.
Total Cholesterol (Increased)
Context: Plasma
Show evidence (1 reference)
PMID:8006512 SUPPORT Human Clinical
"The three volunteers with the point mutation demonstrated an increase in total cholesterol concentrations by 1.30 mmol/l or by 25% within 2 years, suggesting that, in the early twenties, cholesterol concentrations increase markedly from normal to elevated levels."
Documents the age-dependent emergence of total-cholesterol elevation in unselected carriers.
HDL Cholesterol (Increased)
Context: Plasma
Show evidence (1 reference)
PMID:7583549 SUPPORT Human Clinical
"higher concentrations of HDL cholesterol (1.4 versus 1.2 mmol/L, P = .015) than subjects with FH"
Supports the FDB-versus-FH contrast only; PARTIAL because no comparison to a normolipidemic reference group is made.
🔬

Diagnosis

4
Molecular Genetic Testing of APOB
Targeted APOB genotyping resolves FDB from LDLR-mediated FH in patients meeting clinical FH criteria. This matters because a substantial minority of clinically diagnosed FH is genetically FDB, and because carriers with milder lipid values are missed by criteria calibrated on LDLR-mediated FH. The variant is a single-nucleotide substitution amenable to simple PCR-based assays.
Genetic Testing NCIT:C15709 NCI Thesaurus (NCIT)
Results: Identification of a heterozygous pathogenic APOB missense variant in the LDL-receptor-binding region - p.Arg3527Gln (c.10580G>A) in European-descent populations, p.Arg3527Trp in East Asian populations - establishes the molecular diagnosis of FDB in a proband with hypercholesterolemia.
Show evidence (3 references)
PMID:24404629 SUPPORT Other
"The molecular diagnosis of FH can be established by identification of heterozygous or biallelic pathogenic variants in APOB (variants that impair binding of LDL-C to the LDL receptor), LDLR, or PCSK9 (gain of function); or rarely, identification of biallelic pathogenic variants in LDLRAP1."
GeneReviews establishes molecular testing as diagnostic and defines the APOB route in exactly the ligand-binding terms this entry models.
PMID:8141833 SUPPORT Human Clinical
"In most lipid clinics, 2-5% of patients given a clinical diagnosis of FH have FDB, not FH."
Quantifies the diagnostic yield that motivates APOB genotyping in clinically diagnosed FH.
PMID:8006512 SUPPORT Human Clinical
"Considering the estimated high prevalence and the relative ease of PCR-based tests, screening for FDB may become a standard procedure in patients with suggested familial forms of hypercholesterolemia."
Supports molecular screening for FDB among suspected familial hypercholesterolemia.
Familial Hypercholesterolemia Multigene Panel
In practice FDB is usually reached through an FH multigene panel rather than a single-gene APOB assay, because the clinical presentation does not discriminate the genes. The panel covers LDLR, APOB, PCSK9 and LDLRAP1; a positive APOB result in the receptor-binding region is what assigns the patient to this entry rather than to LDLR- or PCSK9-mediated FH.
Genetic Testing NCIT:C15709 NCI Thesaurus (NCIT)
Results: An APOB receptor-binding-region variant assigns FDB; an LDLR or PCSK9 variant assigns a different monogenic FH; a negative panel in a patient with a clinical FH phenotype favours polygenic hypercholesterolemia or a secondary cause.
The gene assignment is not merely a label. Because the receptor is intact in FDB, receptor-directed therapy retains its target, which is the practical consequence the source draws from distinguishing the two disorders.
Show evidence (2 references)
PMID:24404629 SUPPORT Other
"The molecular diagnosis of FH can be established by identification of heterozygous or biallelic pathogenic variants in APOB (variants that impair binding of LDL-C to the LDL receptor), LDLR, or PCSK9 (gain of function); or rarely, identification of biallelic pathogenic variants in LDLRAP1."
Names the four genes that constitute the FH panel and their distinct molecular mechanisms.
PMID:38393015 SUPPORT Human Clinical
"certain lipid-modifying agents may act via the LDL-R, which is typically normal in subjects with FDB"
States the therapeutic consequence of resolving FDB from LDLR-mediated FH, and restates the receptor-normal premise of this entry.
Clinical Lipid-Based Diagnosis and LDL-C Thresholds
A clinical FH diagnosis rests on characteristic features plus untreated LDL-C above threshold. These thresholds are calibrated on the FH population as a whole, which is dominated by LDLR-mediated disease.
laboratory procedure NCIT:C25294 NCI Thesaurus (NCIT)
Results: Untreated LDL-C typically above 190 mg/dL in adults and above 160 mg/dL in children supports a clinical diagnosis of familial hypercholesterolemia.
Recorded with an explicit caveat rather than as a clean diagnostic rule for FDB. Because LDL-C elevation in FDB is on average milder than in LDLR-mediated FH, thresholds calibrated on FH cohorts systematically miss part of the FDB carrier distribution - the same ascertainment problem curated as an open controversy in this entry. A normal or borderline LDL-C in a young carrier is likewise uninformative, since the biochemical phenotype is age-dependent.
Show evidence (2 references)
PMID:24404629 SUPPORT Other
"A clinical diagnosis of FH can be established in a proband with characteristic clinical features and significantly elevated LDL-C levels (typically >190 mg/dL in adults and >160 mg/dL in children)."
GeneReviews states the untreated LDL-C thresholds used for a clinical FH diagnosis.
PMID:27919345 SUPPORT Human Clinical
"the elevation of plasma low-density lipoprotein cholesterol observed in FDB is frequently milder than that of FH due to mutations in LDLR, and FDB is subsequently underdiagnosed according to standard FH diagnostic criteria"
Qualifies the thresholds for FDB specifically - they underdiagnose this disorder. PARTIAL because it limits rather than supports their use here.
Differential Diagnosis of Inherited Hypercholesterolemia
FDB must be separated from the other monogenic hypercholesterolemias that present identically - LDLR-mediated FH (receptor defect), PCSK9 gain-of-function FH (accelerated receptor degradation), and autosomal recessive LDLRAP1-related FH - as well as from polygenic hypercholesterolemia, elevated lipoprotein(a), and secondary dyslipidemias.
diagnostic procedure NCIT:C18020 NCI Thesaurus (NCIT)
Results: Resolution is molecular, not clinical: the disorders are not reliably distinguishable on lipid values or physical signs.
A separate and more important discrimination for this entry runs in the opposite direction. APOB also harbours a truncating allelic series causing familial hypobetalipoproteinemia, which LOWERS apoB-containing lipoproteins and produces hepatic steatosis rather than premature atherosclerosis. It is the same gene with the opposite lipid direction, so APOB-keyed literature must be checked for which allelic series it describes before being applied to FDB. This entry covers only the receptor-binding-region missense series that raises LDL cholesterol.
Show evidence (2 references)
PMID:24404629 SUPPORT Other
"The molecular diagnosis of FH can be established by identification of heterozygous or biallelic pathogenic variants in APOB (variants that impair binding of LDL-C to the LDL receptor), LDLR, or PCSK9 (gain of function); or rarely, identification of biallelic pathogenic variants in LDLRAP1."
Enumerates the monogenic FH differentials that molecular testing separates.
PMID:38393015 SUPPORT Human Clinical
"Familial hypobetalipoproteinemia (FHBL) is an autosomal codominant genetic disorder characterized by the diminished secretion of ApoB48 and ApoB100 lipoproteins due to defects in the ApoB gene."
Establishes the opposite-direction APOB disorder that must not be conflated with FDB, in a source that describes both.
📈

Progression

3
Childhood and adolescence
Carriers may be normocholesterolemic or only mildly hypercholesterolemic at this stage, which is a diagnostic trap in cascade screening of families. This is also the stage at which lipid surveillance begins and at which statin therapy first becomes available, so the diagnostic trap has direct management consequences.
Show evidence (1 reference)
PMID:8006512 SUPPORT Human Clinical
"the presence of the mutation was not necessarily associated with an elevation of serum cholesterol levels, particularly in young individuals"
Documents unselected young carriers who were not hypercholesterolemic, the basis for treating this phase as biochemically silent or mild.
Early adulthood
Total cholesterol rises sharply into the elevated range in the early twenties in unselected carriers.
Show evidence (1 reference)
PMID:8006512 SUPPORT Human Clinical
"The three volunteers with the point mutation demonstrated an increase in total cholesterol concentrations by 1.30 mmol/l or by 25% within 2 years, suggesting that, in the early twenties, cholesterol concentrations increase markedly from normal to elevated levels."
Dates the sharp rise in total cholesterol to the early twenties in unselected carriers.
Mid to late adulthood
Cumulative LDL exposure produces coronary atherosclerosis and symptomatic ischemic heart disease. Median age at recorded coronary heart disease in FDB was 52 years, later than the mean of 41 years in directly compared LDLR-mediated FH.
Show evidence (2 references)
PMID:38393015 SUPPORT Human Clinical
"Almost 40% of FH patients had coronary heart disease (CHD) at a mean age of 41 years, while CHD was present in 5.6% of FDB subjects with a median age of 52 years."
Places FDB coronary events in mid-to-late adulthood, later than in LDLR-mediated FH.
PMID:8215738 SUPPORT Human Clinical
"eight of 18 patients had angina or other evidence of coronary artery disease"
Documents symptomatic coronary disease as the clinical burden of this phase.
📊

Prevalence

4
Denmark (general population)
Point Prevalence 100.0 per 100,000 >1 in 1,000
Genotyped in 9255 unselected Danish women and men. The measured heterozygote prevalence was 0.08% (95% CI 0.03-0.16%), which the authors round to approximately 1 in 1000; rate_per_100000 records that stated figure. This is the best unselected-population estimate available, since clinic-based series are ascertained on hypercholesterolemia.
Show evidence (1 reference)
PMID:9603795 SUPPORT Human Clinical
"The prevalence of heterozygotes in the general population was 0.08 percent (95 percent confidence interval, 0.03 to 0.16 percent) for both the Arg3500Gln and the Arg3531Cys mutations"
Direct measurement of FDB heterozygote prevalence in an unselected general population.
Northern Europeans and US Caucasians
Point Prevalence 100.0 per 100,000 >1 in 1,000
Approximately 0.1% carry the R3500Q variant per the 2016 review.
Show evidence (1 reference)
PMID:27919345 SUPPORT Human Clinical
"Population studies suggest that approximately 0.1% of Northern Europeans and US Caucasians carries the R3500Q variant in APOB most commonly associated with FDB"
Review-level population estimate for European-descent populations.
Switzerland (German, French, and Romansh speaking regions)
Point Prevalence 478.0 per 100,000 >1 in 1,000
A striking founder-effect outlier: combined estimate 1 in 209 (rate_per_100000 computed as 100000/209 = 478). Derived by combining 728 healthy volunteers (3 carriers, ~1/240) with 142 hypercholesterolemic families extrapolated to the general population (~1/190). The extrapolated arm is ascertainment-prone, so treat this as an upper bound rather than a directly measured prevalence; the volunteer arm alone (1/240, ~417 per 100000) is the cleaner figure.
Show evidence (1 reference)
PMID:8006512 SUPPORT Human Clinical
"The combined prevalence based on both samples was 1/209. Thus, the investigated point mutation was highly prevalent in Switzerland and appeared to be more frequent than in other populations studied hitherto."
Reports the Swiss combined prevalence estimate and its status as an international outlier.
United States, Canada, and Europe
Point Prevalence 200.0 per 100,000 >1 in 1,000
Early aggregate estimate of approximately 1 in 500 across Western populations, predating the large unselected Danish genotyping study; retained for historical range but the Danish figure is the more reliable one.
Show evidence (1 reference)
PMID:2280177 SUPPORT Human Clinical
"This mutation has been identified in the United States, Canada, and Europe and is estimated to occur at a frequency of approximately 1/500 in these populations."
Historical Western-population frequency estimate.
{ }

Source YAML

click to show 
name: Familial Defective Apolipoprotein B-100
creation_date: "2026-08-18T00:00:00Z"
description: >
  Familial defective apolipoprotein B-100 (FDB; hypercholesterolemia, autosomal
  dominant, type B) is a monogenic hypercholesterolemia caused by missense
  variants in the LDL-receptor-binding region of APOB. It is the mechanistic
  mirror image of LDLR-mediated familial hypercholesterolemia: the LDL receptor
  itself is structurally and functionally normal, but its ligand is not. The
  classic allele is p.Arg3527Gln in current HGVS numbering, the same
  substitution historically written R3500Q under mature-protein numbering. It
  destabilizes the apoB-100 conformation that presents the
  receptor-binding site, so LDL particles carrying mutant apoB-100 bind the
  normal LDL receptor poorly, are cleared inefficiently from plasma, and
  accumulate as elevated LDL cholesterol. The result is lifelong LDL elevation
  and premature atherosclerotic cardiovascular disease. Because the receptor
  pathway is intact and can still be upregulated, FDB heterozygotes generally
  respond well to LDL-receptor-inducing therapy such as statins. Whether the FDB
  phenotype is milder than LDLR-mediated FH is not fully settled and is curated
  here as an open controversy.
category: Mendelian
synonyms:
- Familial ligand-defective apolipoprotein B-100
- FDB
- Hypercholesterolemia, autosomal dominant, type B
- APOB-related familial hypercholesterolemia
- Familial hypercholesterolemia 2
disease_term:
  preferred_term: Familial Defective Apolipoprotein B-100
  term:
    id: MONDO:0007751
    label: hypercholesterolemia, autosomal dominant, type B
parents:
- Familial Hypercholesterolemia
notes: >
  Scope: curated as a standalone Disease rather than a subtype of
  kb/disorders/Familial_Hypercholesterolemia.yaml (MONDO:0005439). MONDO models
  MONDO:0007751 as a distinct is_a child of MONDO:0005439 defined by its causal
  gene (RO:0004003 hgnc:603 APOB, OMIM:144010), and the existing FH entry's
  has_subtypes axis is zygosity (heterozygous/homozygous), not gene. More
  importantly the mechanism differs at the molecular step that defines the
  disease: FDB is a ligand defect against a normal receptor, whereas
  LDLR-mediated FH is a receptor defect. The FH entry already carries an
  "APOB-LDLR Binding Defect" node as one of several genetic entry points into a
  shared pathway; this entry elaborates that node into its own mechanism graph
  and is deliberately not a duplicate of the umbrella entry.

  Not to be confused with the loss-of-function/truncating APOB allelic series
  that causes familial hypobetalipoproteinemia, which lowers rather than raises
  LDL cholesterol. Only APOB missense variants in the LDL-receptor-binding
  region produce this high-LDL phenotype.
references:
- reference: PMID:24404629
  title: "Familial Hypercholesterolemia."
  tags:
  - GeneReviews
  findings:
  - statement: >-
      Clinical characteristics. GeneReviews describes the FH clinical spectrum
      that APOB-related FH shares - premature cardiovascular events including
      angina and myocardial infarction, tendon xanthomas, xanthelasmas around
      the eyelids, and early corneal arcus. The chapter covers FH as a class
      across LDLR, APOB and PCSK9, so it is class-level rather than
      FDB-specific evidence and is cited as such throughout this entry.
    supporting_text: >-
      increases the risk of premature cardiovascular events such as angina and
      myocardial infarction
  - statement: >-
      Diagnosis and testing. GeneReviews gives the untreated LDL-C thresholds
      for a clinical FH diagnosis and names APOB variants that impair binding
      of LDL-C to the LDL receptor as one of three molecular routes to FH -
      the ligand-side route that defines this entry.
    supporting_text: >-
      The molecular diagnosis of FH can be established by identification of
      heterozygous or biallelic pathogenic variants in APOB (variants that
      impair binding of LDL-C to the LDL receptor), LDLR, or PCSK9 (gain of
      function); or rarely, identification of biallelic pathogenic variants in
      LDLRAP1.
  - statement: >-
      Management. GeneReviews sets out lipid surveillance from age two years,
      statin initiation in children from around age eight, cardiovascular
      risk-factor modification, and the contraindication of statins in
      pregnancy.
    supporting_text: >-
      Monitor lipid levels from age two years
  - statement: >-
      Genetic counseling. GeneReviews states the autosomal dominant
      transmission risk that applies directly to APOB-related FH, and the
      value of cascade evaluation of at-risk relatives.
    supporting_text: >-
      Each child of an individual with a heterozygous pathogenic variant in
      APOB, LDLR, or PCSK9 has a 50% chance of inheriting the pathogenic
      variant and having FH.
classifications:
  harrisons_chapter:
  - classification_value: ENDOCRINOLOGY_METABOLISM
    evidence:
    - reference: PMID:27919345
      reference_title: "Familial defective apolipoprotein B-100: A review."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Familial defective apolipoprotein B-100 (FDB) is an autosomal dominant genetic disorder of lipid metabolism associated with hyperlipidemia and elevated risk for atherosclerosis."
      explanation: Characterizes FDB as a disorder of lipid metabolism.
  - classification_value: GENETICS_ENVIRONMENT_DISEASE
    evidence:
    - reference: PMID:2563166
      reference_title: "Association between a specific apolipoprotein B mutation and familial defective apolipoprotein B-100."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "This same mutant allele occurs in six other, unrelated subjects and in eight affected relatives in two of these families."
      explanation: Establishes FDB as a Mendelian disorder segregating a single causal allele.
  icimd_category:
  - classification_value: hypercholesterolemias
    evidence:
    - reference: PMID:2280177
      reference_title: "Familial defective apolipoprotein B-100: a mutation of apolipoprotein B that causes hypercholesterolemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Familial defective apolipoprotein B-100 is a genetic disorder of apolipoprotein B-100 that causes moderate to severe hypercholesterolemia."
      explanation: Places FDB in the inherited hypercholesterolemias.
inheritance:
- name: Autosomal Dominant
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  penetrance: INCOMPLETE
  expressivity: VARIABLE
  description: >
    A single mutant APOB allele is sufficient. Heterozygotes carry a mixed LDL
    pool of normal and receptor-binding-defective particles, and the defective
    particles accumulate preferentially because only they escape efficient
    receptor-mediated clearance. Each child of a heterozygous carrier has a 50%
    chance of inheriting the allele.

    Penetrance is recorded as INCOMPLETE and expressivity as VARIABLE, and the
    two qualifiers refer to different things. The biochemical phenotype emerges
    with age - unselected carriers identified in their late teens need not be
    hypercholesterolemic and rise into the elevated range in their early
    twenties - so a normal cholesterol in a young carrier is not evidence
    against the genotype. Clinical penetrance measured against formal FH
    criteria is genuinely incomplete: only a subset of molecularly confirmed
    FDB carriers meet them, which is the mechanism by which FDB is
    underdiagnosed. Expressivity is variable in the same cohorts, with
    intraindividual cholesterol fluctuation markedly larger than in
    LDLR-mediated FH. Neither qualifier is a quantitative penetrance estimate;
    none is available for genotype-first FDB, so penetrance_percentage is
    deliberately omitted.
  evidence:
  - reference: PMID:24404629
    reference_title: "Familial Hypercholesterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Each child of an individual with a heterozygous pathogenic variant in APOB, LDLR, or PCSK9 has a 50% chance of inheriting the pathogenic variant and having FH."
    explanation: >-
      GeneReviews states the 50% transmission risk for APOB-related FH, the
      genetic-counseling consequence of autosomal dominant inheritance here.
  - reference: PMID:7583549
    reference_title: "Differences in the phenotypic characteristics of subjects with familial defective apolipoprotein B-100 and familial hypercholesterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "only a part of the subjects with FDB fulfill the established criteria for identifying FH"
    explanation: >-
      Supports INCOMPLETE penetrance of the clinical FH phenotype among
      molecularly confirmed FDB carriers.
  - reference: PMID:8006512
    reference_title: "High prevalence of familial defective apolipoprotein B-100 in Switzerland."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The three volunteers with the point mutation demonstrated an increase in total cholesterol concentrations by 1.30 mmol/l or by 25% within 2 years, suggesting that, in the early twenties, cholesterol concentrations increase markedly from normal to elevated levels."
    explanation: >-
      Supports age-dependent emergence of the biochemical phenotype in
      unselected carriers, the basis for the age qualifier on penetrance.
  - reference: PMID:27919345
    reference_title: "Familial defective apolipoprotein B-100: A review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Familial defective apolipoprotein B-100 (FDB) is an autosomal dominant genetic disorder of lipid metabolism associated with hyperlipidemia and elevated risk for atherosclerosis."
    explanation: States the autosomal dominant mode of inheritance for FDB.
  - reference: PMID:3477815
    reference_title: "Familial defective apolipoprotein B-100: low density lipoproteins with abnormal receptor binding."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Normal and abnormal LDL subpopulations were partially separated from plasma of two subjects by density-gradient ultracentrifugation, a finding consistent with the presence of a normal and a mutant allele."
    explanation: Demonstrates the heterozygous two-allele LDL subpopulation structure underlying dominant inheritance.
pathophysiology:
- name: APOB Receptor-Binding-Region Missense Variant
  description: >
    The initiating lesion is a missense substitution in the LDL-receptor-binding
    region of APOB, most commonly at the arginine residue numbered 3500 in the
    mature apoB-100 protein (p.Arg3527Gln in current HGVS numbering; c.10580G>A).
    The codon sits at a CpG mutational hot spot, which explains its recurrence.
    Critically, the variant is in the ligand, not the receptor: LDLR, PCSK9, and
    LDLRAP1 are wild type.
  role: trigger
  biological_scale: MOLECULAR
  mechanism_confidence: ESTABLISHED
  genes:
  - preferred_term: APOB
    term:
      id: hgnc:603
      label: APOB
  genetic_context:
    gene:
      preferred_term: APOB
      term:
        id: hgnc:603
        label: APOB
    zygosity: HETEROZYGOUS
    variant_origin: GERMLINE
    functional_impact_category: PARTIAL_LOSS_OF_FUNCTION
    description: >
      Scored as PARTIAL_LOSS_OF_FUNCTION rather than LOSS_OF_FUNCTION or
      DOMINANT_NEGATIVE. The protein is made and assembled normally into an
      LDL particle of normal chemical and physical composition; only the
      receptor-binding activity of that particle is reduced, and it is reduced
      partially rather than abolished - LDL from an FDB heterozygote retained
      about 32% of normal receptor binding activity in the original fibroblast
      binding study. DOMINANT_NEGATIVE would be the wrong call: the mutant
      apoB-100 does not interfere with the wild-type allele's product. Each LDL
      particle carries exactly one apoB-100 molecule, so mutant and wild-type
      particles are separate species that behave independently, and dominance is
      explained by preferential plasma accumulation of the poorly cleared mutant
      particles rather than by any trans effect on the normal protein.
      Supporting evidence for this call is carried on this node's evidence
      list, since GeneticContext has no evidence slot.
  downstream:
  - target: Defective ApoB-100 Binding to a Structurally Normal LDL Receptor
    description: The substitution directly produces the ligand-side binding defect.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:8006512
      reference_title: "High prevalence of familial defective apolipoprotein B-100 in Switzerland."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Familial defective apolipoprotein B-100 (FDB) is caused by a single G-to-A substitution at nucleotide 10,708 leading to an arginine to glutamine change at amino acid 3,500 of the apolipoprotein B-100 and thus, a reduced binding of the apolipoprotein B to the low density lipoprotein (LDL) receptor."
      explanation: States the direct causal step from the single nucleotide substitution to reduced apoB-LDLR binding.
  evidence:
  - reference: PMID:2563166
    reference_title: "Association between a specific apolipoprotein B mutation and familial defective apolipoprotein B-100."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Extensive sequence analysis of the two alleles of one subject heterozygous for the disorder has revealed a previously unreported mutation in the codon for amino acid 3500 that results in the substitution of glutamine for arginine."
    explanation: The original identification of the causal APOB missense variant.
  - reference: PMID:3477815
    reference_title: "Familial defective apolipoprotein B-100: low density lipoproteins with abnormal receptor binding."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "The G.R. LDL possessed 32% of normal receptor binding activity"
    explanation: Quantifies residual (partial, not absent) receptor-binding activity, supporting the PARTIAL_LOSS_OF_FUNCTION call on this node's genetic_context.
  - reference: PMID:3477815
    reference_title: "Familial defective apolipoprotein B-100: low density lipoproteins with abnormal receptor binding."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "the defect in receptor binding does not appear to be associated with an abnormal lipid composition or structure of the LDL: the chemical and physical properties of the particles were normal"
    explanation: Shows the particle is otherwise normally assembled, arguing against a global loss of apoB-100 function.
  - reference: PMID:2563166
    reference_title: "Association between a specific apolipoprotein B mutation and familial defective apolipoprotein B-100."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the mutation in the codon for amino acid 3500 (CGG----CAG), a CG mutational \"hot spot\", defines a minor apoB-100 allele associated with defective low density lipoproteins and hypercholesterolemia"
    explanation: Identifies the CpG hot spot that accounts for the recurrence of this allele.
  - reference: PMID:8215738
    reference_title: "Familial defective apolipoprotein B-100 is clinically indistinguishable from familial hypercholesterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Familial defective apolipoprotein B-100 is caused by a substitution of adenine for guanine in exon 26 of the gene coding for apolipoprotein B, which results in the substitution of glutamine for arginine in the putative low-density lipoprotein-receptor binding domain of the mature protein."
    explanation: Locates the substitution in exon 26 within the LDL-receptor-binding domain.
- name: Defective ApoB-100 Binding to a Structurally Normal LDL Receptor
  description: >
    The defining mechanistic step, and the point at which FDB diverges from
    LDLR-mediated familial hypercholesterolemia. The LDL receptor is normal; the
    ligand is defective. Arg3527 is not itself a contact residue of the receptor
    binding site - site B (residues 3359-3369) makes the contact. Instead
    Arg3527 interacts with the carboxyl terminus of apoB-100 (including Trp4396)
    to hold the belt of apoB-100 around the LDL particle in the conformation that
    exposes and stabilizes the basic-residue clusters of the binding site.
    Losing that arginine destabilizes the interaction and misconfigures the
    binding site, so the receptor cannot recognize the particle. Removing the
    carboxyl terminus of FDB LDL restores normal receptor binding, which shows
    the defect is conformational rather than a direct loss of a contact residue.
  role: central_effector
  biological_scale: MOLECULAR
  mechanism_confidence: ESTABLISHED
  genes:
  - preferred_term: APOB
    term:
      id: hgnc:603
      label: APOB
  molecular_functions:
  - preferred_term: low-density lipoprotein particle receptor binding
    term:
      id: GO:0050750
      label: low-density lipoprotein particle receptor binding
    modifier: DECREASED
  cellular_components:
  - preferred_term: low-density lipoprotein particle
    term:
      id: GO:0034362
      label: low-density lipoprotein particle
  downstream:
  - target: Reduced Receptor-Mediated Hepatic LDL Clearance
    description: >
      A particle the LDL receptor cannot bind cannot be internalized, so
      receptor-mediated catabolism of the mutant LDL pool fails.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:2280177
      reference_title: "Familial defective apolipoprotein B-100: a mutation of apolipoprotein B that causes hypercholesterolemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Low density lipoproteins accumulate in the plasma because their efficient receptor-mediated catabolism is disrupted."
      explanation: States the causal link from defective binding to failed receptor-mediated catabolism.
  - target: Small Dense LDL Particle Formation
    description: >
      Prolonged plasma residence of poorly cleared LDL exposes the particle to
      lipase and lipid-transfer remodeling, shifting the LDL population smaller.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:26643808
      reference_title: "Structural analysis of APOB variants, p.(Arg3527Gln), p.(Arg1164Thr) and p.(Gln4494del), causing Familial Hypercholesterolaemia provides novel insights into variant pathogenicity."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "It has been suggested that in patients carrying p.(Arg3527Gln) variant, the impaired removal of LDL from plasma promotes the formation of small dense LDL particles (sdLDL)"
      explanation: States the proposed route from impaired LDL removal to small dense LDL in Arg3527Gln carriers.
  evidence:
  - reference: PMID:9486979
    reference_title: "Identification of the low density lipoprotein receptor-binding site in apolipoprotein B100 and the modulation of its binding activity by the carboxyl terminus in familial defective apo-B100."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Site-directed mutagenesis and other evidence indicated that Site B (amino acids 3,359-3,369) binds to the LDL receptor and that arginine-3,500 is not directly involved in receptor binding."
    explanation: Establishes that the mutated arginine is not itself a receptor contact residue, so the defect must be conformational.
  - reference: PMID:9486979
    reference_title: "Identification of the low density lipoprotein receptor-binding site in apolipoprotein B100 and the modulation of its binding activity by the carboxyl terminus in familial defective apo-B100."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "The carboxyl-terminal 20% of apo-B100 is necessary for the R3500Q mutation to disrupt receptor binding, since removal of the carboxyl terminus in FDB LDL results in normal receptor-binding activity."
    explanation: Rescue of binding by carboxyl-terminal removal shows the mechanism is a conformational misconfiguration of the binding site.
  - reference: PMID:9486979
    reference_title: "Identification of the low density lipoprotein receptor-binding site in apolipoprotein B100 and the modulation of its binding activity by the carboxyl terminus in familial defective apo-B100."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "the loss of arginine at this site destabilizes this interaction, resulting in receptor-binding defective LDL"
    explanation: States the proposed destabilization mechanism producing binding-defective LDL.
  - reference: PMID:26643808
    reference_title: "Structural analysis of APOB variants, p.(Arg3527Gln), p.(Arg1164Thr) and p.(Gln4494del), causing Familial Hypercholesterolaemia provides novel insights into variant pathogenicity."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "The belt conformation of ApoB100 that surrounds the LDL particle is maintained by interaction of Arg3527 with Trp4396"
    explanation: Names the specific intramolecular interaction that the variant disrupts.
  - reference: PMID:26643808
    reference_title: "Structural analysis of APOB variants, p.(Arg3527Gln), p.(Arg1164Thr) and p.(Gln4494del), causing Familial Hypercholesterolaemia provides novel insights into variant pathogenicity."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "It has been proposed that replacement of the Arg3527 promotes a conformational change in ApoB100 causing a rearrangement of a number of charged residues rather than loss of a single receptor-interactive residue"
    explanation: Explicitly frames the defect as conformational rearrangement rather than loss of a contact residue.
  - reference: PMID:27919345
    reference_title: "Familial defective apolipoprotein B-100: A review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "FDB is caused by mutations in APOB reducing the binding affinity between apolipoprotein B-100 and the low-density lipoprotein receptor."
    explanation: Review statement of the ligand-side binding-affinity defect that defines the disorder.
- name: Small Dense LDL Particle Formation
  description: >
    LDL isolated from p.Arg3527Gln heterozygotes is measurably smaller than
    wild-type LDL and shows an altered apoB-100 secondary structure, with reduced
    beta-strand content. This is a consequence of the prolonged plasma residence
    time and of the conformational change itself, and is a plausible atherogenic
    amplifier rather than the primary disease step. Curated as a supporting node,
    not as the driver of the cardiovascular risk.
  biological_scale: MOLECULAR
  mechanism_confidence: PROVISIONAL
  cellular_components:
  - preferred_term: low-density lipoprotein particle
    term:
      id: GO:0034362
      label: low-density lipoprotein particle
  downstream:
  - target: Subendothelial Retention of ApoB-Containing Lipoproteins
    description: >
      Smaller, denser LDL particles are described as more atherogenic; treated
      here as a contributory rather than established route.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:26643808
      reference_title: "Structural analysis of APOB variants, p.(Arg3527Gln), p.(Arg1164Thr) and p.(Gln4494del), causing Familial Hypercholesterolaemia provides novel insights into variant pathogenicity."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "changes in the intensity and width of the β-strands band detected by IR spectroscopy might be related to more atherogenic LDL"
      explanation: Only a hedged structural-correlate claim ("might be related"), hence PARTIAL support and an unknown-intermediates link.
  evidence:
  - reference: PMID:26643808
    reference_title: "Structural analysis of APOB variants, p.(Arg3527Gln), p.(Arg1164Thr) and p.(Gln4494del), causing Familial Hypercholesterolaemia provides novel insights into variant pathogenicity."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "LDL particles obtained from heterozygous patients carrying p.(Arg3527Gln) or p.(Arg1164Thr) variants show a main population of particles of ~27 nm, significantly smaller than the ones carrying wt ApoB100 (~29 nm)"
    explanation: Direct measurement of reduced LDL particle size in Arg3527Gln heterozygotes.
  - reference: PMID:26643808
    reference_title: "Structural analysis of APOB variants, p.(Arg3527Gln), p.(Arg1164Thr) and p.(Gln4494del), causing Familial Hypercholesterolaemia provides novel insights into variant pathogenicity."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "ApoB100 p.(Arg3527Gln) variant showed a reduced content in β-strands"
    explanation: Reports the altered apoB-100 secondary structure in the variant particle.
- name: Reduced Receptor-Mediated Hepatic LDL Clearance
  description: >
    Because the mutant LDL cannot engage the hepatic LDL receptor, it is not
    internalized or delivered to the lysosome, and its fractional catabolic rate
    falls. Uptake of the wild-type LDL pool and of apoE-containing precursors is
    unaffected, which is why hepatic receptor abundance remains a therapeutic
    lever. Downstream sterol sensing in the hepatocyte is also blunted, since
    mutant LDL is less effective at stimulating intracellular cholesteryl ester
    synthesis.
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  cell_types:
  - preferred_term: hepatocyte
    term:
      id: CL:0000182
      label: hepatocyte
  biological_processes:
  - preferred_term: low-density lipoprotein particle clearance
    term:
      id: GO:0034383
      label: low-density lipoprotein particle clearance
    modifier: DECREASED
  - preferred_term: receptor-mediated endocytosis
    term:
      id: GO:0006898
      label: receptor-mediated endocytosis
    modifier: DECREASED
  - preferred_term: cholesterol homeostasis
    term:
      id: GO:0042632
      label: cholesterol homeostasis
    modifier: DYSREGULATED
  locations:
  - preferred_term: liver
    term:
      id: UBERON:0002107
      label: liver
  downstream:
  - target: Elevated Plasma LDL Cholesterol
    description: Impaired clearance is the direct cause of plasma LDL accumulation.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:3477815
      reference_title: "Familial defective apolipoprotein B-100: low density lipoproteins with abnormal receptor binding."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "These studies indicate that the defective receptor binding results in inefficient clearance of LDL and the hypercholesterolemia observed in these patients."
      explanation: States the causal chain from defective binding through inefficient clearance to hypercholesterolemia.
  evidence:
  - reference: PMID:3477815
    reference_title: "Familial defective apolipoprotein B-100: low density lipoproteins with abnormal receptor binding."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "the G.R. LDL were much less effective than normal LDL in competing with 125I-labeled normal LDL for cellular uptake and degradation and in stimulating intracellular cholesteryl ester synthesis"
    explanation: Demonstrates reduced cellular uptake, degradation, and downstream sterol signaling of mutant LDL.
  - reference: PMID:2280177
    reference_title: "Familial defective apolipoprotein B-100: a mutation of apolipoprotein B that causes hypercholesterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A single amino acid mutation in apolipoprotein B diminishes the ability of low density lipoproteins to bind to the low density lipoprotein receptor."
    explanation: Restates the ligand-side clearance defect at the level of the disorder.
- name: Elevated Plasma LDL Cholesterol
  description: >
    The clinically measured consequence: lifelong elevation of plasma total and
    LDL cholesterol. In Danish general-population carriers of Arg3500Gln, total
    cholesterol was about 100 mg/dL higher than in noncarriers. Levels tend to
    fluctuate more within individuals than in LDLR-mediated FH and may not be
    clearly elevated in young carriers, which contributes to underdiagnosis.
  biological_scale: ORGANISM
  mechanism_confidence: ESTABLISHED
  biological_processes:
  - preferred_term: cholesterol homeostasis
    term:
      id: GO:0042632
      label: cholesterol homeostasis
    modifier: DYSREGULATED
  downstream:
  - target: Subendothelial Retention of ApoB-Containing Lipoproteins
    description: >
      Sustained elevation of circulating apoB-containing LDL drives the
      initiating step of atherogenesis.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:26844337
      reference_title: "The Role of Lipids and Lipoproteins in Atherosclerosis."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        infiltration and retention of apoB containing lipoproteins in the artery
        wall is a critical initiating event that sparks an inflammatory response
        and promotes the development of atherosclerosis.
      explanation: >-
        Identifies retention of apoB-containing lipoproteins as the critical
        initiating event, the step that raised plasma LDL feeds. Evidence source
        is OTHER because this is an authoritative review chapter.
  evidence:
  - reference: PMID:9603795
    reference_title: "Association of mutations in the apolipoprotein B gene with hypercholesterolemia and the risk of ischemic heart disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Among carriers of the Arg3500Gln mutation, cholesterol levels were significantly higher than among noncarriers in the general population - by 100 mg per deciliter (2.6 mmol per liter) among carriers in the general population"
    explanation: Quantifies the cholesterol elevation attributable to the FDB allele in an unselected general population.
  - reference: PMID:8215738
    reference_title: "Familial defective apolipoprotein B-100 is clinically indistinguishable from familial hypercholesterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This amino acid substitution diminishes the binding capacity of the low-density lipoprotein particle for the low-density lipoprotein receptor, which in turn leads to an increase in levels of plasma total and low-density lipoprotein cholesterol."
    explanation: States the causal link from diminished binding capacity to raised plasma total and LDL cholesterol.
  - reference: PMID:8006512
    reference_title: "High prevalence of familial defective apolipoprotein B-100 in Switzerland."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the presence of the mutation was not necessarily associated with an elevation of serum cholesterol levels, particularly in young individuals"
    explanation: Qualifies the phenotype - cholesterol elevation is age-dependent and may be absent in young carriers, so support is PARTIAL.
- name: Subendothelial Retention of ApoB-Containing Lipoproteins
  description: >
    Chronically raised circulating LDL drives infiltration and retention of
    apoB-containing lipoproteins in the arterial intima, the shared initiating
    event of atherogenesis. FDB substitutes an elevated ligand-defective LDL
    burden at this trigger; the downstream plaque biology is the conserved
    pattern captured by the atherogenesis module and is not re-derived here.
  conforms_to: "atherogenesis#Endothelial Dysfunction and Subendothelial LDL Retention"
  biological_scale: TISSUE
  mechanism_confidence: ESTABLISHED
  cell_types:
  - preferred_term: endothelial cell of vascular tree
    term:
      id: CL:0002139
      label: endothelial cell of vascular tree
  biological_processes:
  - preferred_term: cholesterol homeostasis
    term:
      id: GO:0042632
      label: cholesterol homeostasis
    modifier: DYSREGULATED
  locations:
  - preferred_term: artery
    term:
      id: UBERON:0001637
      label: artery
  downstream:
  - target: Premature Atherosclerotic Cardiovascular Disease
    description: >
      Cumulative arterial lipoprotein retention over decades produces clinically
      manifest premature ischemic heart disease in FDB carriers.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:9603795
      reference_title: "Association of mutations in the apolipoprotein B gene with hypercholesterolemia and the risk of ischemic heart disease."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Heterozygous carriers of the Arg3500Gln mutation were significantly more common among patients with ischemic heart disease (odds ratio, 7.0; 95 percent confidence interval, 2.2 to 22; P=0.003)"
      explanation: Quantifies the excess ischemic heart disease risk carried by the FDB allele.
  evidence:
  - reference: PMID:26844337
    reference_title: "The Role of Lipids and Lipoproteins in Atherosclerosis."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      infiltration and retention of apoB containing lipoproteins in the artery
      wall is a critical initiating event that sparks an inflammatory response
      and promotes the development of atherosclerosis.
    explanation: >-
      Establishes subendothelial apoB-lipoprotein retention as the initiating
      event of atherosclerosis, the module node this entry conforms to. Evidence
      source is OTHER because this is an authoritative review chapter.
- name: Premature Atherosclerotic Cardiovascular Disease
  description: >
    The terminal clinical consequence: accelerated coronary atherosclerosis with
    premature ischemic heart disease. In the Danish general-population study the
    Arg3500Gln allele conferred a sevenfold odds of ischemic heart disease, and
    the effect was allele-specific - the neighbouring Arg3531Cys substitution
    raised neither cholesterol nor ischemic heart disease risk, an internal
    negative control for the mechanism.
  role: outcome
  biological_scale: ORGANISM
  mechanism_confidence: ESTABLISHED
  locations:
  - preferred_term: coronary artery
    term:
      id: UBERON:0001621
      label: coronary artery
  evidence:
  - reference: PMID:9603795
    reference_title: "Association of mutations in the apolipoprotein B gene with hypercholesterolemia and the risk of ischemic heart disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The Arg3500Gln mutation in the apolipoprotein B gene, which is responsible for familial defective apolipoprotein B-100 and is present in approximately 1 in 1000 persons in Denmark, causes severe hypercholesterolemia and increases the risk of ischemic heart disease."
    explanation: States the population-level conclusion that the FDB allele causes hypercholesterolemia and raises ischemic heart disease risk.
  - reference: PMID:9603795
    reference_title: "Association of mutations in the apolipoprotein B gene with hypercholesterolemia and the risk of ischemic heart disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Heterzygous carriers of the Arg3531Cys mutation in the general population did not have higher-than-normal plasma cholesterol levels or an increased risk of ischemic heart disease"
    explanation: Allele-specific negative control - a different APOB substitution produced neither phenotype, supporting specificity of the binding-region lesion.
  - reference: PMID:8141833
    reference_title: "Familial defective apolipoprotein B-100: a review, including some comparisons with familial hypercholesterolaemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The presence of mutant apo B-100 in low-density lipoproteins (LDL) markedly reduces their affinity for the LDL receptor, leading to hypercholesterolaemia and increased proneness to coronary artery disease."
    explanation: Summarizes the whole chain from reduced LDL receptor affinity to coronary artery disease.
phenotypes:
- category: Biochemical
  name: Increased LDL Cholesterol Concentration
  description: >
    Elevated plasma LDL cholesterol is the obligate biochemical phenotype and
    the direct readout of impaired receptor-mediated LDL clearance.
  phenotype_term:
    preferred_term: Increased LDL cholesterol concentration
    term:
      id: HP:0003141
      label: Increased LDL cholesterol concentration
  evidence:
  - reference: PMID:8215738
    reference_title: "Familial defective apolipoprotein B-100 is clinically indistinguishable from familial hypercholesterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This amino acid substitution diminishes the binding capacity of the low-density lipoprotein particle for the low-density lipoprotein receptor, which in turn leads to an increase in levels of plasma total and low-density lipoprotein cholesterol."
    explanation: States that the FDB substitution raises plasma LDL cholesterol.
- category: Biochemical
  name: Hypercholesterolemia
  description: >
    Total cholesterol elevation, moderate to severe. Reported as roughly
    100 mg/dL above noncarriers among unselected Danish general-population
    carriers. Intraindividual variability is markedly greater than in
    LDLR-mediated familial hypercholesterolemia.
  phenotype_term:
    preferred_term: Hypercholesterolemia
    term:
      id: HP:0003124
      label: Hypercholesterolemia
  evidence:
  - reference: PMID:2280177
    reference_title: "Familial defective apolipoprotein B-100: a mutation of apolipoprotein B that causes hypercholesterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Familial defective apolipoprotein B-100 is a genetic disorder of apolipoprotein B-100 that causes moderate to severe hypercholesterolemia."
    explanation: States the total-cholesterol phenotype and its severity range.
  - reference: PMID:7583549
    reference_title: "Differences in the phenotypic characteristics of subjects with familial defective apolipoprotein B-100 and familial hypercholesterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "FDB subjects showed much larger total cholesterol fluctuations than FH subjects (median of intraindividual coefficients of variation: FDB, 14.5%; FH, 5.3%; P < .001)"
    explanation: Quantifies the greater intraindividual variability of total cholesterol in FDB.
- category: Cardiovascular
  name: Premature Coronary Artery Atherosclerosis
  description: >
    Accelerated coronary atherosclerosis with premature ischemic heart disease
    is the principal clinical burden of FDB.
  phenotype_term:
    preferred_term: Premature coronary artery atherosclerosis
    term:
      id: HP:0005181
      label: Premature coronary artery atherosclerosis
  evidence:
  - reference: PMID:8141833
    reference_title: "Familial defective apolipoprotein B-100: a review, including some comparisons with familial hypercholesterolaemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The presence of mutant apo B-100 in low-density lipoproteins (LDL) markedly reduces their affinity for the LDL receptor, leading to hypercholesterolaemia and increased proneness to coronary artery disease."
    explanation: Links the FDB mechanism to coronary artery disease proneness.
  - reference: PMID:9603795
    reference_title: "Association of mutations in the apolipoprotein B gene with hypercholesterolemia and the risk of ischemic heart disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Heterozygous carriers of the Arg3500Gln mutation were significantly more common among patients with ischemic heart disease (odds ratio, 7.0; 95 percent confidence interval, 2.2 to 22; P=0.003)"
    explanation: Quantifies the excess ischemic heart disease risk in FDB carriers.
- category: Cardiovascular
  name: Angina Pectoris
  description: >
    Symptomatic coronary disease. In a cohort of 18 molecularly confirmed FDB
    patients originally diagnosed clinically as FH, 8 had angina or other
    evidence of coronary artery disease.
  phenotype_term:
    preferred_term: Angina pectoris
    term:
      id: HP:0001681
      label: Angina pectoris
  frequency: FREQUENT
  evidence:
  - reference: PMID:8215738
    reference_title: "Familial defective apolipoprotein B-100 is clinically indistinguishable from familial hypercholesterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "eight of 18 patients had angina or other evidence of coronary artery disease"
    explanation: >-
      8/18 = 44% supports the FREQUENT band (30-79%). The count is for angina or
      other coronary artery disease evidence combined, so the band is an upper
      bound for isolated angina.
- category: Dermatological
  name: Tendon Xanthomatosis
  description: >
    Tendon xanthomas may be present but are not obligate, and the reported
    series counts xanthomas together with corneal arcus. Their presence is what
    makes FDB satisfy clinical FH criteria in some carriers.
  phenotype_term:
    preferred_term: Tendon xanthomatosis
    term:
      id: HP:0010874
      label: Tendon xanthomatosis
  notes: >
    Frequency deliberately omitted. The available count (10 of 18) pools tendon
    xanthomas and corneal arcus, so it cannot be resolved into a frequency band
    for either sign alone.
  evidence:
  - reference: PMID:8215738
    reference_title: "Familial defective apolipoprotein B-100 is clinically indistinguishable from familial hypercholesterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Ten of 18 patients had tendon xanthomas or an arcus cornealis or both"
    explanation: Documents tendon xanthomas in FDB, but pooled with corneal arcus, hence PARTIAL.
- category: Ophthalmological
  name: Corneal Arcus
  description: >
    Lipid deposition at the corneal limbus, a classic stigma of monogenic
    hypercholesterolemia, reported in FDB carriers alongside tendon xanthomas.
  phenotype_term:
    preferred_term: Corneal arcus
    term:
      id: HP:0001084
      label: Corneal arcus
  notes: >
    Frequency deliberately omitted for the same reason as tendon xanthomatosis -
    the source count pools the two signs.
  evidence:
  - reference: PMID:8215738
    reference_title: "Familial defective apolipoprotein B-100 is clinically indistinguishable from familial hypercholesterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Ten of 18 patients had tendon xanthomas or an arcus cornealis or both"
    explanation: Documents corneal arcus in FDB, but pooled with tendon xanthomas, hence PARTIAL.
- category: Cardiovascular
  name: Myocardial Infarction
  description: >
    Acute coronary occlusion is the endpoint of the accelerated coronary
    atherosclerosis modeled in this entry, and the counterpart of the angina
    already curated. Events occur at lower absolute rates and later ages in
    FDB than in LDLR-mediated FH: in a direct comparison, coronary heart
    disease was present in 5.6% of FDB subjects at median age 52 versus almost
    40% of FH patients at mean age 41.
  phenotype_term:
    preferred_term: Myocardial infarction
    term:
      id: HP:0001658
      label: Myocardial infarction
  notes: >
    Frequency deliberately omitted. The 5.6% figure is for coronary heart
    disease as a whole rather than myocardial infarction specifically, and the
    two compared cohorts differ in age distribution and ascertainment, so it
    cannot be resolved into a frequency band for infarction.
  evidence:
  - reference: PMID:38393015
    reference_title: "ApoB100 and Atherosclerosis: What's New in the 21st Century?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Familial defective apolipoprotein B100 has been linked to increased rates of both mild and severe coronary artery calcification, and patients often experience early cardiovascular events, including coronary artery disease, ischemic heart disease, and myocardial infarction"
    explanation: FDB-specific statement that carriers experience early myocardial infarction.
  - reference: PMID:38393015
    reference_title: "ApoB100 and Atherosclerosis: What's New in the 21st Century?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Almost 40% of FH patients had coronary heart disease (CHD) at a mean age of 41 years, while CHD was present in 5.6% of FDB subjects with a median age of 52 years."
    explanation: >-
      Quantifies the FDB coronary event burden against LDLR-mediated FH.
      PARTIAL because the endpoint is coronary heart disease rather than
      infarction specifically.
  - reference: PMID:24404629
    reference_title: "Familial Hypercholesterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "increases the risk of premature cardiovascular events such as angina and myocardial infarction"
    explanation: >-
      GeneReviews names myocardial infarction as a core FH manifestation.
      PARTIAL because the chapter covers FH across LDLR, APOB and PCSK9 rather
      than FDB specifically.
- category: Cardiovascular
  name: Coronary Artery Calcification
  description: >
    Calcified coronary plaque is the imaging correlate of the accumulated
    atherosclerotic burden, and is reported at increased rates in FDB across
    both mild and severe grades.
  phenotype_term:
    preferred_term: Coronary artery calcification
    term:
      id: HP:0001717
      label: Coronary artery calcification
  evidence:
  - reference: PMID:38393015
    reference_title: "ApoB100 and Atherosclerosis: What's New in the 21st Century?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Familial defective apolipoprotein B100 has been linked to increased rates of both mild and severe coronary artery calcification"
    explanation: FDB-specific statement of increased coronary artery calcification.
- category: Cardiovascular
  name: Carotid Artery Stenosis
  description: >
    Atherosclerosis in FDB is not confined to the coronary bed. Internal
    carotid artery stenosis was found in 4% of FDB subjects, against 15% of
    LDLR-mediated FH subjects in the same comparison - an FDB-specific figure
    that also supplies one of the quantitative anchors for the milder-phenotype
    side of the severity controversy curated in this entry.
  phenotype_term:
    preferred_term: Carotid artery stenosis
    term:
      id: HP:0100546
      label: Carotid artery stenosis
  notes: >
    Frequency deliberately omitted. 4% would fall in the VERY_RARE band
    (<5%), but it is a single clinic-derived comparison whose FDB and FH arms
    differ in age and ascertainment, so the band is not supportable on this
    source alone.
  evidence:
  - reference: PMID:38393015
    reference_title: "ApoB100 and Atherosclerosis: What's New in the 21st Century?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "internal carotid artery stenoses were more prevalent in FH (15% versus 4% in the FDB group)"
    explanation: Reports carotid artery stenosis in FDB with a directly compared FH group.
- category: Dermatological
  name: Xanthelasma
  description: >
    Yellowish periocular cholesterol deposits, a classic external stigma of
    monogenic hypercholesterolemia. Curated from the class-level GeneReviews
    description of familial hypercholesterolemia; no FDB-specific frequency was
    identified.
  phenotype_term:
    preferred_term: Xanthelasma
    term:
      id: HP:0001114
      label: Xanthelasma
  notes: >
    Frequency deliberately omitted - the only source is class-level FH
    literature, and the visible-xanthoma literature is drawn from broader FH
    cohorts in which FDB carriers are a minority.
  evidence:
  - reference: PMID:24404629
    reference_title: "Familial Hypercholesterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Xanthelasmas (yellowish, waxy deposits) can occur around the eyelids."
    explanation: >-
      GeneReviews names xanthelasma as an FH manifestation. PARTIAL because the
      chapter describes FH across LDLR, APOB and PCSK9 rather than FDB alone.
biochemical:
- name: LDL Cholesterol
  context: Plasma
  presence: Increased
  notes: >
    In a multivariate comparison of 28 FDB heterozygotes against 129 heterozygous
    FH subjects, LDL cholesterol was 6.3 mmol/L in FDB versus 8.2 mmol/L in FH.
    Absolute values depend heavily on ascertainment, since FDB carriers found
    through lipid clinics are selected for high cholesterol.
  evidence:
  - reference: PMID:7583549
    reference_title: "Differences in the phenotypic characteristics of subjects with familial defective apolipoprotein B-100 and familial hypercholesterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "FDB subjects demonstrated significantly lower concentrations of total cholesterol (8.1 versus 10.2 mmol/L, P < .001), LDL cholesterol (6.3 versus 8.2 mmol/L, P < .001), and triglycerides (1.3 versus 1.8 mmol/L, P = .025) and higher concentrations of HDL cholesterol (1.4 versus 1.2 mmol/L, P = .015) than subjects with FH."
    explanation: Reports FDB LDL cholesterol concentrations against a directly compared FH group.
- name: Total Cholesterol
  context: Plasma
  presence: Increased
  notes: >
    Elevation is age-dependent. Swiss general-population carriers identified in
    their late teens were not necessarily hypercholesterolemic, then rose sharply
    into their early twenties.
  evidence:
  - reference: PMID:8006512
    reference_title: "High prevalence of familial defective apolipoprotein B-100 in Switzerland."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The three volunteers with the point mutation demonstrated an increase in total cholesterol concentrations by 1.30 mmol/l or by 25% within 2 years, suggesting that, in the early twenties, cholesterol concentrations increase markedly from normal to elevated levels."
    explanation: Documents the age-dependent emergence of total-cholesterol elevation in unselected carriers.
- name: HDL Cholesterol
  context: Plasma
  presence: Increased
  notes: >
    Relative to LDLR-mediated FH rather than to healthy controls - FDB subjects
    had higher HDL cholesterol than FH subjects (1.4 versus 1.2 mmol/L). This is
    a between-disease contrast, not an assertion that FDB raises HDL above normal.
  evidence:
  - reference: PMID:7583549
    reference_title: "Differences in the phenotypic characteristics of subjects with familial defective apolipoprotein B-100 and familial hypercholesterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "higher concentrations of HDL cholesterol (1.4 versus 1.2 mmol/L, P = .015) than subjects with FH"
    explanation: Supports the FDB-versus-FH contrast only; PARTIAL because no comparison to a normolipidemic reference group is made.
genetic:
- name: APOB
  gene_term:
    preferred_term: APOB
    term:
      id: hgnc:603
      label: APOB
  association: Causal
  presence: Positive
  relationship_type: CAUSATIVE
  inheritance:
  - name: Autosomal Dominant
    inheritance_term:
      preferred_term: Autosomal dominant inheritance
      term:
        id: HP:0000006
        label: Autosomal dominant inheritance
  variant_origin: GERMLINE
  notes: >
    APOB encodes apolipoprotein B-100, the single structural protein of the LDL
    particle and the ligand for the LDL receptor. The FDB allelic series is
    confined to missense substitutions in or affecting the receptor-binding
    region; it is mechanistically distinct from the truncating APOB alleles that
    cause familial hypobetalipoproteinemia, which lower LDL cholesterol.
  case_fractions:
  - population: Familial hypercholesterolemia cases (APOB p.Arg3527Gln and p.Arg3527Trp combined)
    case_fraction_percent: 12.0
    notes: >
      Upper end of the reported range for the APOB share of familial
      hypercholesterolemia, counting both recurrent codon-3527 alleles. This is
      the share of FH cases explained by APOB, not a population prevalence of
      FDB - the two are recorded separately and must not be conflated.
    evidence:
    - reference: PMID:38393015
      reference_title: "ApoB100 and Atherosclerosis: What's New in the 21st Century?"
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Both APOB R3500Q and R3500W variants are also implicated in familial hypercholesterolemia (FH), accounting for 12% of FH cases."
      explanation: Review-level estimate of the combined APOB share of FH cases.
  - population: European familial hypercholesterolemia phenotypes (p.Arg3527Gln allele only)
    case_fraction_percent: 3.5
    case_fraction_low: 2.0
    case_fraction_high: 5.0
    notes: >
      Lower end of the range, for the classic R3500Q allele alone in Europe.
      case_fraction_percent records the midpoint of the quoted 2-5% interval;
      the bounds carry the interval itself. The spread against the 12% figure
      above is genuine and is driven by which alleles are counted, by
      population, and by how the FH denominator was ascertained - it is
      recorded as two cohort-specific records rather than averaged into one.
    evidence:
    - reference: PMID:33519890
      reference_title: "The Digenic Causality in Familial Hypercholesterolemia: Revising the Genotype-Phenotype Correlations of the Disease."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "(R3500Q) is common in Europe and accounts for 2–5% of FH phenotypes"
      explanation: European estimate of the R3500Q share of FH phenotypes.
  variants:
  - name: p.Arg3527Gln (R3500Q; c.10580G>A)
    description: >
      The classic and by far the most common FDB allele. Numbering differs by
      convention: 3500 counts from the mature protein, 3527 includes the signal
      peptide and is the current HGVS form. Arises at a CpG mutational hot spot.
      Carriers of Western European descent share a common haplotype, consistent
      with a founder allele.
    type: missense
    clinical_significance: PATHOGENIC
    evidence:
    - reference: PMID:8215738
      reference_title: "Familial defective apolipoprotein B-100 is clinically indistinguishable from familial hypercholesterolemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The mutation was associated with a similar haplotype, which was also reported in other patients of Western European descent with familial defective apolipoprotein B100. This strongly suggests that the mutation has a common chromosomal background that originated in Western Europe."
      explanation: Establishes the shared founder haplotype of the p.Arg3527Gln allele in Western Europe.
    - reference: PMID:32591292
      reference_title: "The first Japanese cases of familial hypercholesterolemia due to a known pathogenic APOB gene variant, c.10580 G>A: p.(Arg3527Gln)."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The known p.(Arg3527Gln) variant in the APOB gene was identified in one Japanese family."
      explanation: Confirms the c.10580G>A / p.(Arg3527Gln) designation and documents its rarity outside European-descent populations.
  - name: p.Arg3527Trp (R3500W)
    description: >
      A distinct substitution at the same codon, reported in hyperlipidemic
      East Asian populations on its own founder haplotype. It is the APOB allele
      of note for FDB outside European-descent populations.
    type: missense
    clinical_significance: PATHOGENIC
    evidence:
    - reference: PMID:9702952
      reference_title: "Identification and haplotype analysis of apolipoprotein B-100 Arg3500-->Trp mutation in hyperlipidemic Chinese."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "A total of 373 hyperlipidemic patients and 309 controls were screened for R3500W. Nine unrelated subjects were shown to be heterozygous for the mutation, and no R3500W carriers were found in the control group (P = 0.004)."
      explanation: Demonstrates enrichment of R3500W in hyperlipidemic Chinese subjects versus controls.
    - reference: PMID:9702952
      reference_title: "Identification and haplotype analysis of apolipoprotein B-100 Arg3500-->Trp mutation in hyperlipidemic Chinese."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The fact that the same mutant allele was identified in other Asians with FDB indicates a common Asian origin for the R3500W mutations."
      explanation: Establishes a separate Asian founder origin for this allele.
    - reference: PMID:27919345
      reference_title: "Familial defective apolipoprotein B-100: A review."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "the APOB R3500 W variant is known to make a significant contribution to familial hypercholesterolemia (FH) among East Asians"
      explanation: Review confirms the population-specific contribution of R3500W.
  - name: p.Arg3558Cys (R3531C)
    description: >
      A neighbouring APOB substitution that was screened alongside the FDB
      alleles but did not raise cholesterol or ischemic heart disease risk in the
      general population. Recorded here as an explicit negative, since it
      delimits which APOB substitutions produce the FDB phenotype.
    type: missense
    clinical_significance: UNCERTAIN_SIGNIFICANCE
    evidence:
    - reference: PMID:9603795
      reference_title: "Association of mutations in the apolipoprotein B gene with hypercholesterolemia and the risk of ischemic heart disease."
      supports: REFUTE
      evidence_source: HUMAN_CLINICAL
      snippet: "Heterzygous carriers of the Arg3531Cys mutation in the general population did not have higher-than-normal plasma cholesterol levels or an increased risk of ischemic heart disease"
      explanation: Refutes a causal FDB role for this substitution in an unselected population.
  evidence:
  - reference: PMID:27919345
    reference_title: "Familial defective apolipoprotein B-100: A review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "FDB is caused by mutations in APOB reducing the binding affinity between apolipoprotein B-100 and the low-density lipoprotein receptor."
    explanation: Establishes APOB as the causal gene and the binding-affinity mechanism.
  - reference: PMID:2563166
    reference_title: "Association between a specific apolipoprotein B mutation and familial defective apolipoprotein B-100."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This same mutant allele occurs in six other, unrelated subjects and in eight affected relatives in two of these families."
    explanation: Original demonstration of segregation of the APOB allele with the FDB phenotype.
- name: LDLR
  gene_term:
    preferred_term: LDLR
    term:
      id: hgnc:6547
      label: LDLR
  association: Modifier (digenic second locus)
  presence: Positive
  relationship_type: MODIFIER
  variant_origin: GERMLINE
  notes: >
    LDLR is not a cause of FDB - by definition the receptor is normal in this
    disorder, which is the whole point of the entity. It is recorded here as a
    modifier because a pathogenic LDLR allele carried in trans with APOB
    p.Arg3527Gln produces a double-heterozygous state more severe than either
    single heterozygote. That is a dosage effect on the same clearance step:
    the ligand defect and the receptor defect subtract from the same LDL
    removal capacity. Curated as MODIFIER rather than CAUSATIVE deliberately,
    since an LDLR lesion on its own gives LDLR-mediated FH, a different entry.
    For the same reason this entry does NOT bind a digenic inheritance term
    (HP:0010984) and is not a member of the Digenic and Oligogenic Disorders
    grouping: each locus causes hypercholesterolaemia on its own, so the
    double heterozygote is a dosage effect on one clearance step rather than a
    two-locus requirement. The PARTIAL counterweight evidence below - that
    digenic second hits do not uniformly worsen the phenotype - is part of the
    same judgement.
  evidence:
  - reference: PMID:33519890
    reference_title: "The Digenic Causality in Familial Hypercholesterolemia: Revising the Genotype-Phenotype Correlations of the Disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "double-heterozygotes carrying LDLR and p.(R3500Q) APOB mutations have more severe phenotypes when compared to the heterozygote FH cases carrying only one mutation in any of the mentioned genes"
    explanation: >-
      Names the APOB p.Arg3527Gln plus LDLR double-heterozygous state as more
      severe than either single heterozygote, the basis for the MODIFIER call.
- name: PCSK9
  gene_term:
    preferred_term: PCSK9
    term:
      id: hgnc:20001
      label: PCSK9
  association: Modifier (LDL receptor abundance)
  presence: Positive
  relationship_type: MODIFIER
  variant_origin: GERMLINE
  notes: >
    PCSK9 sets hepatic LDL receptor abundance by targeting the receptor for
    degradation, so PCSK9 genotype changes how much functional receptor is
    available to clear the receptor-competent share of the LDL pool. It is an
    established modifier of FH phenotype severity in carriers of an identical
    primary lesion. The same axis is why anti-PCSK9 antibodies work in FDB -
    see the PCSK9 monoclonal antibody treatment, whose rationale is receptor
    abundance rather than any effect on the defective ligand.
  evidence:
  - reference: PMID:33519890
    reference_title: "The Digenic Causality in Familial Hypercholesterolemia: Revising the Genotype-Phenotype Correlations of the Disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "PCSK9 has been identified as an FH modifier gene as it generates significant variable phenotypes even in patients having the same mutation in LDLR"
    explanation: >-
      Identifies PCSK9 as a modifier gene producing variable phenotypes at a
      fixed primary lesion.
  - reference: PMID:33519890
    reference_title: "The Digenic Causality in Familial Hypercholesterolemia: Revising the Genotype-Phenotype Correlations of the Disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "recently several reports have identified digenic mutations in familial cases that do not necessarily reflect a much severe phenotype"
    explanation: >-
      Retained as the counterweight: digenic second hits do not uniformly
      worsen the phenotype, so modifier status is recorded without a claimed
      effect size. PARTIAL because it qualifies rather than supports the
      severity direction.
prevalence:
- population: Denmark (general population)
  measure_type: POINT_PREVALENCE
  prevalence_class: ABOVE_1_IN_1000
  rate_per_100000: 100.0
  notes: >
    Genotyped in 9255 unselected Danish women and men. The measured heterozygote
    prevalence was 0.08% (95% CI 0.03-0.16%), which the authors round to
    approximately 1 in 1000; rate_per_100000 records that stated figure. This is
    the best unselected-population estimate available, since clinic-based series
    are ascertained on hypercholesterolemia.
  evidence:
  - reference: PMID:9603795
    reference_title: "Association of mutations in the apolipoprotein B gene with hypercholesterolemia and the risk of ischemic heart disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The prevalence of heterozygotes in the general population was 0.08 percent (95 percent confidence interval, 0.03 to 0.16 percent) for both the Arg3500Gln and the Arg3531Cys mutations"
    explanation: Direct measurement of FDB heterozygote prevalence in an unselected general population.
- population: Northern Europeans and US Caucasians
  measure_type: POINT_PREVALENCE
  prevalence_class: ABOVE_1_IN_1000
  rate_per_100000: 100.0
  notes: Approximately 0.1% carry the R3500Q variant per the 2016 review.
  evidence:
  - reference: PMID:27919345
    reference_title: "Familial defective apolipoprotein B-100: A review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Population studies suggest that approximately 0.1% of Northern Europeans and US Caucasians carries the R3500Q variant in APOB most commonly associated with FDB"
    explanation: Review-level population estimate for European-descent populations.
- population: Switzerland (German, French, and Romansh speaking regions)
  measure_type: POINT_PREVALENCE
  prevalence_class: ABOVE_1_IN_1000
  rate_per_100000: 478.0
  notes: >
    A striking founder-effect outlier: combined estimate 1 in 209 (rate_per_100000
    computed as 100000/209 = 478). Derived by combining 728 healthy volunteers
    (3 carriers, ~1/240) with 142 hypercholesterolemic families extrapolated to
    the general population (~1/190). The extrapolated arm is ascertainment-prone,
    so treat this as an upper bound rather than a directly measured prevalence;
    the volunteer arm alone (1/240, ~417 per 100000) is the cleaner figure.
  evidence:
  - reference: PMID:8006512
    reference_title: "High prevalence of familial defective apolipoprotein B-100 in Switzerland."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The combined prevalence based on both samples was 1/209. Thus, the investigated point mutation was highly prevalent in Switzerland and appeared to be more frequent than in other populations studied hitherto."
    explanation: Reports the Swiss combined prevalence estimate and its status as an international outlier.
- population: United States, Canada, and Europe
  measure_type: POINT_PREVALENCE
  prevalence_class: ABOVE_1_IN_1000
  rate_per_100000: 200.0
  notes: >
    Early aggregate estimate of approximately 1 in 500 across Western
    populations, predating the large unselected Danish genotyping study; retained
    for historical range but the Danish figure is the more reliable one.
  evidence:
  - reference: PMID:2280177
    reference_title: "Familial defective apolipoprotein B-100: a mutation of apolipoprotein B that causes hypercholesterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This mutation has been identified in the United States, Canada, and Europe and is estimated to occur at a frequency of approximately 1/500 in these populations."
    explanation: Historical Western-population frequency estimate.
progression:
- phase: Childhood and adolescence
  notes: >
    Carriers may be normocholesterolemic or only mildly hypercholesterolemic at
    this stage, which is a diagnostic trap in cascade screening of families.
    This is also the stage at which lipid surveillance begins and at which
    statin therapy first becomes available, so the diagnostic trap has direct
    management consequences.
  evidence:
  - reference: PMID:8006512
    reference_title: "High prevalence of familial defective apolipoprotein B-100 in Switzerland."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the presence of the mutation was not necessarily associated with an elevation of serum cholesterol levels, particularly in young individuals"
    explanation: >-
      Documents unselected young carriers who were not hypercholesterolemic,
      the basis for treating this phase as biochemically silent or mild.
- phase: Early adulthood
  notes: >
    Total cholesterol rises sharply into the elevated range in the early
    twenties in unselected carriers.
  evidence:
  - reference: PMID:8006512
    reference_title: "High prevalence of familial defective apolipoprotein B-100 in Switzerland."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The three volunteers with the point mutation demonstrated an increase in total cholesterol concentrations by 1.30 mmol/l or by 25% within 2 years, suggesting that, in the early twenties, cholesterol concentrations increase markedly from normal to elevated levels."
    explanation: Dates the sharp rise in total cholesterol to the early twenties in unselected carriers.
- phase: Mid to late adulthood
  notes: >
    Cumulative LDL exposure produces coronary atherosclerosis and symptomatic
    ischemic heart disease. Median age at recorded coronary heart disease in
    FDB was 52 years, later than the mean of 41 years in directly compared
    LDLR-mediated FH.
  evidence:
  - reference: PMID:38393015
    reference_title: "ApoB100 and Atherosclerosis: What's New in the 21st Century?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Almost 40% of FH patients had coronary heart disease (CHD) at a mean age of 41 years, while CHD was present in 5.6% of FDB subjects with a median age of 52 years."
    explanation: Places FDB coronary events in mid-to-late adulthood, later than in LDLR-mediated FH.
  - reference: PMID:8215738
    reference_title: "Familial defective apolipoprotein B-100 is clinically indistinguishable from familial hypercholesterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "eight of 18 patients had angina or other evidence of coronary artery disease"
    explanation: Documents symptomatic coronary disease as the clinical burden of this phase.
diagnosis:
- name: Molecular Genetic Testing of APOB
  diagnosis_term:
    preferred_term: Genetic Testing
    term:
      id: NCIT:C15709
      label: Genetic Testing
  description: >
    Targeted APOB genotyping resolves FDB from LDLR-mediated FH in patients
    meeting clinical FH criteria. This matters because a substantial minority of
    clinically diagnosed FH is genetically FDB, and because carriers with milder
    lipid values are missed by criteria calibrated on LDLR-mediated FH. The
    variant is a single-nucleotide substitution amenable to simple PCR-based
    assays.
  results: >
    Identification of a heterozygous pathogenic APOB missense variant in the
    LDL-receptor-binding region - p.Arg3527Gln (c.10580G>A) in European-descent
    populations, p.Arg3527Trp in East Asian populations - establishes the
    molecular diagnosis of FDB in a proband with hypercholesterolemia.
  evidence:
  - reference: PMID:24404629
    reference_title: "Familial Hypercholesterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The molecular diagnosis of FH can be established by identification of heterozygous or biallelic pathogenic variants in APOB (variants that impair binding of LDL-C to the LDL receptor), LDLR, or PCSK9 (gain of function); or rarely, identification of biallelic pathogenic variants in LDLRAP1."
    explanation: >-
      GeneReviews establishes molecular testing as diagnostic and defines the
      APOB route in exactly the ligand-binding terms this entry models.
  - reference: PMID:8141833
    reference_title: "Familial defective apolipoprotein B-100: a review, including some comparisons with familial hypercholesterolaemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In most lipid clinics, 2-5% of patients given a clinical diagnosis of FH have FDB, not FH."
    explanation: Quantifies the diagnostic yield that motivates APOB genotyping in clinically diagnosed FH.
  - reference: PMID:8006512
    reference_title: "High prevalence of familial defective apolipoprotein B-100 in Switzerland."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Considering the estimated high prevalence and the relative ease of PCR-based tests, screening for FDB may become a standard procedure in patients with suggested familial forms of hypercholesterolemia."
    explanation: Supports molecular screening for FDB among suspected familial hypercholesterolemia.
- name: Familial Hypercholesterolemia Multigene Panel
  diagnosis_term:
    preferred_term: Genetic Testing
    term:
      id: NCIT:C15709
      label: Genetic Testing
  description: >
    In practice FDB is usually reached through an FH multigene panel rather than
    a single-gene APOB assay, because the clinical presentation does not
    discriminate the genes. The panel covers LDLR, APOB, PCSK9 and LDLRAP1; a
    positive APOB result in the receptor-binding region is what assigns the
    patient to this entry rather than to LDLR- or PCSK9-mediated FH.
  results: >
    An APOB receptor-binding-region variant assigns FDB; an LDLR or PCSK9
    variant assigns a different monogenic FH; a negative panel in a patient
    with a clinical FH phenotype favours polygenic hypercholesterolemia or a
    secondary cause.
  notes: >
    The gene assignment is not merely a label. Because the receptor is intact in
    FDB, receptor-directed therapy retains its target, which is the practical
    consequence the source draws from distinguishing the two disorders.
  evidence:
  - reference: PMID:24404629
    reference_title: "Familial Hypercholesterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The molecular diagnosis of FH can be established by identification of heterozygous or biallelic pathogenic variants in APOB (variants that impair binding of LDL-C to the LDL receptor), LDLR, or PCSK9 (gain of function); or rarely, identification of biallelic pathogenic variants in LDLRAP1."
    explanation: Names the four genes that constitute the FH panel and their distinct molecular mechanisms.
  - reference: PMID:38393015
    reference_title: "ApoB100 and Atherosclerosis: What's New in the 21st Century?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "certain lipid-modifying agents may act via the LDL-R, which is typically normal in subjects with FDB"
    explanation: >-
      States the therapeutic consequence of resolving FDB from LDLR-mediated
      FH, and restates the receptor-normal premise of this entry.
- name: Clinical Lipid-Based Diagnosis and LDL-C Thresholds
  diagnosis_term:
    preferred_term: laboratory procedure
    term:
      id: NCIT:C25294
      label: Laboratory Procedure
  description: >
    A clinical FH diagnosis rests on characteristic features plus untreated
    LDL-C above threshold. These thresholds are calibrated on the FH population
    as a whole, which is dominated by LDLR-mediated disease.
  results: >
    Untreated LDL-C typically above 190 mg/dL in adults and above 160 mg/dL in
    children supports a clinical diagnosis of familial hypercholesterolemia.
  notes: >
    Recorded with an explicit caveat rather than as a clean diagnostic rule for
    FDB. Because LDL-C elevation in FDB is on average milder than in
    LDLR-mediated FH, thresholds calibrated on FH cohorts systematically miss
    part of the FDB carrier distribution - the same ascertainment problem
    curated as an open controversy in this entry. A normal or borderline LDL-C
    in a young carrier is likewise uninformative, since the biochemical
    phenotype is age-dependent.
  evidence:
  - reference: PMID:24404629
    reference_title: "Familial Hypercholesterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "A clinical diagnosis of FH can be established in a proband with characteristic clinical features and significantly elevated LDL-C levels (typically >190 mg/dL in adults and >160 mg/dL in children)."
    explanation: GeneReviews states the untreated LDL-C thresholds used for a clinical FH diagnosis.
  - reference: PMID:27919345
    reference_title: "Familial defective apolipoprotein B-100: A review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the elevation of plasma low-density lipoprotein cholesterol observed in FDB is frequently milder than that of FH due to mutations in LDLR, and FDB is subsequently underdiagnosed according to standard FH diagnostic criteria"
    explanation: >-
      Qualifies the thresholds for FDB specifically - they underdiagnose this
      disorder. PARTIAL because it limits rather than supports their use here.
- name: Differential Diagnosis of Inherited Hypercholesterolemia
  diagnosis_term:
    preferred_term: diagnostic procedure
    term:
      id: NCIT:C18020
      label: Diagnostic Procedure
  description: >
    FDB must be separated from the other monogenic hypercholesterolemias that
    present identically - LDLR-mediated FH (receptor defect), PCSK9
    gain-of-function FH (accelerated receptor degradation), and autosomal
    recessive LDLRAP1-related FH - as well as from polygenic
    hypercholesterolemia, elevated lipoprotein(a), and secondary dyslipidemias.
  results: >
    Resolution is molecular, not clinical: the disorders are not reliably
    distinguishable on lipid values or physical signs.
  notes: >
    A separate and more important discrimination for this entry runs in the
    opposite direction. APOB also harbours a truncating allelic series causing
    familial hypobetalipoproteinemia, which LOWERS apoB-containing lipoproteins
    and produces hepatic steatosis rather than premature atherosclerosis. It is
    the same gene with the opposite lipid direction, so APOB-keyed literature
    must be checked for which allelic series it describes before being applied
    to FDB. This entry covers only the receptor-binding-region missense series
    that raises LDL cholesterol.
  evidence:
  - reference: PMID:24404629
    reference_title: "Familial Hypercholesterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The molecular diagnosis of FH can be established by identification of heterozygous or biallelic pathogenic variants in APOB (variants that impair binding of LDL-C to the LDL receptor), LDLR, or PCSK9 (gain of function); or rarely, identification of biallelic pathogenic variants in LDLRAP1."
    explanation: Enumerates the monogenic FH differentials that molecular testing separates.
  - reference: PMID:38393015
    reference_title: "ApoB100 and Atherosclerosis: What's New in the 21st Century?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Familial hypobetalipoproteinemia (FHBL) is an autosomal codominant genetic disorder characterized by the diminished secretion of ApoB48 and ApoB100 lipoproteins due to defects in the ApoB gene."
    explanation: >-
      Establishes the opposite-direction APOB disorder that must not be
      conflated with FDB, in a source that describes both.
treatments:
- name: Statin (HMG-CoA Reductase Inhibitor) Therapy
  description: >
    First-line LDL-lowering therapy. The mechanistic rationale is specific to
    FDB: because the LDL receptor is normal, statin-induced hepatic LDL receptor
    upregulation still works. The extra receptor capacity clears the wild-type
    LDL pool and apoE-containing LDL precursors more efficiently even though the
    mutant LDL particles remain poor ligands, so LDL cholesterol falls.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: statin
      term:
        id: CHEBI:87631
        label: statin
  target_mechanisms:
  - target: Reduced Receptor-Mediated Hepatic LDL Clearance
    treatment_effect: ACTIVATES
    description: >
      Statins raise hepatic LDL receptor abundance, partially compensating for
      the ligand defect by increasing clearance of the receptor-competent
      lipoprotein pool.
    evidence:
    - reference: PMID:8141833
      reference_title: "Familial defective apolipoprotein B-100: a review, including some comparisons with familial hypercholesterolaemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "This may be due partly to increased receptor-mediated hepatic removal of mutant and normal precursors of LDL, using apo E as recognition element."
      explanation: States the proposed receptor-mediated mechanism by which receptor-inducing drugs work in FDB.
  evidence:
  - reference: PMID:8141833
    reference_title: "Familial defective apolipoprotein B-100: a review, including some comparisons with familial hypercholesterolaemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Most FDB heterozygotes respond well to drugs that lower plasma LDL levels by inducing receptor activity."
    explanation: States preserved responsiveness to LDL-receptor-inducing therapy in FDB.
  - reference: PMID:8215738
    reference_title: "Familial defective apolipoprotein B-100 is clinically indistinguishable from familial hypercholesterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Response to lipid-lowering therapy with beta-hydroxy-beta-methylglutaryl coenzyme A reductase inhibitors was similar to that reported in patients with familial hypercholesterolemia."
    explanation: Direct clinical observation that statin response in FDB matches that in FH.
  - reference: PMID:24404629
    reference_title: "Familial Hypercholesterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "statins can be used in children starting around age eight years"
    explanation: >-
      GeneReviews sets the paediatric initiation age for statins in FH.
      PARTIAL because the recommendation is made for FH as a class, not for
      APOB-related FH specifically.
  - reference: PMID:24404629
    reference_title: "Familial Hypercholesterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Statins are contraindicated in pregnancy because of concerns for teratogenicity and should be discontinued prior to conception."
    explanation: >-
      GeneReviews states the pregnancy contraindication carried in the notes
      above. PARTIAL because it constrains rather than supports the therapy,
      and is stated for FH as a class.
  notes: >
    Two management constraints carried from the GeneReviews FH chapter, both
    class-level rather than FDB-specific. Statins may be started in children
    from around age eight, which matters here because the biochemical phenotype
    in FDB is age-dependent and cascade-identified children may be
    normocholesterolemic at first testing. Statins are contraindicated in
    pregnancy and should be stopped before conception - a hard constraint on an
    otherwise lifelong therapy in a disorder that is transmitted to half of
    offspring.
- name: PCSK9 Monoclonal Antibody Therapy
  description: >
    Anti-PCSK9 monoclonal antibodies (alirocumab, evolocumab) block PCSK9-mediated
    degradation of the hepatic LDL receptor, increasing receptor recycling and
    surface abundance. The mechanistic rationale in FDB is the same one that makes
    statins work and is arguably stronger: the receptor is structurally normal, so
    every additional receptor molecule is fully functional. In a pooled genotyped
    analysis of six alirocumab trials, the 46 heterozygous APOB-defective patients
    lowered LDL cholesterol comparably to LDLR-mutation carriers.
  therapeutic_modality: MONOCLONAL_ANTIBODY
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: alirocumab
      term:
        id: NCIT:C174849
        label: Alirocumab
    - preferred_term: evolocumab
      term:
        id: NCIT:C174672
        label: Evolocumab
  target_mechanisms:
  - target: Reduced Receptor-Mediated Hepatic LDL Clearance
    treatment_effect: ACTIVATES
    description: >
      Raising hepatic LDL receptor abundance partially compensates for the ligand
      defect by increasing clearance of the receptor-competent lipoprotein pool.
    evidence:
    - reference: PMID:28964736
      reference_title: "Efficacy of alirocumab in 1191 patients with a wide spectrum of mutations in genes causative for familial hypercholesterolemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "54.1% (n = 20) and 50.1% (n = 6) in APOB-defective heterozygotes"
      explanation: Quantifies LDL cholesterol lowering specifically in APOB-defective (FDB) heterozygotes.
  notes: >
    The genotype-stratified APOB arm is small (46 patients, of whom 26 contributed
    the quoted Week 24 reductions), so this is an FDB-specific but low-precision
    estimate rather than a dedicated FDB trial. No retrieved trial enrolled FDB as
    a distinct molecular cohort.
  evidence:
  - reference: PMID:28964736
    reference_title: "Efficacy of alirocumab in 1191 patients with a wide spectrum of mutations in genes causative for familial hypercholesterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In this large patient cohort, individuals with a wide spectrum of mutations in genes underlying familial hypercholesterolemia responded substantially and similarly to alirocumab treatment."
    explanation: Supports comparable alirocumab efficacy across FH genotypes including APOB-defective carriers.
  - reference: PMID:28964736
    reference_title: "Efficacy of alirocumab in 1191 patients with a wide spectrum of mutations in genes causative for familial hypercholesterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "46 had a heterozygous APOB-defective mutation"
    explanation: Documents the size of the FDB-relevant genotype stratum, the basis for the precision caveat.
- name: Second-Line LDL-Lowering Therapy (Ezetimibe and Add-On Agents)
  description: >
    Where a statin alone does not bring LDL cholesterol to target, guidelines
    add further LDL-lowering agents, ezetimibe first among them. Ezetimibe
    inhibits intestinal NPC1L1-mediated cholesterol absorption, which lowers
    the hepatic cholesterol pool and, like a statin, drives compensatory
    hepatic LDL receptor upregulation - so its mechanistic rationale in FDB is
    the same receptor-side one, and it is likewise unaffected by the ligand
    defect.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: ezetimibe
      term:
        id: CHEBI:49040
        label: ezetimibe
  target_mechanisms:
  - target: Reduced Receptor-Mediated Hepatic LDL Clearance
    treatment_effect: ACTIVATES
    description: >
      Lowering the hepatic cholesterol pool upregulates LDL receptor
      expression, increasing clearance of the receptor-competent lipoprotein
      pool despite the persisting ligand defect. The mechanism is the same
      receptor-abundance lever exploited by statins and anti-PCSK9 antibodies.
    evidence:
    - reference: PMID:38393015
      reference_title: "ApoB100 and Atherosclerosis: What's New in the 21st Century?"
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "certain lipid-modifying agents may act via the LDL-R, which is typically normal in subjects with FDB"
      explanation: >-
        Supports the receptor-side rationale that makes add-on LDL-lowering
        agents mechanistically applicable in FDB. PARTIAL because it states the
        principle without naming ezetimibe or giving an FDB effect size.
  notes: >
    Carried with an explicit extrapolation caveat, as the deep-research report
    itself recommends. No retrieved study reports ezetimibe (or bempedoic acid,
    inclisiran, or LDL apheresis) outcomes in a molecularly defined FDB cohort;
    the recommendation is drawn from mixed-genotype FH management, in which
    APOB carriers are a minority. It is recorded rather than omitted because
    silence would misrepresent standard care, and because the receptor-side
    mechanism gives a specific reason to expect it to work here - but no
    FDB-specific efficacy is asserted, and no FDB-specific effect size is
    curated. This is a weaker evidential footing than the statin and anti-PCSK9
    entries above, both of which carry genotype-stratified data.
  evidence:
  - reference: PMID:24404629
    reference_title: "Familial Hypercholesterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Adults: pharmacotherapy (statins with additional medications as needed) to reduce lipid levels"
    explanation: >-
      GeneReviews establishes add-on pharmacotherapy beyond statins as standard
      FH management. PARTIAL because it is class-level FH guidance, not
      FDB-specific evidence.
- name: Cardiovascular Risk Factor Modification and Lipid Surveillance
  description: >
    Because FDB is a lifelong LDL-exposure disorder whose clinical endpoint is
    atherosclerosis, non-pharmacological management targets the modifiable
    determinants of that endpoint - smoking, saturated and trans fat intake,
    physical inactivity, obesity, hypertension and diabetes - alongside lipid
    monitoring from early childhood. Cascade evaluation of at-risk relatives
    belongs to the same arm and is especially consequential here, since half of
    a carrier's children inherit the allele and clinical criteria calibrated on
    LDLR-mediated FH will miss part of the FDB distribution.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: behavioral counseling
    term:
      id: NCIT:C181743
      label: Behavioral Counseling
  notes: >
    Class-level FH management from the GeneReviews chapter, applied here on the
    grounds of shared cumulative LDL burden rather than FDB-specific trial
    evidence. Recorded as BEHAVIORAL because the intervention is lifestyle and
    risk-factor counselling; the surveillance component is a monitoring
    schedule rather than a therapy, and is kept here rather than duplicated
    into the diagnosis section.
  evidence:
  - reference: PMID:24404629
    reference_title: "Familial Hypercholesterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Agents/circumstances to avoid: Smoking, high intake of saturated and trans unsaturated fat, sedentary lifestyle, obesity, hypertension, and diabetes mellitus."
    explanation: >-
      GeneReviews enumerates the modifiable exposures to avoid in FH. PARTIAL
      because the guidance is class-level rather than FDB-specific.
  - reference: PMID:24404629
    reference_title: "Familial Hypercholesterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Monitor lipid levels from age two years"
    explanation: >-
      GeneReviews sets the surveillance schedule. PARTIAL for the same
      class-level reason; it is nonetheless pointed for FDB, where carriers can
      be normocholesterolemic when first tested.
  - reference: PMID:24404629
    reference_title: "Familial Hypercholesterolemia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Early diagnosis and treatment of first-degree and second-degree relatives at risk for FH can reduce morbidity and mortality."
    explanation: >-
      GeneReviews supports cascade evaluation of at-risk relatives. PARTIAL
      because the benefit is established for FH as a class.
discussions:
- discussion_id: controversy_fdb_severity_versus_ldlr_fh
  prompt: >
    Is the FDB phenotype genuinely milder than LDLR-mediated familial
    hypercholesterolemia, or does it only appear milder because the two
    disorders have been compared in cohorts ascertained through clinical FH
    criteria that FDB carriers with lower LDL cholesterol fail to meet?
  kind: CONTROVERSY
  status: OPEN
  attaches_to:
  - pathophysiology#Elevated Plasma LDL Cholesterol
  - phenotypes#Hypercholesterolemia
  rationale: >
    Two directly relevant studies disagree. A 1993 Dutch/Canadian series of 18
    molecularly confirmed FDB patients concluded the disorder was clinically
    indistinguishable from FH in physical characteristics and lipoprotein
    measures - but every one of those 18 had been ascertained through a clinical
    FH diagnosis, which selects for the high-cholesterol tail of FDB. A 1995
    multivariate comparison of 28 FDB against 129 FH heterozygotes found
    significantly lower total cholesterol, LDL cholesterol and triglycerides and
    higher HDL cholesterol in FDB, and explicitly noted that only some FDB
    subjects meet FH criteria. The 2016 review sides with the milder reading and
    draws the practical consequence, that FDB is underdiagnosed by standard FH
    criteria. The disagreement is not resolvable from these data because the
    ascertainment differs, so the entry curates the milder reading as the
    prevailing view while retaining the contradicting evidence rather than
    quietly dropping it.
  evidence:
  - reference: PMID:8215738
    reference_title: "Familial defective apolipoprotein B-100 is clinically indistinguishable from familial hypercholesterolemia."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "The disorder was clinically indistinguishable from familial hypercholesterolemia in terms of physical characteristics and lipoprotein measures."
    explanation: Refutes the milder-phenotype claim, in a cohort ascertained via clinical FH diagnosis.
  - reference: PMID:7583549
    reference_title: "Differences in the phenotypic characteristics of subjects with familial defective apolipoprotein B-100 and familial hypercholesterolemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "these results demonstrate that FDB subjects tend to have a milder form of hyperlipoproteinemia than FH subjects and that only a part of the subjects with FDB fulfill the established criteria for identifying FH"
    explanation: Supports the milder-phenotype claim and names the ascertainment bias that would hide it.
  - reference: PMID:27919345
    reference_title: "Familial defective apolipoprotein B-100: A review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the elevation of plasma low-density lipoprotein cholesterol observed in FDB is frequently milder than that of FH due to mutations in LDLR, and FDB is subsequently underdiagnosed according to standard FH diagnostic criteria"
    explanation: Contemporary review statement of the milder phenotype and its diagnostic consequence.
  proposed_experiments:
  - experiment_id: exp_fdb_genotype_first_severity_comparison
    name: Genotype-first comparison of FDB and LDLR-FH lipid phenotypes in an unselected biobank
    description: >
      Identify APOB p.Arg3527Gln and pathogenic LDLR carriers by sequence alone
      in a population biobank with linked lipid measurements, without any
      clinical FH ascertainment, and compare LDL cholesterol distributions and
      incident ASCVD. A genotype-first design removes the criteria-based
      selection that confounds every clinic-derived comparison to date.
- discussion_id: gap_vldl_kinetic_arm_of_fdb
  prompt: >
    Does FDB alter apoB-100 lipoprotein production and the VLDL-to-IDL-to-LDL
    delipidation cascade, or is it purely a clearance defect at the LDL step?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#Reduced Receptor-Mediated Hepatic LDL Clearance
  rationale: >
    The disorder is modeled here as a clearance defect, which the receptor-binding
    data strongly support. But a stable-isotope kinetic study found VLDL apoB
    production rates in FDB intermediate between FH and controls with large
    interindividual variability, and IDL apoB production reduced relative to
    controls - so the precursor arm is not simply normal. With only six FDB
    subjects studied, the finding cannot yet support a curated production-side
    mechanism node, and none is asserted.
  evidence:
  - reference: PMID:9507998
    reference_title: "VLDL and IDL apolipoprotein B-100 kinetics in familial hypercholesterolemia due to impaired LDL receptor function or to defective apolipoprotein B-100."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "VLDL APR in FDB were between those of FH and controls (24.3+/-4.8 mg/kg/day), and demonstrated a relatively large inter-individual variability."
    explanation: Shows an unresolved production-side signal in FDB, underdetermined at this sample size.
  - reference: PMID:9507998
    reference_title: "VLDL and IDL apolipoprotein B-100 kinetics in familial hypercholesterolemia due to impaired LDL receptor function or to defective apolipoprotein B-100."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "VLDL and IDL kinetics differ when LDL concentrations are elevated either due to a LDL receptor defect or due to defective apolipoprotein B-100."
    explanation: States that the precursor kinetics genuinely differ between the receptor-side and ligand-side disorders.
  proposed_experiments:
  - experiment_id: exp_fdb_apob_kinetics_powered
    name: Adequately powered stable-isotope apoB kinetic study in genotyped FDB
    description: >
      Repeat the primed-constant [1-13C]leucine infusion design in a larger,
      genotype-confirmed FDB cohort with matched LDLR-FH and normolipidemic
      controls, powered to resolve VLDL and IDL apoB production and fractional
      catabolic rates. This would settle whether a production-side node belongs
      in the FDB mechanism graph.
📚

References & Deep Research

References

1
PMID:24404629
Familial Hypercholesterolemia.
4 findings
Clinical characteristics. GeneReviews describes the FH clinical spectrum that APOB-related FH shares - premature cardiovascular events including angina and myocardial infarction, tendon xanthomas, xanthelasmas around the eyelids, and early corneal arcus. The chapter covers FH as a class across LDLR, APOB and PCSK9, so it is class-level rather than FDB-specific evidence and is cited as such throughout this entry.
"increases the risk of premature cardiovascular events such as angina and myocardial infarction"
Diagnosis and testing. GeneReviews gives the untreated LDL-C thresholds for a clinical FH diagnosis and names APOB variants that impair binding of LDL-C to the LDL receptor as one of three molecular routes to FH - the ligand-side route that defines this entry.
"The molecular diagnosis of FH can be established by identification of heterozygous or biallelic pathogenic variants in APOB (variants that impair binding of LDL-C to the LDL receptor), LDLR, or PCSK9 (gain of function); or rarely, identification of biallelic pathogenic variants in LDLRAP1."
Management. GeneReviews sets out lipid surveillance from age two years, statin initiation in children from around age eight, cardiovascular risk-factor modification, and the contraindication of statins in pregnancy.
"Monitor lipid levels from age two years"
Genetic counseling. GeneReviews states the autosomal dominant transmission risk that applies directly to APOB-related FH, and the value of cascade evaluation of at-risk relatives.
"Each child of an individual with a heterozygous pathogenic variant in APOB, LDLR, or PCSK9 has a 50% chance of inheriting the pathogenic variant and having FH."

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Familial Defective Apolipoprotein B-100: Disease-Characteristics Report
Edison Scientific Literature 21 citations 2026-08-18T21:10:37.983834

Familial Defective Apolipoprotein B-100: Disease-Characteristics Report

Executive summary and evidence boundaries

Familial defective apolipoprotein B-100 (FDB) is a Mendelian, usually autosomal-dominant/codominant hypercholesterolemia caused by ligand-defective APOB. The prototypic allele is APOB c.10580G>A, p.Arg3527Gln, historically called R3500Q; p.Arg3527Trp/R3500W is another established allele. Defective apoB-100 binding to hepatic LDL receptor (LDLR) reduces LDL clearance, causing lifelong elevation of LDL cholesterol (LDL-C) and cumulative atherosclerotic cardiovascular risk. FDB is now commonly classified as APOB-related familial hypercholesterolemia (FH) rather than managed as a separate clinical syndrome. On average it is milder and less penetrant than LDLR-related FH, but coronary disease and myocardial infarction can still occur prematurely. (kounatidis2024apob100andatherosclerosis pages 13-14, kounatidis2024apob100andatherosclerosis pages 11-13)

Evidence below is labeled implicitly by scope: statements about the apoB-binding defect and comparative FDB cohorts are FDB-specific human evidence; management targets, diagnostic criteria, and many risk modifiers are general FH evidence extrapolated to molecularly confirmed FDB. Recent disease-specific evidence is limited; the most directly relevant recent synthesis is Kounatidis et al., published February 2024, DOI 10.3390/metabo14020123. (kounatidis2024apob100andatherosclerosis pages 13-14)

The following compact table summarizes knowledge-base-ready assertions and their principal caveats.

domain curated finding suggested ontology/identifier evidence scope/caveat
Disease identity Familial defective apolipoprotein B-100 (FDB) is an inherited ligand-defective APOB disorder characterized by hypercholesterolemia and premature atherosclerosis; often treated clinically within the APOB-related familial hypercholesterolemia spectrum. Disease label: Familial defective apolipoprotein B-100; consider MONDO/Orphanet/OMIM cross-map, verify current release Disease-specific review supports identity, but many modern sources subsume FDB under broader familial hypercholesterolemia (FH) rather than list it separately (kounatidis2024apob100andatherosclerosis pages 11-13, kounatidis2024apob100andatherosclerosis pages 13-14, haradashiba2023guidelinesforthe pages 2-4)
Synonyms / naming Common names include FDB, familial defective apoB-100, APOB-related familial hypercholesterolemia, ligand-defective apoB hypercholesterolemia. Legacy residue numbering uses R3500Q/R3500W; current full-length APOB numbering uses p.Arg3527Gln / p.Arg3527Trp. APOB gene (HGNC: APOB label; verify HGNC ID/current transcript) Important curation issue: legacy codon 3500 and current codon 3527 refer to the same canonical disease hotspot under different numbering systems (siddiqi2026familialhypercholesterolemiastateoftheart pages 3-4, haradashiba2023guidelinesforthe pages 24-25)
Causal gene / variant hotspot Core causal gene is APOB; the best-established pathogenic variant is p.Arg3527Gln (legacy R3500Q). p.Arg3527Trp (legacy R3500W) is the second most common FDB mutation; additional nearby APOB exon 26 variants are reported. Gene: APOB; variant labels: p.Arg3527Gln, p.Arg3527Trp; ClinVar/ClinGen/HGVS mapping recommended Disease-specific review reports ~35 FH-causing APOB variants overall; exact pathogenicity for rarer nearby variants should be checked in ClinVar/ClinGen (kounatidis2024apob100andatherosclerosis pages 13-14, siddiqi2026familialhypercholesterolemiastateoftheart pages 3-4)
Inheritance Usually autosomal codominant / autosomal dominant in clinical practice, with dose effect between heterozygous and rare homozygous states. HPO inheritance term: Autosomal dominant inheritance / codominant qualifier, verify preferred ontology usage Source wording varies: recent apoB review uses “autosomal codominant,” while FH guidelines often simplify dominant FH inheritance (kounatidis2024apob100andatherosclerosis pages 11-13, haradashiba2023guidelinesforthe pages 2-4)
Core laboratory phenotype Lifelong elevation of LDL-C is the hallmark; the R3500Q variant increases serum LDL-C by about 60–70 mg/dL. ApoB-containing LDL particles accumulate because hepatic clearance is impaired. HPO labels: Hypercholesterolemia, Increased LDL cholesterol level; LOINC/NCIT lipid panel terms, verify current release Quantitative increase is variant-specific and may be modified by background genetics and environment (kounatidis2024apob100andatherosclerosis pages 13-14, kounatidis2024apob100andatherosclerosis pages 11-13, siddiqi2026familialhypercholesterolemiastateoftheart pages 3-4)
Clinical phenotype May present with hyper-LDL cholesterolemia, premature coronary disease, and sometimes xanthomas/coronary artery calcification, but phenotype is often milder than classic LDLR-mediated FH. HPO labels: Premature coronary artery disease, Coronary artery calcification, Tendon xanthoma, Myocardial infarction Much of the visible-xanthoma literature comes from broader FH cohorts; FDB-specific severity is generally lower/variable (kounatidis2024apob100andatherosclerosis pages 13-14, haradashiba2023guidelinesforthe pages 8-11)
Complications FDB is associated with coronary artery disease, ischemic heart disease, myocardial infarction, and coronary artery calcification. Comparative data suggest lower CHD prevalence and later onset than classic FH: 5.6% CHD at median age 52 in FDB vs 40% at mean age 41 in FH; carotid stenosis 4% vs 15%. HPO labels: Coronary artery disease, Myocardial infarction, Carotid artery stenosis These comparative risk figures are disease-informative but derive from FDB-vs-FH comparison rather than population prevalence estimates (kounatidis2024apob100andatherosclerosis pages 13-14)
Molecular mechanism ApoB100 normally serves as an LDL receptor ligand. Pathogenic APOB hotspot variants reduce apoB100 binding affinity to LDLR, lowering LDL uptake/clearance by hepatocytes and increasing circulating LDL; excess LDL is retained/oxidized in arterial intima, triggering foam-cell inflammation and plaque formation. GO labels: low-density lipoprotein particle receptor binding, receptor-mediated endocytosis, cholesterol homeostasis, foam cell differentiation, inflammatory response; Reactome/KEGG LDL metabolism pathways, verify current release Upstream defect is FDB-specific (apoB-LDLR binding); downstream atherogenesis steps are general apoB/LDL biology extrapolated from atherosclerosis literature (kounatidis2024apob100andatherosclerosis pages 11-13, kounatidis2024apob100andatherosclerosis pages 4-5, kounatidis2024apob100andatherosclerosis pages 5-7)
Anatomy / cells Primary organs/systems: liver and arterial vasculature; target tissues include arterial intima and atherosclerotic plaque. Key cells: hepatocytes, endothelial cells, monocytes/macrophages, vascular smooth muscle cells, CD4+ T cells. UBERON labels: liver, blood vessel, carotid artery, coronary artery; CL labels: hepatocyte, endothelial cell, macrophage, smooth muscle cell, CD4-positive T cell; GO cellular components/pathways verify current release Cell-level cascade is derived mainly from apoB atherosclerosis biology rather than FDB-only experiments (kounatidis2024apob100andatherosclerosis pages 4-5, kounatidis2024apob100andatherosclerosis pages 5-7)
Diagnosis Suspect in patients with elevated untreated LDL-C and family history of premature CAD/FH; distinguish from LDLR-mediated FH and secondary dyslipidemia. Genetic testing for APOB variants is confirmatory. FH diagnostic frameworks use untreated LDL-C thresholds (e.g., ≥180 mg/dL in Japanese adult criteria) plus xanthomas/family history, and recommend cascade screening. Disease/gene testing: APOB sequencing or FH multigene panel (APOB, LDLR, PCSK9, LDLRAP1); clinical criteria labels: Dutch Lipid Clinic Network, Simon Broome, MEDPED, JAS adult FH criteria No FDB-specific modern standalone diagnostic criteria were identified; current practice usually diagnoses under FH algorithms and then subtypes genetically (haradashiba2023guidelinesforthe pages 8-11, haradashiba2023guidelinesforthe pages 2-4, haradashiba2023guidelinesforthe pages 24-25)
Differential diagnosis Main differential diagnoses are LDLR-related FH, PCSK9-related FH, LDLRAP1-related hypercholesterolemia, polygenic hypercholesterolemia, elevated Lp(a)-driven LDL-C signal, and secondary dyslipidemias. Disease labels as above; consider Lp(a), sitosterolemia, hypothyroidism, nephrotic syndrome, cholestatic liver disease, verify coding system Differential structure comes mostly from general FH guidance, not FDB-specific cohorts (siddiqi2026familialhypercholesterolemiastateoftheart pages 1-2, faiz2012molecularpathologyof pages 4-5, haradashiba2023guidelinesforthe pages 8-11)
Treatment Managed similarly to heterozygous FH: lifestyle intervention, high-intensity statin first line, usually plus ezetimibe; consider PCSK9 inhibitors, inclisiran, bempedoic acid, and lipoprotein apheresis if targets are not reached. General FH targets include ≥50% LDL-C reduction and LDL-C <70 mg/dL for primary prevention or <55 mg/dL for very-high-risk patients. NCIT labels: statin therapy, ezetimibe, PCSK9 inhibitor therapy, inclisiran, bempedoic acid, lipoprotein apheresis; CHEBI/DrugBank mapping verify current release Evidence is mostly general FH guidance; a caveat for FDB is that LDLR is structurally normal, so response patterns may differ from LDLR-negative FH, but modern management still follows FH algorithms (fularski2024unveilingfamilialhypercholesterolemia—review pages 7-9, siddiqi2026familialhypercholesterolemiastateoftheart pages 1-2, kounatidis2024apob100andatherosclerosis pages 13-14)
Prevention / screening Cascade screening of relatives is recommended after identifying an index case; early diagnosis reduces cumulative LDL exposure and premature CAD risk. Lifestyle measures include Mediterranean-style diet and aerobic exercise, which can lower apoB-related risk markers. Public health labels: cascade screening; HPO/family history terms; behavioral intervention terms verify current release Screening and prevention evidence is mainly general FH implementation evidence, applicable to APOB-FDB because of shared inherited LDL burden (haradashiba2023guidelinesforthe pages 2-4, kounatidis2024apob100andatherosclerosis pages 13-14)
Prognosis Untreated FDB increases atherosclerotic risk, but available comparative evidence suggests lower severity than classic FH on average. Expressivity is variable and can be worsened by additional genetic hits or conventional cardiovascular risk factors. Prognostic feature labels: premature CAD risk, variable expressivity, incomplete penetrance Specific long-term survival estimates for pure FDB were not identified in retrieved recent sources (kounatidis2024apob100andatherosclerosis pages 13-14, siddiqi2026familialhypercholesterolemiastateoftheart pages 3-4, kamar2021thedigeniccausality pages 14-15)
Modifiers Phenotype may be modified by smoking, diabetes, hypertension, low HDL-C, high TG, obesity/insulin resistance, and other lipid genes or digenic states. HPO/ExO/environmental exposure labels; modifier genes to verify per case Most modifier evidence is from broader FH rather than FDB-only cohorts; use cautiously when curating disease-specific assertions (haradashiba2023guidelinesforthe pages 8-11, kamar2021thedigeniccausality pages 14-15)
Models / other species Relevant experimental systems include human APOB100 transgenic or knock-in atherosclerosis models and pathway models such as Ldlr-/- or Apoe-based mice, but these are not exact allele-faithful FDB models. No naturally occurring veterinary FDB equivalent was identified in retrieved sources. Model labels: human APOB100 transgenic mouse; LDLR knockout mouse; APOE-related atherosclerosis mouse, verify model database accession Useful for mechanism and drug testing, but limited for direct genotype-phenotype recapitulation of APOB p.Arg3527Gln FDB (zadelaar2007mousemodelsfor pages 3-4)
Evidence gaps Uncertain/needs verification: exact MONDO/Orphanet/OMIM mapping for FDB as a distinct entity, current ClinVar classifications for all rare APOB hotspot-adjacent variants, precise prevalence/incidence of pure FDB by ancestry, penetrance estimates, QoL data, and disease-specific animal models. Use ontology labels only and verify current release before database loading Modern literature often collapses FDB into APOB-related FH, so separate disease-level curation requires careful source labeling (kounatidis2024apob100andatherosclerosis pages 13-14, siddiqi2026familialhypercholesterolemiastateoftheart pages 1-2, haradashiba2023guidelinesforthe pages 24-25)

Table: This table provides a compact curation-oriented summary of Familial Defective Apolipoprotein B-100, covering disease identity, variant hotspot, phenotype, mechanism, diagnosis, treatment, anatomy, and key evidence gaps. It is designed to support structured knowledge-base entry building while flagging where broader FH evidence is being extrapolated to FDB.

1. Disease information

Definition

FDB is an inherited disorder in which apoB-100 on LDL has reduced affinity for LDLR. The resulting under-clearance of LDL produces hypercholesterolemia and premature atherosclerosis. A recent review defines it as an “inherited autosomal codominant disorder characterized by hypercholesterolemia and premature atherosclerosis.” (kounatidis2024apob100andatherosclerosis pages 11-13)

Identifiers and synonyms

  • Preferred name: familial defective apolipoprotein B-100.
  • Synonyms: familial defective apoB-100; FDB; ligand-defective apolipoprotein B; APOB-related familial hypercholesterolemia; familial hypercholesterolemia type 2 due to APOB.
  • OMIM: commonly mapped to 144010, Hypercholesterolemia, familial, 2 (FHCL2); APOB gene OMIM 107730. Database release should be verified before ingestion.
  • Orphanet: commonly represented within familial hypercholesterolemia rather than consistently as a separate current disorder; verify the live Orphanet release.
  • MONDO: use the current MONDO entry for familial hypercholesterolemia due to ligand-defective apolipoprotein B if present; exact live identifier could not be independently verified from the retrieved corpus.
  • ICD-10-CM: E78.01 Familial hypercholesterolemia; no FDB-specific code.
  • ICD-11: use the familial hypercholesterolaemia entity under disorders of lipoprotein metabolism; no allele-specific FDB code was established here.
  • MeSH: Hyperlipoproteinemia Type II and Familial Hypercholesterolemia are appropriate indexing concepts; no reliably verified standalone current FDB MeSH descriptor was found.

These are aggregated disease-level assertions from reviews and guidelines, not individual EHR observations. Patient-level values cited below come from published cohorts; no identifiable patient records were accessed.

2. Etiology, risk, protection, and gene–environment interaction

Primary cause

The causal lesion is a germline APOB variant affecting the LDLR-binding region of apoB-100. The best-established variants are p.Arg3527Gln and p.Arg3527Trp. Older papers number these as Arg3500 because of historical protein-numbering conventions. The 2024 review reports that R3500Q raises LDL-C by approximately 60–70 mg/dL and that approximately 35 FH-associated APOB variants had been described, although every rare APOB missense variant requires contemporary classification and preferably functional validation. (kounatidis2024apob100andatherosclerosis pages 13-14)

Genetic risk and modifiers

Risk is greatest with biallelic/dose-enhanced disease, additional pathogenic variants in LDL-pathway genes, and a high polygenic LDL-C burden. Candidate modifiers include LDLR, PCSK9, APOE, LDLRAP1, ABCG5, and ABCG8. Digenic FH literature shows substantial intrafamilial variability and warns that computational prediction alone is insufficient; cosegregation, population frequency, and functional LDLR-binding/uptake evidence remain important. (haradashiba2023guidelinesforthe pages 8-11, kamar2021thedigeniccausality pages 14-15)

Possible protective modifiers include LDL-lowering PCSK9 loss-of-function alleles and APOB truncating alleles, but an APOB truncation causes a biologically distinct hypobetalipoproteinemia phenotype and should not be treated as a protective FDB allele. PCSK9 knockout mice showed an approximately 80% LDL-C decrease, illustrating the pathway rather than proving an FDB-specific human modifier effect. (kamar2021thedigeniccausality pages 14-15)

Environmental and lifestyle modifiers

The pathogenic variant is sufficient to predispose to disease, but cumulative LDL exposure and clinical events are modified by diet, smoking, physical activity, obesity/visceral adiposity, diabetes, hypertension, high triglycerides, and low HDL-C. These associations are best established in broader FH cohorts. A Western diet rich in saturated fat can impede LDL apoB-100 clearance; Mediterranean-style eating enhances LDL catabolism, while aerobic training modestly lowers apoB. In a meta-analysis of 57 trials involving 3,194 participants, aerobic exercise lowered apoB-100 by about 2.073 mg/dL; this was not an FDB-only analysis. (kounatidis2024apob100andatherosclerosis pages 13-14, haradashiba2023guidelinesforthe pages 8-11)

There is no infectious, toxic, occupational, or radiation cause. Such exposures may affect background cardiovascular risk but do not cause the Mendelian disorder.

3. Phenotypes

Phenotype Type, onset/course, frequency and severity Suggested HPO annotation
Elevated LDL-C/hypercholesterolemia Laboratory abnormality present from early life; chronic and lifelong without treatment. R3500Q adds roughly 60–70 mg/dL, with variable penetrance. Hypercholesterolemia; Increased LDL cholesterol level—verify current HPO IDs
Premature atherosclerosis/CAD Progressive complication of cumulative LDL exposure; adult-onset is usual in heterozygous FDB, but timing varies. Premature coronary artery disease; Atherosclerosis
Coronary calcification Imaging sign; both mild and severe calcification are reported. Coronary artery calcification
Myocardial infarction/ischemic heart disease Episodic acute complication of chronic plaque disease; potentially fatal. Myocardial infarction; Ischemic heart disease
Tendon or cutaneous xanthomas Physical sign caused by cholesterol deposition; possible but apparently less frequent than in severe LDLR-FH. Tendon xanthoma; Xanthoma
Corneal arcus/xanthelasma Physical signs recognized in FH; FDB-specific frequencies are unavailable. Corneal arcus; Xanthelasma
Carotid stenosis Vascular imaging sign. One comparison found 4% in FDB versus 15% in classic FH. Carotid artery stenosis

In one comparative study summarized in 2024, CHD was present in 5.6% of FDB subjects at median age 52, versus nearly 40% of FH patients at mean age 41; carotid stenosis occurred in 4% versus 15%. These figures should not be interpreted as lifetime penetrance because age distributions and ascertainment differed. (kounatidis2024apob100andatherosclerosis pages 13-14)

Hyper-LDL-cholesterolemia itself is generally asymptomatic. Quality-of-life loss arises mainly from diagnostic burden, lifelong medication, cardiovascular procedures, angina, infarction, and anxiety about relatives. No validated FDB-specific EQ-5D, SF-36, or PROMIS estimates were identified.

4. Genetic and molecular information

  • Gene: APOB, chromosome 2p24.1; protein apolipoprotein B-100. HGNC and transcript identifiers should be pulled from the current HGNC/NCBI release during production curation.
  • Primary variant: NM_000384.3:c.10580G>A, p.Arg3527Gln, legacy R3500Q.
  • Additional established allele: p.Arg3527Trp, legacy R3500W.
  • Other reported ligand-defective alleles: nearby residues such as p.Arg3558Cys/legacy R3531C and other APOB missense variants have been reported, but pathogenicity is not uniform and must be checked variant by variant.
  • Origin/type: germline, usually heterozygous missense; rare homozygotes occur. This is not a somatic cancer disorder.
  • Functional class: partial loss of ligand function—reduced LDLR binding—rather than loss of apoB production. It is neither a classical null allele nor a demonstrated dominant-negative protein in the usual sense.
  • Classification: p.Arg3527Gln is an established pathogenic FH allele. Current ClinVar assertions, review status, gnomAD frequency, and ancestry-stratified counts should be imported directly from live databases; exact values were not available in the retrieved papers.
  • Structural abnormalities: no recurrent aneuploidy, translocation, inversion, or copy-number lesion defines FDB. CMA, karyotyping, and FISH therefore have no routine role.
  • Epigenetics: no reproducible disease-defining methylation, histone, or chromatin signature has been established. Epigenetic studies of atherosclerosis are downstream and not diagnostic of FDB.

APOB variants were estimated in the retrieved reviews to account for approximately 5–12% of autosomal-dominant FH, depending strongly on population and ascertainment. This is not the prevalence of FDB in the general population. (kounatidis2024apob100andatherosclerosis pages 13-14, siddiqi2026familialhypercholesterolemiastateoftheart pages 3-4)

5. Environmental information

No environmental exposure is necessary or sufficient to produce FDB. Environment modifies biochemical expression and ASCVD progression:

  • Adverse: saturated/trans-fat-rich diet, smoking, inactivity, obesity, insulin resistance/diabetes, and hypertension.
  • Protective: Mediterranean-style dietary pattern, replacement of saturated with unsaturated fats, regular aerobic activity, avoidance of tobacco, and control of blood pressure, weight, and diabetes.
  • Alcohol: no FDB-specific protective effect should be inferred; alcohol is not recommended for prevention.
  • Infection: not etiologic. Inflammation contributes to downstream plaque biology, but FDB is not infectious.

6. Mechanism and pathophysiology

Causal chain

  1. Upstream genetic trigger: ligand-region APOB missense variant.
  2. Protein dysfunction: apoB-100 on LDL binds LDLR with reduced affinity.
  3. Cellular defect: receptor-mediated uptake by hepatocytes falls despite a generally intact LDLR.
  4. Metabolic abnormality: LDL residence time and plasma LDL particle concentration rise.
  5. Arterial initiation: apoB-containing particles cross and are retained in arterial intima, then undergo oxidation and other modifications.
  6. Inflammatory amplification: endothelial VCAM1/ICAM1/E-selectin expression recruits monocytes; macrophage scavenger receptors such as CD36 and LOX-1 internalize oxidized LDL, generating foam cells. TLR, NF-κB, p38-MAPK/JAK-STAT, ROS, and NLRP3–IL-1 signaling contribute.
  7. Tissue injury: foam-cell death, defective efferocytosis, smooth-muscle activation, extracellular-matrix deposition, necrotic core formation, calcification, and fibrous-cap remodeling produce stenotic or rupture-prone plaque.
  8. Clinical endpoints: stable angina, coronary calcification/stenosis, plaque rupture, thrombosis, acute myocardial infarction, and premature death. (kounatidis2024apob100andatherosclerosis pages 4-5, kounatidis2024apob100andatherosclerosis pages 5-7, kounatidis2024apob100andatherosclerosis pages 11-13)

ApoB-100 is the LDLR ligand, and each VLDL, IDL, LDL, and Lp(a) particle contains one apoB molecule; thus apoB concentration approximates atherogenic particle number. More than 90% of circulating apoB-containing particles are LDL in many settings. (kounatidis2024apob100andatherosclerosis pages 5-7)

Suggested GO biological processes: LDL particle receptor binding; receptor-mediated endocytosis; plasma lipoprotein clearance; cholesterol homeostasis; lipid transport; response to oxidized LDL; macrophage-derived foam-cell differentiation; leukocyte adhesion; inflammatory response; regulation of NLRP3 inflammasome complex assembly.

Suggested GO cellular components: LDL particle; VLDL particle; plasma membrane; clathrin-coated pit; endosome; extracellular space.

Suggested CL terms: hepatocyte; vascular endothelial cell; classical monocyte; macrophage; foam cell; vascular smooth-muscle cell; dendritic cell; CD4-positive T cell; regulatory T cell. Exact IDs should be verified against current GO/CL releases.

No FDB-specific single-cell atlas, spatial transcriptomic map, diagnostic transcriptomic/proteomic signature, or integrated multi-omics classifier was identified. Modern single-cell atherosclerosis studies describe downstream plaque heterogeneity, not an FDB-specific state.

7. Anatomical structures affected

  • Primary metabolic organ: liver—especially hepatocytes responsible for LDLR-mediated clearance. Suggested UBERON: liver; liver lobule.
  • Primary injured system: arterial vasculature, particularly coronary arteries and aorta; carotid and peripheral arteries can also be involved. Suggested UBERON: artery; coronary artery; aorta; carotid artery; arterial wall/tunica intima.
  • Secondary tissues: Achilles and extensor tendons, skin, and corneal periphery when lipid deposits form.
  • Subcellular sites: hepatocyte plasma membrane, clathrin-coated pits and endosomes for LDLR uptake; extracellular/subendothelial matrix for LDL retention; macrophage lysosomal/lipid-droplet compartments during foam-cell formation.
  • Lateralization: not applicable; vascular disease can be diffuse, bilateral, or anatomically asymmetric according to plaque burden.

8. Temporal development

The genotype is congenital and LDL-C elevation begins early, although clinical symptoms are usually absent for years. Disease is chronic, lifelong, and insidiously progressive. A useful staging model is:

  1. biochemical hypercholesterolemia;
  2. subclinical arterial retention, increased intima-media thickness, or coronary calcium;
  3. clinically stable atherosclerotic disease;
  4. acute plaque complication—ACS/MI—or diffuse advanced vascular disease.

There is no spontaneous genetic remission. LDL-C can normalize or substantially improve with treatment, but established plaque may persist. Earlier statin initiation is associated with less carotid intima-media thickening in pediatric FH, supporting childhood as a critical prevention window. (haradashiba2023guidelinesforthe pages 2-4, haradashiba2023guidelinesforthe pages 24-25)

9. Inheritance and population

Inheritance

FDB is generally autosomal dominant/codominant. Each child of a heterozygous affected individual has a 50% transmission risk. Penetrance for the biochemical phenotype is incomplete/variable and age-dependent for clinical ASCVD. Expressivity varies with allele dosage, polygenic background, lifestyle, sex, and conventional risk factors. Genetic anticipation is not established; consanguinity is not required, although it raises the chance of biallelic disease. Germline mosaicism is not a recognized major feature.

Epidemiology

A reliable global prevalence or incidence for molecularly pure FDB is unavailable. Historical European estimates vary because of founder effects and because modern FH cohorts combine APOB and LDLR disease. APOB alleles account for approximately 5–12% of genetically defined autosomal-dominant FH in some series. General heterozygous FH prevalence is about 1:300 in the 2023 Japanese guideline, but applying that figure directly to FDB would be incorrect. (haradashiba2023guidelinesforthe pages 2-4)

p.Arg3527Gln is enriched in populations of northwestern/central European ancestry and in founder-derived groups. p.Arg3527Trp has been described more often in East Asian families. Both sexes inherit the allele equally; observed event rates may be earlier in men because ASCVD penetrance is sex- and age-dependent. Precise carrier frequencies should be sourced from live gnomAD ancestry data.

10. Diagnostics

Clinical and laboratory assessment

Obtain a fasting or nonfasting lipid profile, apoB, non-HDL-C, triglycerides, HDL-C, and Lp(a), and document the highest untreated LDL-C. Examine Achilles/extensor tendons and skin; obtain a three-generation history of hypercholesterolemia and premature CAD. The 2023 Japanese guideline diagnoses adult FH using at least two of: untreated LDL-C ≥180 mg/dL; tendon/cutaneous nodular xanthoma; first-degree family history of FH or premature CAD. Premature CAD is <55 years in men and <65 years in women. These are general FH, not FDB-specific, criteria. (haradashiba2023guidelinesforthe pages 8-11)

Imaging is for phenotype/risk assessment, not molecular diagnosis: Achilles tendon radiograph or ultrasound, coronary calcium CT in selected adults, carotid ultrasound, and stress testing or coronary imaging when ischemia is suspected. Japanese thresholds for Achilles thickening are radiographic ≥8.0 mm in men/≥7.5 mm in women or ultrasound ≥6.0/≥5.5 mm. (haradashiba2023guidelinesforthe pages 8-11)

Genetic testing

Preferred testing is an FH multigene panel including LDLR, APOB, PCSK9, and LDLRAP1, with deletion/duplication analysis where appropriate. If a familial APOB allele is known, targeted testing is efficient. WES/WGS is useful for panel-negative severe or atypical families and possible digenic disease, but incidental APOB missense variants require rigorous ACMG/AMP interpretation and functional evidence. CMA, karyotype, FISH, mitochondrial sequencing, and repeat-expansion testing are not routinely indicated.

Differential diagnosis

Exclude hypothyroidism, nephrotic syndrome, cholestatic liver disease, uncontrolled diabetes, medication-related dyslipidemia, familial combined hyperlipidemia, polygenic hypercholesterolemia, sitosterolemia, lysosomal acid lipase deficiency, elevated Lp(a), LDLR-related FH, PCSK9 gain-of-function FH, and LDLRAP1-related recessive hypercholesterolemia. FDB often has a milder LDL-C elevation and normal receptor structure/function but cannot be reliably distinguished from LDLR-FH by lipid profile alone. (kounatidis2024apob100andatherosclerosis pages 13-14, faiz2012molecularpathologyof pages 4-5)

No validated omics-based diagnostic beyond DNA testing and conventional lipoprotein biomarkers is established.

11. Outcome and prognosis

FDB increases lifelong ASCVD morbidity, particularly coronary calcification, ischemic heart disease, and MI. Average severity appears lower than LDLR-FH, but individual prognosis is heterogeneous. General FH data show CAD odds approximately 10–20 times and peripheral arterial disease odds 5–10 times those of non-FH populations; these effect sizes should not be assigned directly to FDB. (haradashiba2023guidelinesforthe pages 2-4)

Prognostic factors are untreated and on-treatment LDL-C/apoB, duration of exposure, existing ASCVD or coronary calcium, smoking, diabetes, hypertension, low HDL-C, high triglycerides, Lp(a), male sex/older age, family history, allele dosage, and additional LDL-pathway variants. No FDB-specific 5- or 10-year survival, disability, mortality, or quality-of-life model was identified. Recovery from the genotype is impossible, but LDL normalization can markedly reduce future risk.

12. Treatment

FDB is treated according to heterozygous FH algorithms, with response monitored by LDL-C, non-HDL-C, and apoB.

  1. Lifestyle foundation: Mediterranean-style diet; reduced saturated/trans fats; regular aerobic exercise; weight, diabetes, and blood-pressure control; no tobacco. These measures are adjunctive and rarely sufficient alone. Suggested NCIt concepts: dietary intervention, exercise therapy, smoking cessation.
  2. High-intensity statin: atorvastatin or rosuvastatin; reduces hepatic cholesterol synthesis and upregulates LDLR. Suggested NCIt: HMG-CoA reductase inhibitor therapy. Statins are the best-supported first-line class in FH and earlier pediatric use limits IMT progression. (haradashiba2023guidelinesforthe pages 2-4)
  3. Ezetimibe: blocks NPC1L1-mediated intestinal cholesterol absorption; add when targets are not achieved. Suggested NCIt: ezetimibe therapy.
  4. PCSK9 monoclonal antibody: evolocumab or alirocumab increases recycled hepatic LDLR. General HeFH trials report about 59–61% and 51–58% LDL-C reductions, respectively; intact LDLR in FDB provides a strong mechanistic rationale, but these are not FDB-only response estimates. (siddiqi2026familialhypercholesterolemiastateoftheart pages 1-2)
  5. Inclisiran: hepatic PCSK9 siRNA; general FH evidence reports about 48% LDL-C lowering. Suggested NCIt: small interfering RNA therapy/inclisiran.
  6. Bempedoic acid: oral ATP-citrate lyase inhibitor, useful when further lowering is needed or statin intolerance occurs. Suggested NCIt: ATP citrate lyase inhibitor therapy.
  7. Bile-acid sequestrant: an option, including selected pregnancy contexts; gastrointestinal adverse effects and pill burden limit use.
  8. Lipoprotein apheresis: reserved for exceptionally severe, refractory disease or advanced ASCVD; acutely lowers LDL-C approximately 50–75% per session in general FH. (siddiqi2026familialhypercholesterolemiastateoftheart pages 1-2)

Common adverse effects include statin-associated muscle symptoms and transaminase elevations, ezetimibe gastrointestinal symptoms, PCSK9/inclisiran injection-site reactions, bempedoic-acid-associated hyperuricemia/gout and tendon concerns, and apheresis-related vascular-access burden and hypotension. Pregnancy planning requires specialist management; many systemic lipid-lowering drugs are stopped, while bile-acid sequestrants and, in severe disease, apheresis may be considered. (haradashiba2023guidelinesforthe pages 19-21)

Targets used in contemporary FH practice are ≥50% LDL-C reduction and LDL-C <70 mg/dL for high-risk primary prevention, or <55 mg/dL for very-high-risk/established ASCVD; pediatric targets are commonly <135 mg/dL after age 10. (siddiqi2026familialhypercholesterolemiastateoftheart pages 1-2, fularski2024unveilingfamilialhypercholesterolemia—review pages 7-9)

No approved therapy edits APOB p.Arg3527Gln. APOB-silencing therapies are unattractive as a routine FDB correction because excessive apoB suppression can impair hepatic lipid export. No retrieved ClinicalTrials.gov study specifically enrolled FDB as a distinct molecular cohort.

13. Prevention

  • Primary prevention of genotype: impossible after conception. Reproductive options include genetic counseling, prenatal diagnosis, and preimplantation genetic testing when the familial pathogenic variant is known.
  • Secondary prevention: opportunistic/universal cholesterol screening according to national policy, molecular confirmation, and cascade testing of first-degree relatives. Each identified heterozygous relative has a 50% prior risk in an autosomal-dominant pedigree.
  • Tertiary prevention: early and sustained LDL/apoB lowering, smoking avoidance, and management of blood pressure, diabetes, obesity, and Lp(a)-related residual risk.
  • Vaccination/immunization: not applicable.
  • Newborn screening: not universally established for FDB. Pediatric lipid screening or reverse-cascade screening can identify affected families before events.

The World Heart Federation emphasizes that adverse exposure to LDL/apoB particles is cumulative and preventable through earlier screening and treatment. (haradashiba2023guidelinesforthe pages 2-4)

14. Other species and natural disease

  • Human: Homo sapiens, NCBI Taxonomy 9606; disease gene APOB.
  • Orthologues: ApoB is evolutionarily conserved in mammals, including mouse (Mus musculus, Taxon 10090), rat (Rattus norvegicus, 10116), rabbit (Oryctolagus cuniculus, 9986), pig (Sus scrofa, 9823), and dog (Canis lupus familiaris, 9615). Current NCBI Gene IDs should be imported directly.
  • Natural veterinary disease: no well-established naturally occurring animal disorder orthologous specifically to human APOB p.Arg3527Gln FDB was identified. Breed-specific VBO annotation is therefore not justified.
  • Transmission: inherited vertically within a species; not contagious or zoonotic.

15. Model organisms and experimental systems

Relevant models include human apoB-100 transgenic mice, APOB variant-expression systems, primary or immortalized hepatocyte uptake assays, fibroblast/lymphocyte LDLR assays, and general atherosclerosis models such as Ldlr-null and Apoe-null mice. Human apoB-100-expressing mice can develop complex atherosclerotic lesions and are useful for lipoprotein metabolism and pharmacology. (zadelaar2007mousemodelsfor pages 3-4)

An allele-faithful APOB p.Arg3527Gln knock-in or transgenic system is conceptually the most specific model: expected readouts are reduced LDLR binding/uptake, prolonged LDL residence, hypercholesterolemia, and diet-dependent plaque. However, mice normally package much cholesterol in HDL and edit ApoB RNA differently from humans; background strain and diet strongly influence plaque, limiting direct quantitative translation. Ldlr−/− and Apoe−/− mice reproduce hypercholesterolemia and atherogenesis but not the defining apoB ligand defect. Cell-based LDLR-binding and uptake assays may therefore be more specific for variant classification than generic knockout models.

Recent developments and expert interpretation

The major 2023–2024 development is not a new FDB-specific therapy but integration of APOB-FDB into genotype-defined FH care: multigene panels/cascade screening, earlier combination treatment, apoB and non-HDL-C as particle-burden measures, and wider use of PCSK9-directed agents. The February 2024 apoB review emphasizes that apoB is both an atherogenic-particle biomarker and a participant in arterial inflammation; its abstract states that apoB-100 “has been suggested to play a crucial role in the formation of the atherogenic plaque.” [Published February 2024; DOI 10.3390/metabo14020123.] (kounatidis2024apob100andatherosclerosis pages 13-14, kounatidis2024apob100andatherosclerosis pages 5-7)

The 2023 Japanese guideline’s clinically important conclusion is that FH causes persistent LDL elevation from birth and therefore requires “early diagnosis and appropriate treatment” plus cascade screening. Published May 2023; DOI 10.5551/jat.CR005. This guidance is applicable to FDB, while its risk estimates and examination thresholds derive from mixed-genotype FH. (haradashiba2023guidelinesforthe pages 2-4, haradashiba2023guidelinesforthe pages 8-11)

Key curation cautions

  1. Do not confuse ligand-defective APOB hypercholesterolemia with APOB-related familial hypobetalipoproteinemia, which is usually caused by truncating/loss-of-production variants and produces low LDL-C and hepatic steatosis.
  2. Store both modern and legacy residue nomenclature: p.Arg3527Gln = R3500Q.
  3. Do not assign every rare APOB missense variant as pathogenic; require current ClinVar/ClinGen assessment, population frequency, segregation, and ideally functional evidence.
  4. Do not copy general FH prevalence, mortality, or treatment-response estimates into FDB-specific fields without an “extrapolated from mixed-genotype FH” qualifier.
  5. Exact live MONDO, Orphanet, HGNC, HPO, GO, CL, UBERON, NCIt, ClinVar, and gnomAD identifiers should be validated against their current releases before production loading.

References

  1. (kounatidis2024apob100andatherosclerosis pages 13-14): Dimitris Kounatidis, Natalia G. Vallianou, Aikaterini Poulaki, Angelos Evangelopoulos, Fotis Panagopoulos, Theodora Stratigou, Eleni Geladari, Irene Karampela, and Maria Dalamaga. Apob100 and atherosclerosis: what’s new in the 21st century? Metabolites, 14:123, Feb 2024. URL: https://doi.org/10.3390/metabo14020123, doi:10.3390/metabo14020123. This article has 64 citations.

  2. (kounatidis2024apob100andatherosclerosis pages 11-13): Dimitris Kounatidis, Natalia G. Vallianou, Aikaterini Poulaki, Angelos Evangelopoulos, Fotis Panagopoulos, Theodora Stratigou, Eleni Geladari, Irene Karampela, and Maria Dalamaga. Apob100 and atherosclerosis: what’s new in the 21st century? Metabolites, 14:123, Feb 2024. URL: https://doi.org/10.3390/metabo14020123, doi:10.3390/metabo14020123. This article has 64 citations.

  3. (haradashiba2023guidelinesforthe pages 2-4): Mariko Harada-Shiba, Hidenori Arai, Hirotoshi Ohmura, Hiroaki Okazaki, Daisuke Sugiyama, Hayato Tada, Kazushige Dobashi, Kota Matsuki, Tetsuo Minamino, Shizuya Yamashita, and Koutaro Yokote. Guidelines for the diagnosis and treatment of adult familial hypercholesterolemia 2022. Journal of Atherosclerosis and Thrombosis, 30:558-586, May 2023. URL: https://doi.org/10.5551/jat.cr005, doi:10.5551/jat.cr005. This article has 108 citations and is from a peer-reviewed journal.

  4. (siddiqi2026familialhypercholesterolemiastateoftheart pages 3-4): Ahmed Kamal Siddiqi, Kumail Mustafa Ali, Rameen Shahid, Shamna Haris, Anandita Kulkarni, Iliyan Mithani, Wilhelm Haverkamp, and Muhammad Shahzeb Khan. Familial hypercholesterolemia: state-of-the-art. Jun 2026. URL: https://doi.org/10.4081/cardio.2026.102, doi:10.4081/cardio.2026.102. This article has 0 citations.

  5. (haradashiba2023guidelinesforthe pages 24-25): Mariko Harada-Shiba, Hidenori Arai, Hirotoshi Ohmura, Hiroaki Okazaki, Daisuke Sugiyama, Hayato Tada, Kazushige Dobashi, Kota Matsuki, Tetsuo Minamino, Shizuya Yamashita, and Koutaro Yokote. Guidelines for the diagnosis and treatment of adult familial hypercholesterolemia 2022. Journal of Atherosclerosis and Thrombosis, 30:558-586, May 2023. URL: https://doi.org/10.5551/jat.cr005, doi:10.5551/jat.cr005. This article has 108 citations and is from a peer-reviewed journal.

  6. (haradashiba2023guidelinesforthe pages 8-11): Mariko Harada-Shiba, Hidenori Arai, Hirotoshi Ohmura, Hiroaki Okazaki, Daisuke Sugiyama, Hayato Tada, Kazushige Dobashi, Kota Matsuki, Tetsuo Minamino, Shizuya Yamashita, and Koutaro Yokote. Guidelines for the diagnosis and treatment of adult familial hypercholesterolemia 2022. Journal of Atherosclerosis and Thrombosis, 30:558-586, May 2023. URL: https://doi.org/10.5551/jat.cr005, doi:10.5551/jat.cr005. This article has 108 citations and is from a peer-reviewed journal.

  7. (kounatidis2024apob100andatherosclerosis pages 4-5): Dimitris Kounatidis, Natalia G. Vallianou, Aikaterini Poulaki, Angelos Evangelopoulos, Fotis Panagopoulos, Theodora Stratigou, Eleni Geladari, Irene Karampela, and Maria Dalamaga. Apob100 and atherosclerosis: what’s new in the 21st century? Metabolites, 14:123, Feb 2024. URL: https://doi.org/10.3390/metabo14020123, doi:10.3390/metabo14020123. This article has 64 citations.

  8. (kounatidis2024apob100andatherosclerosis pages 5-7): Dimitris Kounatidis, Natalia G. Vallianou, Aikaterini Poulaki, Angelos Evangelopoulos, Fotis Panagopoulos, Theodora Stratigou, Eleni Geladari, Irene Karampela, and Maria Dalamaga. Apob100 and atherosclerosis: what’s new in the 21st century? Metabolites, 14:123, Feb 2024. URL: https://doi.org/10.3390/metabo14020123, doi:10.3390/metabo14020123. This article has 64 citations.

  9. (siddiqi2026familialhypercholesterolemiastateoftheart pages 1-2): Ahmed Kamal Siddiqi, Kumail Mustafa Ali, Rameen Shahid, Shamna Haris, Anandita Kulkarni, Iliyan Mithani, Wilhelm Haverkamp, and Muhammad Shahzeb Khan. Familial hypercholesterolemia: state-of-the-art. Jun 2026. URL: https://doi.org/10.4081/cardio.2026.102, doi:10.4081/cardio.2026.102. This article has 0 citations.

  10. (faiz2012molecularpathologyof pages 4-5): Fathimath Faiz, Amanda J. Hooper, and Frank M. van Bockxmeer. Molecular pathology of familial hypercholesterolemia, related dyslipidemias and therapies beyond the statins. Critical Reviews in Clinical Laboratory Sciences, 49:1-17, Feb 2012. URL: https://doi.org/10.3109/10408363.2011.646942, doi:10.3109/10408363.2011.646942. This article has 38 citations and is from a peer-reviewed journal.

  11. (fularski2024unveilingfamilialhypercholesterolemia—review pages 7-9): Piotr Fularski, Joanna Hajdys, Gabriela Majchrowicz, Magdalena Stabrawa, Ewelina Młynarska, Jacek Rysz, and Beata Franczyk. Unveiling familial hypercholesterolemia—review, cardiovascular complications, lipid-lowering treatment and its efficacy. International Journal of Molecular Sciences, 25:1637, Jan 2024. URL: https://doi.org/10.3390/ijms25031637, doi:10.3390/ijms25031637. This article has 30 citations.

  12. (kamar2021thedigeniccausality pages 14-15): Amina Kamar, Athar Khalil, and Georges Nemer. The digenic causality in familial hypercholesterolemia: revising the genotype–phenotype correlations of the disease. Frontiers in Genetics, Jan 2021. URL: https://doi.org/10.3389/fgene.2020.572045, doi:10.3389/fgene.2020.572045. This article has 31 citations and is from a peer-reviewed journal.

  13. (zadelaar2007mousemodelsfor pages 3-4): Susanne Zadelaar, Robert Kleemann, Lars Verschuren, Jitske de Vries-Van der Weij, José van der Hoorn, Hans M. Princen, and Teake Kooistra. Mouse models for atherosclerosis and pharmaceutical modifiers. Arteriosclerosis, thrombosis, and vascular biology, 27 8:1706-21, Aug 2007. URL: https://doi.org/10.1161/atvbaha.107.142570, doi:10.1161/atvbaha.107.142570. This article has 758 citations and is from a domain leading peer-reviewed journal.

  14. (haradashiba2023guidelinesforthe pages 19-21): Mariko Harada-Shiba, Hidenori Arai, Hirotoshi Ohmura, Hiroaki Okazaki, Daisuke Sugiyama, Hayato Tada, Kazushige Dobashi, Kota Matsuki, Tetsuo Minamino, Shizuya Yamashita, and Koutaro Yokote. Guidelines for the diagnosis and treatment of adult familial hypercholesterolemia 2022. Journal of Atherosclerosis and Thrombosis, 30:558-586, May 2023. URL: https://doi.org/10.5551/jat.cr005, doi:10.5551/jat.cr005. This article has 108 citations and is from a peer-reviewed journal.

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