Week 8 Part 2 - Familial Hypercholesterolaemia

Question 1

Common Risk Factors for Cardiovascular Disease

Answer 1

Age
Sex
Smoking
Hypertension
BMI
Hyperlipidaemia
Previous Family history
Diabetes

Question 2

Lipoproteins

Answer 2

Macromolecular complexes of phospholipids, triglyceride, cholesterol, cholesterol ester, and protein - known as apoproteins
They serve a vehicle for the transport of fats and cholesterol in the plasma
- cholesterol and triglycerides are highly water insoluble
- lipoproteins have a role in the transport of these lipids in plasma
The synthesis and metabolism of the different lipoproteins is dictated by their apoprotein content and by lipoprotein receptors-which serve as cargo receptors

Question 3

Lipoprotein Classes Found in Plasma - LDL and HDL

Answer 3

Cholesterol transport to (forward) and from (reverse) peripheral tissues respectively
Often referred to as “bad” and “good” cholesterol

Question 4

Lipoprotein Classes Found in Plasma - Very Low Density Lipoprotein (VLDL)

Answer 4

Synthesised in liver
Transports Tg to peripheral tissues, and metabolized to LDL
Apo B is an important protein constituent
Very large

Question 5

Lipoprotein Classes Found in Plasma - Chylomicrons

Answer 5

Very similar function to VLDL but lipids come from the diet,sheds Tg and metabolised to
Very large

Question 6

Lipoprotein Classes Found in Plasma - Others

Answer 6

Intermediate density lipoprotein (IDL) - transient intermediate in metabolism of VLDL to LDL
Lp(a) - like LDL but with apo(a)
Chylomicron remnants - formed from chylomicrons

Question 7

Apolipoproteins

Answer 7

Proteins associated with lipoprotein particles are known as apolipoproteins (apo for short)
Amphipathic regions
Not receptors in the traditional sense
A number have multiple functions
Synthesis, secretion and catabolism of lipoproteins depends in large part on apolipoproteins- e.g. Apo A-I, Apo B-100

Question 8

Apolipoproteins - Functions

Answer 8

Mediate binding of lipoproteins to lipoprotein receptors
- apo B-100: ligand for binding of LDL to the LDL receptor
- apo E: ligand for binding of VLDL, chylomicron remnants to the LDL receptor
- apo A-1: ligand for binding of HDL to ABCA1 receptor
Structural proteins required for synthesis of lipoproteins
- apoB-100, apoB-48, apo A-I
Co-factors for enzymes involved in metabolism and remodelling of lipoprotein particles
- apo CII (lipoprotein lipase activator), apo A-I (LCAT activator)

Question 9

Metabolism of VLDL and LDL - Hepatic Synthesis of VLDL

Answer 9

Associates with lipoprotein lipase and becomes IDL and FFA (which are transported to muscle and adipose tissue)
IDL can be directly removed from the liver via LDL-R
IDL associates with hepatic lipase to become LDL
LDL uptake by tissue and the liver via LDL-R

Question 10

Metabolism of VLDL and LDL - GIT Synthesis of Chylomicron

Answer 10

GIT synthesis of chylomicron
Associates with lipoprotein lipase to produce CR and FFA (which are transported to muscle and adipose tissue)
CR binds with hepatic receptor (LDL-R/LRP)

Question 11

WHO Classification of Hyperlipidaemia

Answer 11

Increased cholesterol
Increased LDL-C
Triglyceride normal
Lipoproteins primarily elevated = LDL

Question 12

Causes of Hyperlipidaemia

Answer 12

Causes can be genetic
- inherent, known as primary cause
But more often polygenic
- effect of multiple genes + environmental/behavioural influences
- imbalance between calorie intake vs expenditure
Very often also secondary to other disorders
- hypothyroidism, diabetes, alcohol intake, obesity, renal failure

Question 13

Genetics of Familial Hypercholesterolaemia (FH)

Answer 13

Inheritance patterns
- autosomal dominant disorder*
- heterozygotes & homozygotes affected
- homozygotes are affected much more seriously
Molecular Genetics
- most commonly due to LDL receptor (LDLR) defects
- can be due to “some” apo B-100 mutations, and “some” PCSK9 mutations

Question 14

Genetics of Familial Hypercholesterolaemia - Phenotype

Answer 14

Elevated plasma/blood LDL & cholesterol
Lipid accumulation in numerous sites, including the vascular wall
Premature obstructive coronary artery disease
Homozygotes severely affected, most often found in cosanguinous unions

Question 15

LDL Receptor Domains

Answer 15

1. 292 aa
   - ligand binding
   - cysteine rich
2. 417 aa
   - EGFP homology
3. 58 aa
   - O-linked sugars
   Also has receptor domains 4 and 5

Question 16

Where is the LDL Gene Found?

Answer 16

Chromosome 19 at 19p13.2
Has 18 exons

Question 17

Founder Effects - Afrikaner Population

Answer 17

3 founder mutations introduced by European settlers
Heterozygote frequency as high as 1/70

Question 18

Founder Effects - Christian Lebanese

Answer 18

10x higher homozygous frequency
82% with Cys681X which removes 180 aa (ligand binding and part of EGF homolgy domains retained)

Question 19

Founder Effects - French Canadians

Answer 19

5 founder mutations, responsible for 3/4 of cases.
15 Kb deletion removes promoter & exon 1: ~60% of cases

Question 20

Genetic Heterogeneity

Answer 20

The term Familial Hypercholesterolaemia has usually referred to hypercholesterolalemia associated with defects in the LDL-receptor
However, mutations in a number of the proteins involved in LDL-receptor mediated upatke of LDL from circulation have similar phenotypic effects.
Example
- defects in the protein apo B-100, the component of LDL which mediates binding of LDL to the LDL-receptor
- PCSK9 mutation

Question 21

Variable Expressivity

Answer 21

Patients inheriting the same mutation may not exhibit the same clinical phenotype: there may be variation in the way the condition is expressed
Some may show differing degrees of hypercholesterolaemia
At risk of developing symptomatic coronary artery disease, but not all amy present with this.
Hence the absence of CAD or tendon xanthomas doesn’t “rule out” the condition
Some may even have plasma cholesterol which is not that different from “normal”
Makes it problematic to identify affected individuals on the basis of phenotype

Question 22

Genetic Testing and Cascade Testing

Answer 22

Genetic Testing not usually the first step.
Measurement of lipid levels, extent of hypercholesterolaemia, evidence of lipid deposition, family history
The genetic testing
- NGS followed by Sanger sequencing and if indicated MLPA analysis
- any of the 18 exons could be affected
OR
- screening using kits, e.g. using ARMS, reverse hybridisation (requires lower skill level), and does not require scale of operations

Question 23

When do we Measure Lipids?

Answer 23

Lipid disorders are common
- as part of assessment of risk for coronary heart disease
- to monitor efficacy of drug/dietary or lifestyle modifications/interventions
Routinely checked in adults and especially in those with metabolic disorders, including being overweight/obese,
Subjects with a history or family history of hyperlipidamia or heart disease
Subjects with metabolic disorders
Conversely, finding of hyperlipidaemia may be a trigger for investigations for numerous possible causes