F AMILIAL H YPERLIPIDEMIAS Julia Creider, PGY4 Endocrine.

FAMILIAL HYPERLIPIDEMIASJulia Creider, PGY4 Endocrine

OBJECTIVES

Review lipid and lipoprotein classification and nomenclature

Understand the pathways of cholesterol biosynthesis and metabolism

Review primary disorders of hyper- and hypolipidemia Low-Density Lipoprotein (LDL) Triglycerides (TG) High-Density Lipoprotein (HDL)

LIPIDS

Group of naturally occurring molecules Biologically important lipids

Free cholesterol Cholesterol esters (CE) Triglycerides (TG) Phospholipids Fat soluble vitamins (A, D, E, and K)

Main biological functions include Storing energy Signaling Structural components of cell membranes

LIPOPROTEINS

Large macromolecular complexes that transport hydrophobic lipids surrounded by hydrophilic phospholipids and proteins

LIPOPROTEINS

Type Site of Origin Major Lipids Major Apolipoprotiens

Chylomicrons (CM)

Intestine 85% TG B48, A1, AIV

CM Remnant Intestine 60% TG, 20% C B48, E

VLDL Liver 55% TG, 20% C B100, E, C1, CII, CIII

IDL From VLDL 35% C, 25% TG B100, E

LDL From IDL 60% C, 5% TG B100

HDL Liver, intestine, plasma

25% PL, 20% C, 5% TG (50% protein)

A1, AII, C1, CII, CIII, E

Lp(a) Liver 60% C, 5% TG B100, (a)

LIPOPROTEINS

Lp(a)

APOLIPOPROTEINS

Protein component of lipoproteins Function

Activate enzymes important to lipid metabolism Act as ligands for cell surface receptors

Apolipoprotein Site of Synthesis Major Functions

ApoA-1 Liver, intestineStructural protein of HDLCofactor for LCATLigand for ABCA-1 and SR-B1

ApoA-II Liver Inhibits apo-E binding to receptors

ApoA-IV IntestineActivator of LCATFacilitates lipid secretion from intestine

ApoA-V Liver Activator of LPL lipolysis

ApoB-100 Liver Protein for VLDL, IDL, LDL

ApoB-48 Intestine Protein for CMs

ApoC-1 LiverModulates apo-E mediated binding of remnantsActivate LCAT

ApoC-II Liver Cofactor for LPL

ApoC-III LiverRemnant binding to receptorsInhibitor of LPL

ApoELiver, brain, skin,

spleen, testesLigand for LDL & remnant receptorReverse cholesterol transport

Apo(a) Liver Unknown

RECEPTORS

Low-Density Lipoprotein Receptor (LDLR) Present on cells throughout the body Mediates uptake of cholesterol-rich lipoproteins Requires specific proteins on lipoprotein surface

ApoB-100 (LDL) ApoE (CM remnants, VLDL, IDL, and HDL)

Number of LDLR on cell surface is tightly regulated by intracellular cholesterol content

Low-Density Lipoprotein Receptor-Related Protein (LRP) Aka Remnant receptor Binds with high affinity to ApoE (CM remnants, VLDL) Does not bind LDL

IMPORTANT ENZYMES

Lipoprotein Lipase (LPL) Bound to capillary endothelial cells Mediates hydrolysis of TGs to release FFA from

CMs and VLDL Requires ApoC-II as cofactor Activated by ApoA-V Inhibited by ApoC-III Activated by insulin in adipocytes Activated by glucagon and adrenaline in muscle

and myocardial tissues

IMPORTANT ENZYMES

Hepatic Lipase (HL) Hydrolyzes TGs in final processing of CM

remnants Completes processing of IDL to LDL Facilitates interaction of remnant lipoproteins

with LRP for internalization by hepatocytes Participates in conversion of HDL2 back to HDL3

WHAT HAPPENS WHEN YOU EAT? (1)

Brush boarder of epithelial cells of small intestine (duodenum and proximal jejunum) synthesize CMs from dietary fat and cholesterol

CMs enter mesenteric lymph and are absorbed into general circulation by the thoracic duct

Newly synthesized CMs have: ApoB-48 ApoA-1 ApoA-IV

They acquire ApoE and C-apolipoproteins (primarily from HDL)

WHAT HAPPENS WHEN YOU EAT? (2)

LPL catalyzes release of FFAs from CM TG’s and converts them to CM remnants

FFAs are: Stored in adipose tissue Oxidized as energy source Reutilized in hepatic lipoprotien TG synthesis (VLDL)

Hepatic lipase helps in final preparation of CMs for uptake by hepatocytes

CM remnants are rapidly cleared by the liver by either LDL or LRP receptors, mediated by ApoE

ApoB-48ApoA-1ApoA-IV

ApoE, C1, CII, CIII from HDLApoB-48ApoE

VLDL

Synthesized by the liver Production stimulated by increased delivery of FFA

to hepatocytes Microsomal triglyceride transfer protein (MTP)

Transfers TG and PL to nascent ApoB containing lipoproteins (ApoB-100, and B48)

Deficiency of MTP causes Abetalipoproteinemia VLDL triglycerides are hydrolyzed by LPL and HL Converted to smaller particles that are

increasingly rich in cholesterol

IDL

Metabolic product of VLDL catabolism by LPL Primary proteins are ApoE and ApoB-100 Fate:

Further processed by LPL and HL to LDL Removed from plasma by the LDLR (uptake

mediated by ApoE

LDL

50% of VLDL makes it to LDL

70% of total plasma cholesterol is in LDL

Major apolipoprotein is ApoB-100

Uptake by the LDLR is mediated by ApoB-100

Delivers cholesterol to cells

B100, E, C1, CII, CIII B100, E B100

LIPID DISORDERS

Previously classified according to Fredrickson phenotype

Categorized by type of lipoprotein particle that accumulated in the blood

Does not include HDL Not take into account cause Not distinguish between primary and secondary

causes

FREDRICKSON PHENOTYPES

Phenotype Lipoprotein Lipid Elevation

Type I CMs TG

Type IIa LDL TC

Type IIb LDL and VLDL TC and TG

Type III IDL TC and TG

Type IV VLDL TG

Type V VLDL and CMs TC and TG

PRIMARY DISORDERS OF HYPERLIPIDEMIA

Increased Cholesterol

Increased Cholesterol and Triglycerides

Increased Triglycerides


Increased Cholesterol Familial Hypercholesterolemia (FH) Familial Defective Apolipoprotein B100 (FDB) Autosomal Recessive Hypercholesterolemia (ARH) Sitosterolemia Polygenic Hypercholesterolemia

FAMILIAL HYPERCHOLESTEROLEMIA (FH)

Autosomal dominant Caused by mutation in LDL receptor gene

>900 described mutations Markedly elevated LDL (>95%ile) Heterozygous 1 in 500, homozygous 1 in 106

60-80, 000 Canadians French Canadian, Christian Lebanese, Dutch

South Afrikaners ~5% of men with MI age < 60

High Risk of CAD if untreated > 60% men, > 30% women by age 60

Katz P. 2014. Lipid metabolism & clinical lipid disorders.

WHEN TO CONSIDER FH

Very high LDL (typically > 5.0 mmol/L) Personal history of early cardiovascular

disease Typical physical findings:

Xanthelasmas Arcus cornealis Tendon xanthomas

Family history: Early cardiovascular disease Marked hyperlipidemia

XANTHALASMAS

ARCUS CORNEALIS

TENDON XANTHOMAS

HOMOZYGOUS FH Very rare, 1 in 106

Marked hypercholesterolemia from birth TC 15 – 25 mmol/L, LDL 14 - 25 mmol/L

Symptomatic CHD < 10 years of age, MI as

young as 18 months If untreated, usually die

in 20’s of CHD Xanthomas,

xanthelasma early in life

Tuberous xanthomasKatz P. 2014. Lipid metabolism & clinical lipid disorders.

SCREENING FOR FH

1. Targeted screening to identify FH index cases with at least 1 feature:

Personal or family history of clinical stigmata or premature CVD

Family history of significant hypercholesterolemia

2. Cascade screening to detect affected members

Opportunistic screening should be done around time of cardiovascular event

Canadian-specific cascade screening www.fhcanada.net

CCS: Position Statement on FH. 2014

FH DIAGNOSIS

Homozygous Suspect in child with TC > 12.9 or xanthomas

Heterozygotes Primarily clinical diagnosis Family history very important Standard criteria have been suggested:

Simon-Broome Criteria Dutch Lipid Clinic Criteria

If family member has known FH and mutation can do cascade testing in family members

Yuan G et al. 2006. CMAJ 17(8):1124

Yuan G et al. 2006. CMAJ 17(8):1124

FH DIAGNOSIS

Yuan G et al. 2006. CMAJ 17(8):1124

WHEN TO DO GENETIC TESTING?

Cases of diagnostic uncertainty Unavailable family history Borderline lipid levels Screened as possible or probable FH Will change management

CCS: Position Statement on FH. 2014

TREATMENT OF FH

Global risk factor assessment and management HTN, DM, smoking, obesity

Homozygotes – plasmapharesis or LDL apheresis

Heterozygotes – statins +/- other agents Ezetimibe, bile acid sequestrants, niacin Newer agents PSCK9 monoclonal antibodies

Treatment goal is at least a 50% reduction in LDL or less than 2.0 mmol/L

Feldman D et al. 2015. CurrAtherosclerRep 17(1):473.

TREATMENT OF FH

Feldman D et al. 2015. CurrAtherosclerRep 17(1):473.

CHOLESTEROL BIOSYNTHESIS

Cholesterol is either absorbed from diet or synthesized by cells in the body

3 molecules of acetate are condensed to form 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA)

HMG-CoA is converted to mevalonic acid by HMG-CoA reductase ➔ RATE LIMITING STEP

Feedback regulation: intracellular cholesterol ➔ HMG-CoA reductase Cholesterol deficiency ➔ upregulate enzyme

ENTEROHEPATIC CIRCULATION

Either excreted as free cholesterol (FC) in bile or converted to bile acids (BA)

50% of FC and 97% of secreted BA are reabsorbed

Reabsorbed cholesterol and BA regulate de novo cholesterol and bile acid synthesis in the liver

7-αHydroxylase RATE LIMITING STEP in BA synthesis Feedback regulation by recirculating BA Closely coupled to HMG-CoA reductase

activity

Niacin:

1. Direct inhibition of DGAT2

2. Decrease lipolysis and FFA influx to liver

3. Increase ApoB catabolism

Statins: inhibit HMG CoA reductasedecrease cholesterolupregulate LDL-R expressionincreased clearance of LDL-C

Bile acid sequestrants:Decrease FC/BA reabsorptionEzetimibe:inhibit intestinal absorption

PCSK9

Mullard A. 2012. Nature Rev Drug Discovery 11:817

FAMILIAL DEFECTIVE APOLIPOPROTEIN B100

Mutation in ApoB-100 that impairs its ability to bind to the LDL receptor

Single mutation accounts for almost all cases Substitution of glutamine for arginine at aa3500

Phenotypically similar to FH Isolated elevated LDL Tendon xanthomas and xanthelasma Premature CVD

Generally less severe Clear remnant particles through LDL receptor via ApoE

Kronenberg HM et al. Williams Textbook of Endocrinology. 12th Ed

AUTOSOMAL RECESSIVE HYPERCHOLESTEROLEMIA

Mutation in ARH which encodes LDLR adaptor protein 1

Mediates endocytosis of LDL receptor in hepatocyte cells

Very rare – described in Sardinia, Lebanon

✕

Soutar A. 2010. IUBMB Life. 62(2):125.

SITOSTEROLEMIA

Very rare autosomal recessive Mutations in Adenosine triphosphate binding

cassette transporter (ABCG) 5 or 8 Function to remove passively absorbed plant

sterols Leads to elevations of sitosterol and

campesterol Presentation:

Xanthomas Premature CHD Arthralgias, hemolysis, thrombocytopenia

Requires liquid or gas chromatography to identify

Treatment – restriction of dietary plant sterols and ezetimibe

Othman R et al. 2013. Atherosclerosis. 231(2):291.

POLYGENIC HYPERCHOLESTEROLEMIA

A cholesterol value > 95th percentile for population

Exclude other primary genetic causes by absence of tendon xanthomas and family history in ≤ 10% of first degree relatives



Increased Cholesterol and Triglyceride Familial Combined Hyperlipidemia Familial Dysbetalipoproteinemia

FAMILIAL COMBINED HYPERLIPIDEMIA (FCH)

Autosomal dominant Common disorder (5-7%) Unknown genetic cause, likely multiple genes Overproduction of ApoB ➔ ⇧ VLDL, LDL or

both Moderate elevations of cholesterol or TG or

both Predominant lipid abnormality can vary

among affected family member or a single person over time


FAMILIAL COMBINED HYPERLIPIDEMIA (FCH)

Clinically Overlapping features of metabolic syndrome

Insulin resistance Obesity Hyperuricemia Low HDL

No xanthomas Increased susceptibility to CHD

11% of male survivors of MI < age 60

Treatment Lifestyle – diet, exercise, weight loss Pharmacotherapy – targeted at specific lipid

abnormality


FAMILIAL DYSBETALIPOPROTEINEMIA/TYPE III HYPERLIPOPROTEINEMIA

Mutations in ApoE Results in impaired binding of ApoE to lipoprotein

receptors and accumulations of remnant particles (CM remnants and IDL)

Moderate to severe ⇧ TG and TC LDL reduced (cleared by LDL receptor via ApoB-

100)

Marais A et al. 2014.CritRevClinLabSci. 51(1):46.

ApoB-48ApoE

B100, E B100

×

×

FAMILIAL DYSBETALIPOPROTEINEMIA/TYPE III HYPERLIPOPROTEINEMIA

Typically autosomal recessive (rarely dominant) 1 in 10, 000 Majority are homozygous of ApoE-2 genotype ~ 1% population is ApoE2/E2, only 0.01% have type

III Requires 2nd exacerbating metabolic factor Hypothyroidism, menopause, alcohol, diabetes

Premature vascular disease (including PVD) Treatment

Treat exacerbating factor Diet Fibrates +/- statins

Marais A et al. 2014.CritRevClinLabSci. 51(1):46.

PALMAR XANTHOMAS

Yellowish plaques on palms – especially creases and flexural surface of fingers


Increased Triglceride Lipoprotein Lipase Deficiency Apolipoprotein CII Deficiency Familial hypertriglyceridemia

LIPOPROTEIN LIPASE DEFICIENCY

Autosomal recessive Mutation in the LPL gene

Absence or inactivation of LPL protein Impaired clearance of TG-rich lipoproteins from

plasma Accumulation of CMs and VLDL

Chylomicronemia Syndrome Marked hypertriglyceridemia (TG >22.6 mmol/L) Recurrent abdominal pain /pancreatitis

Rahalkar A et al. 2009.Can J Physiol Pharmacol. 87(3):151.

LIPOPROTEIN LIPASE DEFICIENCY

Usually present in infancy or childhood Clinically

Eruptive xanthomas Lipemia retinalis Hepatosplenomegaly Neurological manifestations Dyspnea

Biochemically Lipemic plasma Pseudohyponatremia

Treatment – diet, fibrates

Rahalkar A et al. 2009.Can J Physiol Pharmacol. 87(3):151.

ERUPTIVE XANTHOMA

LIPEMIA RETINALIS

LIPEMIC PLASMA

APOLIPOPROTEIN CII DEFICIENCY

Rare autosomal recessive disorder (< 1 in 106)

ApoC-II necessary cofactor for LPL activity Chylomicronemic syndrome similar to LPL

deficiency Severely elevated TG Lipemic serum Recurrent pancreatitis/abdominal pain Eruptive xanthmoas and lipemia retinalis

Absent ApoC-II on electrophoresis Treatment – diet, fibrates


FAMILIAL HYPERTRIGLYCERIDEMIA

Overproduction of VLDL with near normal ApoB production Typically TG 2.3-5.6 Normal LDL Often low HDL

Must be present in half of 1st degree relatives to diagnose

Eruptive xanthomas usually not present Obesity, insulin resistance common

Exacerbating factors – hypothyroid, estrogen therapy, alcohol

Uncertain if increase CHD risk


HIGH-DENSITY LIPOPROTEIN (HDL)

Redistribution of lipids among lipoproteins and cells by REVERSE CHOLESTEROL TRANSPORT: HDL acquires cholesterol from cells and transports it

to other cells that require cholesterol or to the liver for excretion

ORIGIN OF HDL

1. Liver makes ApoA-I phspholipid disc (nascent, pre-beta HDL)

2. Small intestine directly synthesizes small ApoA-I containing HDL particles

3. Derived from surface material (ApoA-I and PL) that comes from CM and VLDL during lipolysis by LPL

ACQUISITION OF CHOLESTEROL BY HDL

1. Aqueous transfer from cells Free cholesterol moves by passive desorption from

high concentration in membranes of cells with excess cholesterol to low concentration on HDL surface

2. Transport by a cell-surface binding proteins SR-B1 – transfers CEs through hydrophilic channel ABCA1

Binds ApoA-1 or a pre-beta HDL disc to the cell membrane and facilitates transfer of FC and PL from cell to HDL precursor

ATP binding cassette transporters (ABCG1 and ABCG4) stimulate cholesterol efflux to mature HDL (HDL2 and HDL3)

MATURATION OF HDL

Nascent HDL particles (ApoA-I phospholipid discs) are excellent acceptors of excess cholesterol from cells or other lipoproteins

Lecithin-cholesterol acyltransferase (LCAT) Converts free cholesterol to cholesterol esters (CEs) LCAT is activated by ApoA-I

CEs are more hydrophobic, turn disc ➔ sphere (HDL3)

HDL3 accepts and esterifies free cholesterol increases in size ➔ HDL2

FATE OF HDL2

1. Reconverted to HDL3 by hepatic lipase

2. Exchange CE for TG with VLDL, IDL, LDL and remnants via CETP which are then indirectly delivered to the liver and taken up by remnant receptor (MAJOR)

3. Via SR-B1 receptor deposit CEs directly to liver, adrenal, gonads

4. Is further enriched with CE and acquires ApoE becoming HDL1 thereby allowing interaction with LDL receptor allowing excretion of cholesterol by the liver (MINOR)

PRIMARY DISORDERS OF HDL METABOLISM

DisorderMutant Gene

AD vs AR Frequency HDL Corneal

opacification

Early vascular disease

Familial Hypo-alphalipoproteinemia Unknown AD ~1/400 0.5-0.8 No Yes

Familial ApoA-I and ApoC-II deficiency

ApoA-I or ApoC-II AR Rare <0.1 Yes Yes

ApoA-I Milano ApoA-I AD Rare ~0.3 No No

LCAT deficiency LCAT AR Rare <0.3 Yes Yes

Fish-eye disease LCAT AR Rare <0.3 Yes No

Tangier disease ABCA1 AR Rare <0.1 Yes Yes

CETP deficiency CETP AR Rare >2.6 No No


FAMILIAL HYPOALPHALIPOPROTEINEMIA

HDL < 10% in men (<0.77 mmol/L), < 15% in women (<1.04 mmol/L)

Normal LDL and TG Increased risk of premature CHD No characteristic findings Often family history


APOLIPOPROTEIN A1 MUTATIONS

Mutations in ApoA-I results in poor LCAT activation

HDL < 0.3 Corneal opacities Increased CHD ApoA-I Milano

rare variant of ApoA-I Autosomal dominant Low HDL Not associated with premature CHD


LCAT DEFICIENCY

Decreased esterification of cholesterol to cholesterol esters in HDL particles

Free cholesterol accumulates on lipoprotein particles and in peripheral tissues

Features Corneal opacities Normochromic anemia Renal failure Decreased HDL Increased free cholesterol

Saeedi R et al. 2014. Clin Biochem. Aug:Epub

FISH-EYE DISEASE

Variant of LCAT deficiency Phenotype is less severe

Able to esterfy cholesterol on ApoB-containing lipoproteins just not HDL

Low HDL Corneal opacities No anemia, renal disease, or premature CHD


TANGIER DISEASE

Mutations in ABCA1 Loss of cholesterol efflux from cells such as

macrophages ➔ massive accumulation of CEs Hypolipidemia

Decreases in plasma HDL and LDL Features

Orange tonsils Corneal opacities Hepatosplenomegaly Peripheral neuropathy Premature CHD

Kolovou G et al. 2006. Curr Med Chem. 13(7):771.

CHOLESTERYL ESTER TRANSFER PROTEIN (CETP) DEFICIENCY

Diminished CETP activity Decreased transfer of CE from HDL to

ApoB containing lipoproteins (VLDL, IDL, LDL)

HDL increased More common in Japanese population Homozygotes have marked elevation of

HDL (> 2.6 mmol/L) Effect on CHD risk is unclear


OBJECTIVES

Review lipid and lipoprotein classification and nomenclature

Understand the pathways of cholesterol biosynthesis and metabolism

Review primary disorders of hyper- and hypolipidemia Low-Density Lipoprotein (LDL) Triglycerides (TG) High-Density Lipoprotein (HDL)

QUESTIONS?

REFERENCES CCS: Position Statement on FH. 2014 Feldman D et al. 2015. CurrAtherosclerRep 17(1):473. Katz P. 2014. Lipid metabolism & clinical lipid disorders.

CSEM. Kolovou G et al. 2006. Curr Med Chem. 13(7):771. Kronenberg HM et al. Williams Textbook of

Endocrinology. 12th Ed Marais A et al. 2014.CritRevClinLabSci. 51(1):46. Mullard A. 2012. Nature Rev Drug Discovery 11:817. Othman R et al. 2013. Atherosclerosis. 231(2):291. Rahalkar A et al. 2009.Can J Physiol Pharmacol.

87(3):151. Saeedi R et al. 2014. Clin Biochem. Aug:Epub. Soutar A. 2010. IUBMB Life. 62(2):125. Yuan G et al. 2006. CMAJ 17(8):1124.

F AMILIAL H YPERLIPIDEMIAS Julia Creider, PGY4 Endocrine.

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