Causes of elevated LDL

Causes of elevated LDL

1. Familial Hypercholesterolaemia (FH) MUTATIONS IN LDL-R

   • Class 1- null/negligible LDL-R synthesis in ER
   • Class 2
     • Class 2A - defective transport to Golgi body
       • Lebanese mutation- single nucleotide mutation, leads to truncated LDL-R. Prevents dimerisation of two Cys residues leading to misfolding of the R which is thendegreaded rather than transported to the Golgi
     • Class 2B- receptor is secreted
   • Class 3- Mutations in the ligand binding repeats prevent binding of LDL
   • Class 4
     • Normal LDL binding. Unable to internalise R-Ligand complexes
       • Y807C substitution- mutation in NPXY motif of LDL-R in cytoplasmic tail, prevents interaction with clathrin
   • Class 5- R's cannot recycle properly
     • Relatively mild phenotype. LDL-R is still present in cell surface but must be newly synthesised
   • AUTOSOMNAL DOMINANT

2. Familial defective ApoB100 (FDB)

   • R3500Q mutation most common
     • C-terminal 11% crosses the belt of the carboxyl tail at AA 3500 close to binding site (3359-3369)
       • Mutated R3500, and w4369, prevent normal Arg3500 and Trp4369 interaction which is essential for correct folding of C-terminal ApoB100 and normal LDL-binding to LDL-R
         • R3500Q mutation masks the binding site of ApoB100
   • Causes reduction in LDL binding and hypobetalipoproteinamia (synthesis is impaired)

3. Autosomal recessive hypercholestrolaemia (ARH)

   • Decreased LDL-catabolism, not LDL-R or ApoB genes
   • LDLRAP1
     • ARH locus was mapped to chromosome 1p35 and a gene encoding LDLRAP1- this contained a PTB domain (which binds the NPXY motif in cytoplasmic LDL-R)
       • In liver (and lymphocytes) ARH is essential for LDL-R to internalise bound LDL, but not in fibroblasts- another adaptor protien, Dab2, may substitute for ARH in fibroblasts
     • Reduces LDL clearance
     • LDLRAP1 has a pentapeptide clatherin box (LLDLE) which binds to clatherin
   • Heterozygous ARH patients have normal plasma LDL levels, unless heterozygote FH patients who have 2.5 fold increases

4. Protein convertase subtilisin/kexin 9 (PCSK9)

   • Identified as third gene involved in autosomal DOMINANT hypercholesterolaemia (ADH)
   • Degrades LDL-R
     • In 2003 S127R and F216L mutations in PCKS9 were linked to increase in LDL levels
       • GAIN of FUNCTION mutations, increase LDL-R degradation therefore plasma LDL levels rise. LOSS of FUNCTION mutations result in reduced plasma LDL (cardioprotective)
     • Doesn't directly degrade LDL-Rs
   • Largely autocatalytic activity. Cleaved prodomain stays associated with catalytic and C-terminal domains
     • Cleaved protein secreted into circulation, acts as ligand for LDL-R binding specificially to the N terminal region of EGF-A domain in a Ca2+ dependent manner
       • Arg194 forms salt bridge with EGFA. Phe379 makes hydrophobic contact.
         • Mutations in either decrease binding by >90%
   • Ways to target PCSK9 for therapeutics
     • Intracellular inhibition of autocatalytic processing (prevents PCSK9 secretion from ER)
     • Degrading PCSK9 mRNA with antisense oligonucleotides or small interfering RNA
     • Inhibit PCSK9, which functions primarily at the cell surface, binding to LDL-Rs using drugs, peptides or antibodies