LDLR Processing Dysregulation by Statin Treatment of Class II Transport Mutant Cells

The familial hypercholesterolemia (FH) Class II transport mutation is believed to be the most common defect in the low‐density lipoprotein receptor (LDLR). Class II mutations cause mis‐folding of the LDLR precursor preventing its transport from the ER for maturation in the Golgi and transport to the plasma membrane. The homozygous 3‐nucleotide deletion, c.654_656delTGG (FH‐Piscataway), in LDLR exon 4, caused a Class II mutation resulting in serum LDL over 800mg/dl in a 9yo female (GM03040, Coriell). Induced pluripotent stem cells (iPSC) derived from this patient's fibroblasts showed restored LDL‐receptor mediated endocytosis in homozygous CRISPR corrected clones. When statin treated, LDLR in non‐corrected FH‐iPSC (FH‐iPSC‐NC) persisted in the precursor (120kDa) form, while corrected cells (FH‐iPSC‐C) processed the LDLR to the mature form (160kDa). Quantification of total LDLR (immature + mature) showed 2.2‐fold more protein in NC cells. LDLR transcript quantification of statin treated NC, C and LDLR normal cells (H1 human embryonic stem cells, hESC) showed no difference, indicating LDLR accumulation is a dysregulation of post‐translational protein processing. This data and evidence in the literature led to our hypothesis that statin‐treatment of Class II LDLR mutant cells causes ER accumulation of mis‐folded LDLR and ER stress, leading to the unfolded protein response activation, which is reversed by LDLR correction. Since LDL cholesterol is primarily regulated by hepatocytes, we differentiated FH‐NC, C and H1 to hepatocyte‐like cells (HLC) using a modified version of Hay's protocol (PMID: 25081564) where stage 3 HLC were formed into spheroids of ~50/aggregate using AggreWell 400 plates. The HLC spheroids were treated with 5 mM Lovastatin or ethanol control in 5% lipoprotein‐deficient serum media overnight after which protein or RNA lysate was collected for Western blotting or qPCR, respectively. LDLR transcript levels were unchanged between groups, indicating that dysregulation is occurring at the post‐translational level in HLC. This work will provide new information as to whether statin treatment is implicated in inducing ER stress in Class II LDLR mutations and contribute to understanding the downstream effects and significance of statin treatment in patients with Class II LDLR FH. Support or Funding Information Jewish Heritage Fund for Excellence (NB) This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .