In a previous post (http://evomed.org/?p=1034), I discussed a study from Stuart Orkin’s lab that illustrated the exploitation of genetic variants that influence a disease-related phenotype to design a possible therapy for a murine version of sickle cell disease.  Increased fetal hemoglobin expression had been demonstrated to diminish the severity of sickle cell disease in mice, as is true also in humans.  Orkin and colleagues showed that eliminating a gene (BCL11A) that mediated decreased fetal hemoglobin expression, thereby achieving increased fetal hemoglobin expression, improved hematologic parameters in mice that serve as a model of sickle cell disease.  They speculated about the prospects for developing therapeutic agents that could inhibit BCL11A function and thereby facilitate fetal hemoglobin expression thereby decreasing adult hemoglobin expression and the manifestations of sickle cell disease.

Another example of this general approach has just been profiled in Nature (2013) in connection with cardiovascular disease, which remains the number one cause of mortality in the world.  Helen Hobbs, Jonathan Cohen, and their colleagues at the University of Texas Southwestern believed that it was critical to correlate genotypes and phenotypes and in particular to identify the genotypic correlates of extreme phenotypes.  The particular phenotypes of interest to Hobbs and Cohen were serum cholesterol concentrations, and especially low-density lipoprotein (LDL) cholesterol concentrations, due to their tendencies to correlate with risks of cardiovascular pathology.

In 2003, Abifadel et al.  showed that mutations in the gene encoding proprotein convertase subtilisin/kexin type 9 (initially called neural apoptosis regulated convertase, NARC-1), or PCSK9, were associated with autosomal dominant hypercholesterolemia, along with some mutations in the genes encoding the LDL receptor (LDLR) and apolipoprotein B (APOB).  These investigators showed that PCSK9 was highly expressed in liver.

Hobbs and Cohen then found African-American individuals in their study cohort with exceptionally low serum concentrations of LDL cholesterol.  Investigation of the genetic basis for this phenotype revealed two different nonsense mutations in the PCSK9 gene, (Cohen et al., 2006).  Further investigation demonstrated that absence PCSK9 function was associated with reduced serum concentrations of LDL cholesterol and substantially reduced risk (90%) of coronary heart disease in African Americans.  Different variants of PCSK9 were found in the Caucasian segment of the study population, and these variants were also associated with reduced serum LDL cholesterol and incidence of coronary heart disease.

Additional experiments by Cohen and Hobbs (Wang et al., 2012) addressed the mechanisms by which PCSK9 regulates serum LDL cholesterol concentration.  It appears that PCSK9 complexes with and mediates internalization and degradation of the LDL receptor by mechanisms independent of autophagy or proteasomes and not involving ubiquitination.  Therefore, decreased PCSK9 function allowed more LDL receptors to remain on the cell surface where they could clear LDL cholesterol from the blood.

Pharmaceutical companies have already developed candidate therapeutic agents, in the form of monoclonal antibodies, targeting PCSK9.  Phase III clinical trials are in process for two of these monoclonal antibodies.

It remains to be demonstrated that effective therapeutic inhibition of PCSK9 will improve outcomes in patients at high risk of coronary artery disease sufficient to justify addition of this potentially expensive modality to the available therapies.  If small-molecule inhibitors of PCSK9 can be developed, they might be more convenient and cost-effective to use, especially on a broad segment of the cardiovascular patient population.  In either case, success in this particular line of research will offer a strong demonstration of the value of using naturally occurring genetic variation to identify therapeutic approaches to genetically-complex diseases.

References

Greenspan, N. http://evomed.org/?p=1034. December 28, 2011.

Hall SS. Genetics: a gene of rare effect. Nature. 2013 Apr 11;496(7444):152-5. doi: 10.1038/496152a. PubMed PMID: 23579660.

Abifadel M, Varret M, Rabès JP, Allard D, Ouguerram K, Devillers M, Cruaud C,  Benjannet S, Wickham L, Erlich D, Derré A, Villéger L, Farnier M, Beucler I, Bruckert E, Chambaz J, Chanu B, Lecerf JM, Luc G, Moulin P, Weissenbach J, Prat A, Krempf M, Junien C, Seidah NG, Boileau C. Mutations in PCSK9 cause autosomal dominant hypercholesterolemia. Nat Genet. 2003 Jun;34(2):154-6. PubMed PMID: 12730697.

Cohen JC, Boerwinkle E, Mosley TH Jr, Hobbs HH. Sequence variations in PCSK9,  low LDL, and protection against coronary heart disease. N Engl J Med. 2006 Mar 23;354(12):1264-72. PubMed PMID: 16554528.

Wang Y, Huang Y, Hobbs HH, Cohen JC. Molecular characterization of proprotein  convertase subtilisin/kexin type 9-mediated degradation of the LDLR. J Lipid Res. 2012 Sep;53(9):1932-43. doi: 10.1194/jlr.M028563. Epub 2012 Jul 4. PubMed PMID: 22764087; PubMed Central PMCID: PMC3413232.