Tuesday, March 31, 2015

PharmGKB and Stanford Law School student collaboration

The Stanford Law School blog recently highlighted the collaboration between Stanford law students and PharmGKB to write a response to the FDA proposed regulation of laboratory developed tests.  Read more about the Juelsgaard Intellectual Property and Innovation Clinic students and their work.

Friday, March 27, 2015

NAT2 plays a role in insulin sensitivity

N-acetyltransferase 2 (NAT2) has long been known to metabolize xenobiotics but the endogenous substrates of NAT2 remain unknown. A new study published in “The Journal of Clinical Investigation” provides convincing evidence that NAT2 plays a role in the regulation of insulin sensitivity. The authors of the study reported that several SNPs, including the A allele at the single nucleotide polymorphism (SNP) rs1208 (c.803G>A, p.R268K) were associated with increased likelihood of insulin resistance (IR). The authors conducted a genome wide association study (GWAS) meta-analysis using genomic data and several measures of insulin sensitivity in multiple non-diabetic cohorts, including three European, one Hispanic and one East Asian cohort totaling 5,624 individuals. Although the A allele at rs1208 was not associated with IR at the genome wide significance level (P<0.05 x10^-8) in the discovery or replication cohorts it consistently showed the strongest association (P<6.4 x 10^-7) with increased degree of IR in all cohorts studied.

The authors confirmed the role of NAT2 as a modulator of insulin sensitivity by conducting in vitro studies in mouse adipocytes (3T3-L1) and myotubes. Administration of insulin caused a 50% decrease in expression of Nat1 (the mouse ortholog of NAT2) in both 3T3 cells and myotubes and siRNA mediated silencing of Nat1 caused a decrease in insulin stimulated glucose uptake, and an increase in basal lipolysis. These effects were reversed by over-expression of Nat1. In addition, Nat1 KO (-/-) and heterozygous (-/+) mice had higher fasting plasma glucose, insulin and triglyceride levels as well as decreased response to insulin during insulin tolerance tests as compared to wild-type (+/+) mice. 

These analyses provide convincing evidence that NAT2 plays a role in mediating insulin sensitivity, even though none of the NAT2 SNPs that were identified in the GWAS were significantly associated with IR at the genome wide level. Future studies could include more detailed functional analyses of individual SNPs on NAT2 expression and function, and may provide additional clues to identify the endogenous substrates of NAT2.

Read the original paper:

Knowles JW, Xie W, Zhang Z, Chennemsetty I, Assimes TL, Paananen J, Hansson O, Pankow J, Goodarzi MO, Carcamo-Orive I, Morris AP, Chen YI, Mäkinen VP, Ganna A, Mahajan A, Guo X, Abbasi F, Greenawalt DM, Lum P, Molony C, Lind L, Lindgren C, Raffel LJ, Tsao PS, Schadt EE, Rotter JI, Sinaiko A, Reaven G, Yang X, Hsiung CA, Groop L, Cordell HJ, Laakso M, Hao K, Ingelsson E, Frayling TM, Weedon MN, Walker M, Quertermous T.

J Clin Invest. 2015 Mar 23. pii: 74692. doi: 10.1172/JCI74692. [Epub ahead of print]

Thursday, March 26, 2015

New CPIC Guideline: CYP3A5 and Tacrolimus

Guidelines regarding the use of pharmacogenetic tests for CYP3A5 in dosing tacrolimus have been published in Clinical Pharmacology and Therapeutics by the Clinical Pharmacogenetics Implementation Consortium (CPIC).

Tacrolimus is an immunosuppressive agent administered to transplant recipients to prevent and treat allograft rejection. Clinical use of tacrolimus is complicated by its high inter-patient variability in pharmacokinetics and a narrow therapeutic index. As a result, management of tacrolimus usually includes therapeutic drug monitoring (TDM). Concentrations of tacrolimus are strongly influenced by CYP3A5 genotype - individuals with the CYP3A5 *1/*1 or CYP3A5 *1/*3 genotype (also known as extensive and intermediate metabolizers, respectively) have significantly lower concentrations of tacrolimus as compared to those with the *3/*3 genotype (poor metabolizers). In addition to standard TDM, adjusting the starting dose of tacrolimus based on CYP3A5 genotype may allow for a more rapid achievement of therapeutic drug concentrations. 

In the newly published guidelines, CPIC recommends increasing the starting dose by 1.5 to 2 times the recommended starting dose in CYP3A5 extensive and intermediate metabolizers. This particular CPIC dosing recommendation is unusual in that those with the extensive metabolizer phenotype (typically referred to as the "normal" metabolizer phenotype in other CYP enzymes) require an increase in dose, while those with the poor metabolizer phenotype do not require any change in dose. This is because, in the case of CYP3A5, extensive metabolizers are actually the minority in most worldwide populations (excluding those of African descent), while those with the poor metabolizer phenotype constitute the majority.

For details, see the CPIC guideline on PharmGKB.

For other CPIC guidelines see the list of CPIC publications and guidelines in progress.

Thursday, March 5, 2015

ABCB5 and haloperidol-induced toxicity: Results from a new study

Approximately 50% of patients treated with the antipsychotic drug haloperidol will develop extrapyramidal symptoms, a category that includes tremors, parkinsonism and decreased spontaneous movement. However, studies looking into the genetic variations associated with the development of these symptoms have been limited.

In a study recently published in PLOS Medicine, Zheng et al. used murine models and a human genetic association study to show a link between the ABCB5 gene and haloperidol-induced extrapyramidal symptoms (referred to as haloperidol-induced toxicity (HIT), and indicated in the murine models by "latency", or the time required for a mouse to move all four paws after being placed on an inclined wire-mesh screen). In the human genetic association study, it was the missense SNP rs17143212 in particular that was associated with haloperidol toxicities during the first 7 days of treatment, both before and after correcting for multiple testing using a permutation test.

ABCB5 is a member of the ATP-binding cassette (ABC) transporter family, and is responsible for the movement of substrates across cell membranes. Zheng et al. also used murine models to show that ABCB5 mRNA is expressed in brain capillaries, the location of the blood-brain barrier. This provides a possible mechanistic explanation for the association between the gene and HIT in mice - mouse strains with genetic variations that result in reduced ABCB5 activity may be more susceptible to HIT due to increased haloperidol concentrations in the brain. Furthermore, the authors suggest that this toxicity may actually be due to a metabolite of haloperidol, HPP+, which can induce mitochondrial toxicity that results in Parkinsonian-like symptoms.

While the authors conclude the paper by noting that other genetic factors are likely involved in the development of HIT in humans, the results from this study shed further light on the pharmacogenetics behind haloperidol-induced toxicity.

Read the original article:
The role of abcb5 alleles in susceptibility to haloperidol-induced toxicity in mice and humans.
Zheng M, Zhang H, Dill DL, Clark JB, Tu S, Yablonovitch AL, Tan MH, Zhang R, Rujescu D, Wu M, Tessarollo L, Vieira W, Gottesman MM, Deng S, Eberlin LS, Zare RN, Billard JM, Gillet JP, Li JB, Peltz G. PLoS Medicine. 2015 Feb 3;12(2):e1001782. PMID 25647612.

See the annotation for this paper on PharmGKB: