Transplantation Genetics: Current Status and Prospects
Transplantation Genetics: Current Status and Prospects
Incompatibility across key HLA alleles has traditionally been considered the main factor influencing rejection in stem cell and solid organ transplants, and has therefore been the focus of extensive investigation. Because of high genetic variability in the HLA locus, a detailed characterization of HLA mismatches between donors and recipients is not routinely achieved. The effect of specific HLA mismatches in kidney transplantation have been characterized for sometime, whereas the importance of HLA matching on outcomes in organs such as the liver is still under debate. Even in HLA identically matched kidney transplantation, some degree of rejection is still evident. Non-HLA or minor histocompatibility antigens (mHAs) resulting from a range of functional polymorphisms in the genome have been suggested to be capable of inducing strong cellular immune responses (reviewed in). In contrast to the established role of HLA, Terasaki estimated that over twice as many graft failures in HLA identical siblings at 10 years posttransplantation (38%) were due to immunological reactions to non-HLA factors compared to graft failures attributable to HLA (18%). Although our current knowledge of non-HLA antigens is still limited to a small number of loci, if one extrapolates the large number of genetic variants observed between two unrelated individuals within a donor–recipient pair, then the number of non-HLA discrepancies between any given donor and recipient would be expected to be very large.
The most rigorous way of investigating the association of mHAs with organ survival is within HLA-identical sibling donor–recipients, which is mostly restricted to hematopoietic stem cell transplantation (HSCT) and living kidney transplants. Therefore, the role of mHAs in other solid organ rejection may be more difficult to resolve due to incomplete or no matching.
The first source of mHAs was identified in the Y chromosome with at least six Y chromosome genes encoding various antigens presented by multiple MHC alleles recognized by donor T cells as foreign peptides after gender mismatched transplantation. Autosomal chromosomes also contain mHAs, with more than 40 described to date (reviewed in). Such antigens arise as a consequence of common genetic variation, especially nonsynonymous SNPs in coding regions of the genome, leading to differences in the amino acid sequence proteins between donor and recipient. The number of identified autosomal mHAs identified to date is small considering the abundance of functional variants in the human genome, but it will undoubtedly grow with the recent genomic advances. Recently, Tennessen et al applied whole exome sequencing to 15 585 human protein-coding genes in more than 2000 individuals of European and African ancestry and described more than 500 000 single nucleotide variants (SNVs) among the entire sample with an average of 13 595 variants per individual. The majority of these variants had not been previously described and were rare (minor allele frequency below 0.5%) and population-specific, with ~2% predicted to impact the function of more than 300 genes per genome assessed, highlighting the need for a deeper genome-wide examination of the donor and recipient polymorphisms for potential mHAs.
A distinct form of loss-of-function (LoF) genetic variants, homozygous-deleted CNVs (hdCNVs), is gaining traction as playing a role in transplant rejection. hdCNVs have been identified through large-scale GWAS and sequencing data sets. Possession of hdCNVs may manifest in a "normal" phenotype, with the term "disposable" or "dispensable," genes often used to describe such variants. Cases where unaltered phenotypes are observed with carriage of hdCNVs suggest that there is either compensation for the hdCNV(s), or their function is no longer necessary. For example, hdCNVs of UGT2B17, a gene expressed in graft-versus-host disease-affected tissues, have been characterized as mHAs in HSCT. McCarroll et al recently analyzed six common CNV deletions spanning genes in three HSCT cohorts (totaling 1345 HLA-identical sibling donor–recipient pairs). The authors found that risk of acute graft-versus-host disease was greater in recipients where UGT2B17 hdCNV was mismatched, that is zero gene copies present in the donor but one or two copies present in recipient (OR = 2.5; 95% CI 1.4–4.6). However, they only used markers for CNVs that were detectable on the initial GWAS arrays, which were limited in content, as many CNV regions known now were not adequately probed or captured. Common hdCNVs may thus be important as they may play an active role in graft longevity.
Recent whole exome and genome sequencing indicates that each individual carries numerous genetic variants predicted to cause LoF of protein-coding genes. MacArthur et al recently studied 2951 putative LoF variants obtained from whole genome sequencing from 185 human individuals from the 1000 Genome Project (derived from typical population samples), identifying and validating rare and likely deleterious LoF alleles, as well as common LoF variants in nonessential genes in this data set. They estimated that a typical human genome contains ~100 genuine LoF variants with ~20 genes inactivated in both copies, indicating unexpected redundancy in the human genome and suggesting that there are numerous mutations that are "private" to each personal genome. Since the immune system of an individual carrying LoF variants in both copies of a given gene may have had no previous exposure to protein(s) encoded by that gene(s), cellular or humoral immune recognition of that protein as an alloantigen in the grafted organ could very plausibly contribute to risk of rejection. LoF impacting the expression or translation of both copies of a given gene, especially in the transplanted organ of interest, is thus a plausible source of donor–recipient genomic incompatibilities, and may underpin rejection.
The emergence of large numbers of potential non-HLA incompatibilities with varying potential levels of immunogenicity highlights the clear need for deeper capture and assessment of SNVs, SNPs and CNVs in donors and recipients. Deep sequencing technologies will undoubtedly be key to unraveling common and rare LoFs and hdCNVs in population-scale diversity as well as capturing "private" individual-level polymorphisms to characterize the histocompatibility determinants involved in the biological mechanisms of alloimmunity. It should be noted that while there is low statistical power to detect such LoFs and hdCNVs at an association level, there is a lot of promise to follow up putative individual donor–recipient genomic incompatibility through the use of autoantibody testing in sera and/or tissues from the recipient postoperatively. Such approaches offer the promise of clinical utility through closer monitoring of a priori mHAs or potentially tolerizing recipients to a given gene product.
Over the last decade there have been unprecedented advances in the assessment of human genomic diversity across the major human populations through the development of high-throughput genotyping and deep sequencing technologies, as well as the development of population scale genomic maps such as the International HapMap Project. These tools have been applied to deeply characterize the genetic architecture of common and rare diseases and evoked conditions such as drug severe adverse events. Unfortunately, to date, the field of organ transplantation has not benefited from many of these advances. While over a thousand genetic association studies on organ rejection have been published so far, they are primarily candidate-gene based and suffer from many of the usual pitfalls of genetic association studies such as lack of adequate sample sizes, retrospective study designs, noninclusion of appropriate covariates such as ethnicity, lack of replication and proper statistical correction when multiple hypotheses are tested. Table 1 shows a review of appropriately designed published studies on organ transplantation outcomes. These studies were initially collated from PubMed and filtered for appropriate study size and design, defining appropriate as having at least a sample size of 200 individuals, including covariates, accounting for ethnicity and adjusting for multiple testing, when necessary. As observed in Table 1, genetic variation in up to 47 genes encoding cytokines, chemokines, cell-adhesion molecules, components of the renin–angiotensin–aldosterone pathway, coagulation and aggregation factors has been widely investigated and associated with outcomes such as delayed graft function, acute or chronic rejection or graft failure.
However, results do not consistently replicate across studies. This lack of replication can be attributed to both the above-mentioned limitations of candidate-gene study designs and the complexity and diversity of clinical phenotypes. In addition to differences in protocols for immunosuppressive regimes, ascertainment of outcomes may differ by study—e.g. the criteria used to define acute rejection and chronic allograft dysfunction might vary among transplant centers: these all introduce further heterogeneity between studies. Specifically, in the case of rejection, studies may not always base the diagnosis on biopsy results and biopsy results themselves may be inaccurate due to interoperator differences. It is essential, therefore, to use objective and standard definitions for transplantation outcomes in order to obtain consistent results to homogenize studies.
Thus future studies should aim to define phenotypes with precision and use a rigorous genetic approach, which preferably incorporates a hypothesis-free design such as GWAS to gain the most insight into genetic risk factors for organ transplantation.
Genetic background is thought to account for as much as 95% of the variability in drug disposition and therapeutic effects. Regimes for immunosuppressants are typically characterized by wide pharmacokinetic interindividual variability and narrow therapeutic indexes, which often makes the ideal balance between sufficient immunosuppression and drug toxicity difficult to achieve. Thus, the discovery of genetic markers responsible for the interindividual variation in response to immunosuppressive therapy is an intensive area of ongoing research in transplantation. Ekberg et al evaluated the immunosuppressant drug exposures in 1645 renal transplant patients randomly assigned to four treatment groups: (1) standard-dose cyclosporine, (2) low-dose cyclosporine, (3) low-dose tacrolimus and (4) low-dose sirolimus, and observed that up to 90% of patients experienced at least one adverse event during treatment. Both biopsy-proven acute rejection and severe adverse events were similar in groups, ranging from 2.3% to 37% and 43% to 53%, respectively, depending on the drug, with the highest events observed in the low sirolimus dose group. In efforts to avoid such issues, therapeutic drug monitoring is being routinely performed, but it is currently assessed posttransplant and thus is not used for determining the optimal immunosuppressant starting dose, which is still established by an iterative postoperative approach.
Alternative strategies incorporating pharmacogenetics hold great promise as complementary tools in drug monitoring to better guide individual therapies and doses. While the number of studies focusing on the pharmacogenetics of immunosuppressants has increased dramatically over the last few years, to date, many of these are underpowered that suffer from the typical candidate gene association study pitfalls. One clear exception is the robust association observed for rs776746 in CYP3A5, the primary enzyme involved in the metabolism of tacrolimus, the most frequently prescribed immunosuppressant drug worldwide. While nongenetic factors such as recipient gender, age, diabetes status and calcium channel blockers, such as some newer class of antifungals and grapefruit juice, influence tacrolimus levels, the rs776746 SNP is very important in tacrolimus clearance, with dosing requirements as well as time to therapeutic concentrations explaining up to 45% of the dose and 30% of clearance variability. The rs776746-A form is classically referred to as CYP3A5*1, while rs776746-G is referred to as CYP3A5*3. The latter is a noncoding variant that results in a cryptic splicing site, which causes 131 nucleotides of the intronic sequence to be inserted in the mRNA. The consequence is the introduction of a premature stop codon that truncatesCYP3A5 and leads to complete lack of CYP3A5 translation in *3 homozygotes. The prevalence of this CYP3A5*3 allele is as high as 90% in individuals of European ancestry while ~90% of African Americans carry at least one of the common fully functional CYP3A5*1 alleles (*1/*1 or *1/*3). These frequency differences in CYP3A5*3 make it one the most important genetic markers of interindividual and interethnic differences observed in CYP3A-dependent drug responses and clearance. There is also evidence of an additional rare variant, CYP3A5*6, which is associated with tacrolimus trough levels.
Table 2, Table 3 and Table 4 review genetic association studies for tacrolimus, cyclosporine and mycophenolic acid regarding pharmacokinetics, transplant outcomes and adverse events. Due to the previously mentioned limitations of the candidate gene approach, we only focus on studies with over 100 individuals and appropriate quality control for phenotype and genotype data. Several studies have confirmed the major impact of CYP3A5 on tacrolimus dose requirements and renal clearance; however, similar to genetic association studies on graft outcomes, it has been challenging to robustly replicate additional findings. Apart from CYP3A5, to date, only the associations of ABCB1 rs1045642, rs1128503, rs2032582 and cyclosporine pharmacokinetics and CYP3A4 rs2740574 and rs3559936 with tacrolimus pharmacokinetics appear to be consistent.
All of the rigorously designed studies to date conclude that there is a need to dose tacrolimus patients by the CYP3A5 rs776746 genotype. Thervet et al, during the first 6 days posttransplantation, studied the effect of guiding tacrolimus dosing based on the number of CYP3A5 functional alleles, on plasma drug concentration. Patients were randomly assigned to either a standard initial dose of tacrolimus (0.2 mg/kg/day) or a genotype-adjusted dose, who received 0.3 mg/kg/day if carrying allele *1 and 0.15 mg/kg/day if *3/*3. This randomized clinical trial found significantly higher number of patients achieving optimal dosing in the genotype-based dosing group, more rapidly and with fewer dose modifications. However, the trial was not designed to investigate hard clinical outcomes such as graft failure—the acid test of a pharmacogenetic test.
Passey et al established a dosing algorithm for tacrolimus including clinical, genetic and ethnic information. From a cohort of more than 600 kidney recipients, compared to those with G/G genotype, those with the rs776746 A/G genotype experienced a 69% increase in tacrolimus clearance and A/A genotype a 100% increase in clearance. The final dosing algorithm included CYP3A5 rs776746 genotype, days posttransplantation, age, steroid and calcium channel blocker use as independent predictors of the outcome.
The incorporation of CYP3A5 genetic information into tacrolimus dosing algorithm will likely be a first major step toward precision genotype-guided dosing in the transplantation setting. Optimal use of pharmacogenetics will require a deeper knowledge of additional genetics factors governing drug disposition, efficacy and toxicity. The application of the genomic advances to this field has the real potential of optimizing dosing strategies for immunosuppressive drugs, avoiding serious adverse and improving patient management after organ transplantation.
It is also worth noting the large impact that donor and recipient ancestry has in transplantation. Many studies have reported greater risks of rejection and mortality in African Americans when their organs are used as donor grafts or when they are a recipient of a graft. Callender et al also confirmed lower kidney survival rates among African Americans compared to all other ethnic groups and showed that this ethnic group also harbored the shortest half-life after kidney transplantation: only 5.3 years versus 12.2 years for Asians, 10.2 years for individuals of European descent and 9.0 years for Hispanics.
In 2010, Li et al published a pivotal study showing that Chinese and African human genomes differed by approximately 5 Mb of unique sequence and over 240 potential genes differing between these sequences. Such "pan-genome" data sets illustrate the differences between populations and emphasize the need for further characterization of such genomic differences in the transplantation arena. In addition to the highly polymorphic differences in HLA loci already evident in the different ethnic groups, variants like LoFs or hdCNVs with the potential of acting as incompatibility antigens, or genetic risk factors for postoperative transplant outcomes as well as pharmacogenetic markers, exhibit substantial variability interethnically.
Genetics of Transplantation Outcomes
Incompatibility across key HLA alleles has traditionally been considered the main factor influencing rejection in stem cell and solid organ transplants, and has therefore been the focus of extensive investigation. Because of high genetic variability in the HLA locus, a detailed characterization of HLA mismatches between donors and recipients is not routinely achieved. The effect of specific HLA mismatches in kidney transplantation have been characterized for sometime, whereas the importance of HLA matching on outcomes in organs such as the liver is still under debate. Even in HLA identically matched kidney transplantation, some degree of rejection is still evident. Non-HLA or minor histocompatibility antigens (mHAs) resulting from a range of functional polymorphisms in the genome have been suggested to be capable of inducing strong cellular immune responses (reviewed in). In contrast to the established role of HLA, Terasaki estimated that over twice as many graft failures in HLA identical siblings at 10 years posttransplantation (38%) were due to immunological reactions to non-HLA factors compared to graft failures attributable to HLA (18%). Although our current knowledge of non-HLA antigens is still limited to a small number of loci, if one extrapolates the large number of genetic variants observed between two unrelated individuals within a donor–recipient pair, then the number of non-HLA discrepancies between any given donor and recipient would be expected to be very large.
The most rigorous way of investigating the association of mHAs with organ survival is within HLA-identical sibling donor–recipients, which is mostly restricted to hematopoietic stem cell transplantation (HSCT) and living kidney transplants. Therefore, the role of mHAs in other solid organ rejection may be more difficult to resolve due to incomplete or no matching.
The first source of mHAs was identified in the Y chromosome with at least six Y chromosome genes encoding various antigens presented by multiple MHC alleles recognized by donor T cells as foreign peptides after gender mismatched transplantation. Autosomal chromosomes also contain mHAs, with more than 40 described to date (reviewed in). Such antigens arise as a consequence of common genetic variation, especially nonsynonymous SNPs in coding regions of the genome, leading to differences in the amino acid sequence proteins between donor and recipient. The number of identified autosomal mHAs identified to date is small considering the abundance of functional variants in the human genome, but it will undoubtedly grow with the recent genomic advances. Recently, Tennessen et al applied whole exome sequencing to 15 585 human protein-coding genes in more than 2000 individuals of European and African ancestry and described more than 500 000 single nucleotide variants (SNVs) among the entire sample with an average of 13 595 variants per individual. The majority of these variants had not been previously described and were rare (minor allele frequency below 0.5%) and population-specific, with ~2% predicted to impact the function of more than 300 genes per genome assessed, highlighting the need for a deeper genome-wide examination of the donor and recipient polymorphisms for potential mHAs.
A distinct form of loss-of-function (LoF) genetic variants, homozygous-deleted CNVs (hdCNVs), is gaining traction as playing a role in transplant rejection. hdCNVs have been identified through large-scale GWAS and sequencing data sets. Possession of hdCNVs may manifest in a "normal" phenotype, with the term "disposable" or "dispensable," genes often used to describe such variants. Cases where unaltered phenotypes are observed with carriage of hdCNVs suggest that there is either compensation for the hdCNV(s), or their function is no longer necessary. For example, hdCNVs of UGT2B17, a gene expressed in graft-versus-host disease-affected tissues, have been characterized as mHAs in HSCT. McCarroll et al recently analyzed six common CNV deletions spanning genes in three HSCT cohorts (totaling 1345 HLA-identical sibling donor–recipient pairs). The authors found that risk of acute graft-versus-host disease was greater in recipients where UGT2B17 hdCNV was mismatched, that is zero gene copies present in the donor but one or two copies present in recipient (OR = 2.5; 95% CI 1.4–4.6). However, they only used markers for CNVs that were detectable on the initial GWAS arrays, which were limited in content, as many CNV regions known now were not adequately probed or captured. Common hdCNVs may thus be important as they may play an active role in graft longevity.
Recent whole exome and genome sequencing indicates that each individual carries numerous genetic variants predicted to cause LoF of protein-coding genes. MacArthur et al recently studied 2951 putative LoF variants obtained from whole genome sequencing from 185 human individuals from the 1000 Genome Project (derived from typical population samples), identifying and validating rare and likely deleterious LoF alleles, as well as common LoF variants in nonessential genes in this data set. They estimated that a typical human genome contains ~100 genuine LoF variants with ~20 genes inactivated in both copies, indicating unexpected redundancy in the human genome and suggesting that there are numerous mutations that are "private" to each personal genome. Since the immune system of an individual carrying LoF variants in both copies of a given gene may have had no previous exposure to protein(s) encoded by that gene(s), cellular or humoral immune recognition of that protein as an alloantigen in the grafted organ could very plausibly contribute to risk of rejection. LoF impacting the expression or translation of both copies of a given gene, especially in the transplanted organ of interest, is thus a plausible source of donor–recipient genomic incompatibilities, and may underpin rejection.
Need for Second-generation Sequencing
The emergence of large numbers of potential non-HLA incompatibilities with varying potential levels of immunogenicity highlights the clear need for deeper capture and assessment of SNVs, SNPs and CNVs in donors and recipients. Deep sequencing technologies will undoubtedly be key to unraveling common and rare LoFs and hdCNVs in population-scale diversity as well as capturing "private" individual-level polymorphisms to characterize the histocompatibility determinants involved in the biological mechanisms of alloimmunity. It should be noted that while there is low statistical power to detect such LoFs and hdCNVs at an association level, there is a lot of promise to follow up putative individual donor–recipient genomic incompatibility through the use of autoantibody testing in sera and/or tissues from the recipient postoperatively. Such approaches offer the promise of clinical utility through closer monitoring of a priori mHAs or potentially tolerizing recipients to a given gene product.
Genetic Risk Factors for Transplantation Outcomes Beyond HLA
Over the last decade there have been unprecedented advances in the assessment of human genomic diversity across the major human populations through the development of high-throughput genotyping and deep sequencing technologies, as well as the development of population scale genomic maps such as the International HapMap Project. These tools have been applied to deeply characterize the genetic architecture of common and rare diseases and evoked conditions such as drug severe adverse events. Unfortunately, to date, the field of organ transplantation has not benefited from many of these advances. While over a thousand genetic association studies on organ rejection have been published so far, they are primarily candidate-gene based and suffer from many of the usual pitfalls of genetic association studies such as lack of adequate sample sizes, retrospective study designs, noninclusion of appropriate covariates such as ethnicity, lack of replication and proper statistical correction when multiple hypotheses are tested. Table 1 shows a review of appropriately designed published studies on organ transplantation outcomes. These studies were initially collated from PubMed and filtered for appropriate study size and design, defining appropriate as having at least a sample size of 200 individuals, including covariates, accounting for ethnicity and adjusting for multiple testing, when necessary. As observed in Table 1, genetic variation in up to 47 genes encoding cytokines, chemokines, cell-adhesion molecules, components of the renin–angiotensin–aldosterone pathway, coagulation and aggregation factors has been widely investigated and associated with outcomes such as delayed graft function, acute or chronic rejection or graft failure.
However, results do not consistently replicate across studies. This lack of replication can be attributed to both the above-mentioned limitations of candidate-gene study designs and the complexity and diversity of clinical phenotypes. In addition to differences in protocols for immunosuppressive regimes, ascertainment of outcomes may differ by study—e.g. the criteria used to define acute rejection and chronic allograft dysfunction might vary among transplant centers: these all introduce further heterogeneity between studies. Specifically, in the case of rejection, studies may not always base the diagnosis on biopsy results and biopsy results themselves may be inaccurate due to interoperator differences. It is essential, therefore, to use objective and standard definitions for transplantation outcomes in order to obtain consistent results to homogenize studies.
Thus future studies should aim to define phenotypes with precision and use a rigorous genetic approach, which preferably incorporates a hypothesis-free design such as GWAS to gain the most insight into genetic risk factors for organ transplantation.
Pharmacogenetics of Immunosuppressant Responses
Genetic background is thought to account for as much as 95% of the variability in drug disposition and therapeutic effects. Regimes for immunosuppressants are typically characterized by wide pharmacokinetic interindividual variability and narrow therapeutic indexes, which often makes the ideal balance between sufficient immunosuppression and drug toxicity difficult to achieve. Thus, the discovery of genetic markers responsible for the interindividual variation in response to immunosuppressive therapy is an intensive area of ongoing research in transplantation. Ekberg et al evaluated the immunosuppressant drug exposures in 1645 renal transplant patients randomly assigned to four treatment groups: (1) standard-dose cyclosporine, (2) low-dose cyclosporine, (3) low-dose tacrolimus and (4) low-dose sirolimus, and observed that up to 90% of patients experienced at least one adverse event during treatment. Both biopsy-proven acute rejection and severe adverse events were similar in groups, ranging from 2.3% to 37% and 43% to 53%, respectively, depending on the drug, with the highest events observed in the low sirolimus dose group. In efforts to avoid such issues, therapeutic drug monitoring is being routinely performed, but it is currently assessed posttransplant and thus is not used for determining the optimal immunosuppressant starting dose, which is still established by an iterative postoperative approach.
Alternative strategies incorporating pharmacogenetics hold great promise as complementary tools in drug monitoring to better guide individual therapies and doses. While the number of studies focusing on the pharmacogenetics of immunosuppressants has increased dramatically over the last few years, to date, many of these are underpowered that suffer from the typical candidate gene association study pitfalls. One clear exception is the robust association observed for rs776746 in CYP3A5, the primary enzyme involved in the metabolism of tacrolimus, the most frequently prescribed immunosuppressant drug worldwide. While nongenetic factors such as recipient gender, age, diabetes status and calcium channel blockers, such as some newer class of antifungals and grapefruit juice, influence tacrolimus levels, the rs776746 SNP is very important in tacrolimus clearance, with dosing requirements as well as time to therapeutic concentrations explaining up to 45% of the dose and 30% of clearance variability. The rs776746-A form is classically referred to as CYP3A5*1, while rs776746-G is referred to as CYP3A5*3. The latter is a noncoding variant that results in a cryptic splicing site, which causes 131 nucleotides of the intronic sequence to be inserted in the mRNA. The consequence is the introduction of a premature stop codon that truncatesCYP3A5 and leads to complete lack of CYP3A5 translation in *3 homozygotes. The prevalence of this CYP3A5*3 allele is as high as 90% in individuals of European ancestry while ~90% of African Americans carry at least one of the common fully functional CYP3A5*1 alleles (*1/*1 or *1/*3). These frequency differences in CYP3A5*3 make it one the most important genetic markers of interindividual and interethnic differences observed in CYP3A-dependent drug responses and clearance. There is also evidence of an additional rare variant, CYP3A5*6, which is associated with tacrolimus trough levels.
Table 2, Table 3 and Table 4 review genetic association studies for tacrolimus, cyclosporine and mycophenolic acid regarding pharmacokinetics, transplant outcomes and adverse events. Due to the previously mentioned limitations of the candidate gene approach, we only focus on studies with over 100 individuals and appropriate quality control for phenotype and genotype data. Several studies have confirmed the major impact of CYP3A5 on tacrolimus dose requirements and renal clearance; however, similar to genetic association studies on graft outcomes, it has been challenging to robustly replicate additional findings. Apart from CYP3A5, to date, only the associations of ABCB1 rs1045642, rs1128503, rs2032582 and cyclosporine pharmacokinetics and CYP3A4 rs2740574 and rs3559936 with tacrolimus pharmacokinetics appear to be consistent.
All of the rigorously designed studies to date conclude that there is a need to dose tacrolimus patients by the CYP3A5 rs776746 genotype. Thervet et al, during the first 6 days posttransplantation, studied the effect of guiding tacrolimus dosing based on the number of CYP3A5 functional alleles, on plasma drug concentration. Patients were randomly assigned to either a standard initial dose of tacrolimus (0.2 mg/kg/day) or a genotype-adjusted dose, who received 0.3 mg/kg/day if carrying allele *1 and 0.15 mg/kg/day if *3/*3. This randomized clinical trial found significantly higher number of patients achieving optimal dosing in the genotype-based dosing group, more rapidly and with fewer dose modifications. However, the trial was not designed to investigate hard clinical outcomes such as graft failure—the acid test of a pharmacogenetic test.
Passey et al established a dosing algorithm for tacrolimus including clinical, genetic and ethnic information. From a cohort of more than 600 kidney recipients, compared to those with G/G genotype, those with the rs776746 A/G genotype experienced a 69% increase in tacrolimus clearance and A/A genotype a 100% increase in clearance. The final dosing algorithm included CYP3A5 rs776746 genotype, days posttransplantation, age, steroid and calcium channel blocker use as independent predictors of the outcome.
The incorporation of CYP3A5 genetic information into tacrolimus dosing algorithm will likely be a first major step toward precision genotype-guided dosing in the transplantation setting. Optimal use of pharmacogenetics will require a deeper knowledge of additional genetics factors governing drug disposition, efficacy and toxicity. The application of the genomic advances to this field has the real potential of optimizing dosing strategies for immunosuppressive drugs, avoiding serious adverse and improving patient management after organ transplantation.
Ancestry as a Genetic Risk Factor for Transplantation Outcomes
It is also worth noting the large impact that donor and recipient ancestry has in transplantation. Many studies have reported greater risks of rejection and mortality in African Americans when their organs are used as donor grafts or when they are a recipient of a graft. Callender et al also confirmed lower kidney survival rates among African Americans compared to all other ethnic groups and showed that this ethnic group also harbored the shortest half-life after kidney transplantation: only 5.3 years versus 12.2 years for Asians, 10.2 years for individuals of European descent and 9.0 years for Hispanics.
In 2010, Li et al published a pivotal study showing that Chinese and African human genomes differed by approximately 5 Mb of unique sequence and over 240 potential genes differing between these sequences. Such "pan-genome" data sets illustrate the differences between populations and emphasize the need for further characterization of such genomic differences in the transplantation arena. In addition to the highly polymorphic differences in HLA loci already evident in the different ethnic groups, variants like LoFs or hdCNVs with the potential of acting as incompatibility antigens, or genetic risk factors for postoperative transplant outcomes as well as pharmacogenetic markers, exhibit substantial variability interethnically.
Source...