Concordance between glucose-6-phosphate dehydrogenase (G6PD) genotype and phenotype and rasburicase use in patients with hematologic malignancies

Abstract

Phenotypic rather than genotypic tests remain the gold standard for diagnosing glucose-6-phosphate dehydrogenase (G6PD) deficiency. However, with increasing use of genomic arrays and whole exome or genome sequencing, G6PD genetic data are increasingly available. We examined the utility of G6PD genetic data in patients with hematologic malignancies and the association of G6PD genotype and phenotype with rasburicase-induced methemoglobinemia. We analyzed G6PD activity for 990 patients. Genotype data were available from the Affymetrix DMET array (n = 379), whole exome sequencing (n = 374), and/or the Illumina exome array (n = 634) for 645 patients. Medical records of 341 patients with methemoglobin measures were assessed for the administration of rasburicase. We observed 5 non-synonymous SNPs, 4 of which were known to be associated with deficient G6PD activity (WHO Class I-III). Genotyping 367 males resulted in a positive predictive value of 81.8% (47.8-96.8%), and two males with a Class I-III allele having normal activity both received a red blood cell transfusion prior to the activity assay. However, genotyping males had only 39.1% (20.5-61.2%) sensitivity. Two of the 12 heterozygous females had deficient G6PD activity. Rasburicase-induced methemoglobinemia occurred in 6 patients, 5 of whom had at least one Class I-III allele, despite 2 of these having normal G6PD activity. We conclude that although an apparent nondeficient genotype does not necessarily imply a normal phenotype, a deficient genotype result indicates a deficient phenotype in those without transfusions, and may be a useful adjuct to phenotype to prevent adverse drug reactions.

Figure 1.. G6PD patient cohort.

990 patients with hematological malignancies had at least one G6PD activity measurement. We removed 8 patients who had high leukocyte counts (>100 × 10³ cells/µl) who were measured before December 1996 because we could not confirm that the buffy coat free method was performed. Four more patients were removed because they had two conflicting lab values (one normal and one deficient activity), for a total of 978 patients with evaluable G6PD activity. 645 of these patients had genotyping information and assigned G6PD genotype.341 patients with G6PD activity measurements had at least one methemoglobin lab value in their medical records. 320 of these patients had normal G6PD activity and 21 had deficient G6PD activity.

Figure 2.. 5 nonsynonymous SNPs in the G6PD gene were observed.

Lines represent G6PD variants reported in the CPIC guidelines (6), and color indicates the WHO classification. SNPs interrogated in our cohort are listed by their codon number, amino acid substitution, and common name. The colored circles indicate which platforms include each SNP. The number in the circles indicates how many patients interrogated on that platform harbored that variant SNP. The gray line represents the whole exome sequencing coverage plotted against the left y-axis. The purple shading indicates the N-terminus domain, and the green indicates the C-terminus domain. We observed four Class I-III variants and one Class IV variant.

Figure 3.. Genotyping G6PD has 81.8% positive predictive value and 39.1% sensitivity to predict G6PD phenotypic deficiency.

The assigned phenotype based on genotype according to the CPIC guidelines is in parentheses in the flowchart. Nine of the 23 males with deficient activity were also deficient according to genotype, resulting in a sensitivity of 39.1% (20.5–61.2%). Nine of the 11 males with a Class I-III allele had deficient G6PD activity, resulting in a positive predictive value of 81.8% (47.8–96.8%). The two patients with a Class I-III allele with normal G6PD activity had received a transfusion (**) prior to the activity measure. The 95% confidence interval is listed in parentheses.

Figure 4.. G6PD genotype in females does not correspond well with phenotype.

Of 279 females, 268 had normal and 11 had deficient activity. No female was homozygous for a Class I-III allele, and 10/268 (3.7%) of those with normal activity and 2/11 (18%) of those with deficient activity carried one Class I-III allele. Of the 12 “variable” females (those heterozygous for a Class I-III allele), only 2 had deficient activity (16.7%). 258 out of 267 patients with only Class IV alleles had normal G6PD activity, resulting in a negative predictive value of 96.6% (93.5–98.3%).

Figure 5.. 60 patients with G6PD activity also had at least one methemoglobin measurement and received rasburicase at some point in therapy.

10 of these patients had deficient G6PD activity, and 50 had normal activity. The patients were then classified according to genotype. 18 patients had a methemoglobin level at the time of rasburicase administration: 3 patients with deficient activity and 15 patients with normal activity. The three patients with deficient activity all had a Class I-III allele and developed methemoglobinemia (>3%) after rasburicase administration. Of the 15 patients with normal activity who had evaluable methemoglobin after rasburicase, 3 developed methemoglobinemia: one male patient hemizygous for a Class I-III allele (this is the patient (**) who had received a transfusion prior to the G6PD activity assay); one was a female patient heterozygous for a Class I-III allele; and one was a female who did not have a Class I-III. Thus, of the 18 patients who had methemoglobin measurements at the time of rasburicase administration, 6 patients developed methemoglobinemia after rasburicase, and 5 of these 6 had a Class I-III allele.

Figure 6.. 341 patients had at least one methemoglobin measurement in their medical record, 21 of whom had deficient G6PD activity.

27 patients developed methemoglobinemia at some point during therapy (12 males and 15 females). All 3 patients with deficient G6PD activity developed elevated methemoglobinemia (>3%) after rasburicase administration, and all had a class I-III allele. Of the 24 patients with normal G6PD activity who developed methemoglobinemia at any time during therapy, 3 experienced methemoglobinemia after rasburicase administration. The one male was hemizygous for a Class I-III allele (he had received a transfusion prior to the activity assay**). One female was heterozygous for a Class I-III allele, and one female did not have a Class I-III allele (but was interrogated by the Exomechip array only). Of the other 21 patients with normal G6PD activity who experienced methemoglobinemia, 10 patients received another medication associated with methemoglobinemia.