Mixture models of serum iron measures in population screening for hemochromatosis and iron overload

Bivariate mixture models for the joint distribution of repeated serum ferritin and transferrin saturation measured 12 years apart in a cohort of healthy middle-aged Australians

Homozygosity for the p.C282Y substitution in the HFE protein encoded by the hemochromatosis gene on chromosome 6p (HFE) is a common genetic trait that increases susceptibility to iron overload. McLaren et al. used bivariate mixture modeling to analyze the joint population distribution of transferrin saturation (TS) and serum ferritin concentration (SF) measured for participants in the Hemochromatosis and Iron Overload Screening (HEIRS) Study. They identified four components (C1, C2, C3, and C4) with successively increasing means for TS and SF. They demonstrated that bivariate mixture modeling in TS and SF reflect the genetic locus of HFE and may isolate p.C282Y homozygotes from the general population. In the current study we used data from the another large cohort, the Australian HealthIron study of genetic and environmental modifiers of hereditary hemochromatosis, to validate the component analysis approach, to examine stability of component proportions over time and to determine if TS and SF values from an individual move between components at baseline and follow-up. Because sampling fractions from each p.C282Y / p.H63D genotype stratum are not equal, we used frequency weights based on the inverse of the probability of selection for invitation to participate. In the weighted female analytic cohorts, C4 captured most of C282Y homozygotes, and C2 was the largest component. We identified four components from the weighted male analytic cohort and C4 captured most of p.C282Y homozygotes. The bivariate mixture modeling approach suggested that the model is transferable from one white population to another, although estimated means within components may differ.

Evaluation of a 7-Gene Genetic Profile for Athletic Endurance Phenotype in Ironman Championship Triathletes

Polygenic profiling has been proposed for elite endurance performance, using an additive model determining the proportion of optimal alleles in endurance athletes. To investigate this model’s utility for elite triathletes, we genotyped seven polymorphisms previously associated with an endurance polygenic profile (ACE Ins/Del, ACTN3 Arg577Ter, AMPD1 Gln12Ter, CKMM 1170bp/985+185bp, HFE His63Asp, GDF8 Lys153Arg and PPARGC1A Gly482Ser) in a cohort of 196 elite athletes who participated in the 2008 Kona Ironman championship triathlon. Mean performance time (PT) was not significantly different in individual marker analysis. Age, sex, and continent of origin had a significant influence on PT and were adjusted for. Only the AMPD1 endurance-optimal Gln allele was found to be significantly associated with an improvement in PT (model p = 5.79 x 10⁻¹⁷, AMPD1 genotype p = 0.01). Individual genotypes were combined into a total genotype score (TGS); TGS distribution ranged from 28.6 to 92.9, concordant with prior studies in endurance athletes (mean±SD: 60.75±12.95). TGS distribution was shifted toward higher TGS in the top 10% of athletes, though the mean TGS was not significantly different (p = 0.164) and not significantly associated with PT even when adjusted for age, sex, and origin. Receiver operating characteristic curve analysis determined that TGS alone could not significantly predict athlete finishing time with discriminating sensitivity and specificity for three outcomes (less than median PT, less than mean PT, or in the top 10%), though models with the age, sex, continent of origin, and either TGS or AMPD1 genotype could. These results suggest three things: that more sophisticated genetic models may be necessary to accurately predict athlete finishing time in endurance events; that non-genetic factors such as training are hugely influential and should be included in genetic analyses to prevent confounding; and that large collaborations may be necessary to obtain sufficient sample sizes for powerful and complex analyses of endurance performance.

Should we screen for hereditary hemochromatosis in healthy Lebanese: a pilot study

Hereditary hemochromatosis (HHC) is a genetic disorder of iron metabolism characterized by abnormal accumulation of iron that may lead to organ damage and death. Diagnosis is usually based on various genetic and phenotypic criteria. The study goals were to perform mutation analysis for 18 different mutations associated with HHC in healthy Lebanese, determine their allele frequency, and compare iron-overload status in identified carriers versus those found to be wild-type for mutations analyzed. 116 healthy adults (59 males and 57 females) underwent DNA testing for 18 different HHC mutations, and biochemical testing for percent transferrin saturation (%TS) and ferritin. C282Y mutation was not detected. Only H63D mutation (rs1799945) was found with an overall carrier frequency of 25.8% (24.1% heterozygous and 1.7% homozygous). %TS and ferritin differed significantly between genders. %TS and ferritin were significantly higher in males with H63D mutation when compared to males with wild-type (P=0.001, 0.019; respectively); but not in females. The proportion of subjects with increased %TS and serum ferritin was not statistically different between those with H63D mutation and the wild-type in either gender. In addition, none of the subjects had concurrent increase in %TS and ferritin. In conclusion, the H63D carrier frequency in healthy Lebanese is comparable to other populations in the region, and it does not result in significant biochemical iron overload. Moreover, in the absence of the C282Y mutation, genetic screening for HHC is not recommended according to this preliminary study in healthy Lebanese.

Hemochromatosis genotypes and risk of iron overload--a meta-analysis

PURPOSE

The incomplete phenotypic penetrance of high iron Fe genotypes in relation to hemochromatosis poses a practical problem in the interpretation of the genotyping results by clinicians. We carried out meta-analyses of the associations between hemochromatosis genotypes C282Y/C282Y, C282Y/H63D, C282Y/wild-type, H63D/H63D, H63D/wild-type, versus wild-type/wild-type and iron overload, both provisional (elevated serum iron markers) and documented (elevated serum iron markers associated with evidence of iron excess based on liver biopsy and/or quantitative phlebotomy).

METHODS

After reviewing 3572 article titles and evaluating 92 articles in detail, odds ratios were pooled from 43 study populations (9986 cases and 25,492 controls) using a random-effects model.

RESULTS

Homozygosity for either variant or compound heterozygosity was associated with both provisional and documented iron overload. Single heterozygosity conferred no risk for elevated hepatic iron index and/or mobilizable iron by quantitative phlebotomy. In patients with clinical hereditary hemochromatosis, no evidence of provisional and documented iron overload with transferrin saturation (TS) values greater than 55% was evidenced for C282Y and H63D single heterozygotes whereas documented iron overload including TS of 45% to 50% was weakly associated with C282Y/wild-type genotype; H63D/H63D genotype was not associated with documented iron overload in patients with TS values of 45% to 50%.

CONCLUSIONS

The results, mainly from case-control studies, cannot necessarily be extrapolated to the general population.

Hereditary hemochromatosis: insights from the Hemochromatosis and Iron Overload Screening (HEIRS) Study

Hemochromatosis comprises a group of inherited disorders resulting from mutations of genes involved in regulating iron metabolism. The multicenter, multi-ethnic Hemochromatosis and Iron Overload Screening (HEIRS) Study screened approximately 100,000 participants in the US and Canada, testing for HFE mutations, serum ferritin and transferrin saturation. As in other studies, HFE C282Y homozygosity was common in Caucasians but rare in other ethnic groups, and there was a marked heterogeneity of disease expression in C282Y homozygotes. Nevertheless, this genotype was often associated with elevations of serum ferritin and transferrin saturation and with iron stores of more than four grams in men but not in women. If liver biopsy was performed, in some cases because of evidence of hepatic dysfunction, fibrosis or cirrhosis was often found. Combined elevations of serum ferritin and transferrin saturation were observed in non-C282Y homozygotes of all ethnic groups, most prominently Asians, but not often with iron stores of more than four grams. Future studies to discover modifier genes that affect phenotypic expression in C282Y hemochromatosis should help identify patients who are at greatest risk of developing iron overload and who may benefit from continued monitoring of iron status to detect progressive iron loading.

Bivariate mixture modeling of transferrin saturation and serum ferritin concentration in Asians, African Americans, Hispanics, and whites in the Hemochromatosis and Iron Overload Screening (HEIRS) Study

Bivariate mixture modeling was used to analyze joint population distributions of transferrin saturation (TS) and serum ferritin concentration (SF) measured in the Hemochromatosis and Iron Overload Screening (HEIRS) Study. Four components (C1, C2, C3, and C4) with successively age-adjusted increasing means for TS and SF were identified in data from 26,832 African Americans, 12,620 Asians, 12,264 Hispanics, and 43,254 whites. The largest component, C2, had normal mean TS (21% to 26% for women, 29% to 30% for men) and SF (43-82 microg/L for women, 165-242 microg/L for men), which consisted of component proportions greater than 0.59 for women and greater than 0.68 for men. C3 and C4 had progressively greater mean values for TS and SF with progressively lesser component proportions. C1 had mean TS values less than 16% for women (<20% for men) and SF values less than 28 microg/L for women (<47 microg/L for men). Compared with C2, adjusted odds of iron deficiency were significantly greater in C1 (14.9-47.5 for women, 60.6-3530 for men), adjusted odds of liver disease were significantly greater in C3 and C4 for African-American women and all men, and adjusted odds of any HFE mutation were increased in C3 (1.4-1.8 for women, 1.2-1.9 for men) and in C4 for Hispanic and white women (1.5 and 5.2, respectively) and men (2.8 and 4.7, respectively). Joint mixture modeling identifies a component with lesser SF and TS at risk for iron deficiency and 2 components with greater SF and TS at risk for liver disease or HFE mutations. This approach can identify populations in which hereditary or acquired factors influence metabolism measurement.

Detection of the C282Y and H63D Polymorphisms Associated With Hereditary Hemochromatosis Using the ABI 7500 FAST Real Time PCR Platform

Classic hereditary hemochromatosis is an autosomal recessive disorder characterized by iron overload and sequence variants in the HFE gene. The HFE gene is located at 6p21.3 and contains 2 common single nucleotide polymorphisms (SNPs) C282Y and H63D, which are routinely tested for in the molecular diagnostics laboratory. In this study, we used DNA samples from 59 patients in which clinicians wanted to confirm or rule-out hereditary hemochromatosis that had been previously tested for the HFE SNPs using a polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assay and the ABI 7700 real time PCR assay with a MGB Eclipse ASR Probe system. The new assay used TAQman SNP Genotyping Assays, which were performed on the ABI 7500 FAST real time PCR platform. Allelic discrimination was determined during a postamplification plate read. Of the 59 samples genotyped, 7 were homozygous for C282Y, 6 were heterozygous for C282Y, 9 were homozygous for H63D, 10 were heterozygous for H63D, 6 were compound heterozygotes, and 20 were wild type. With the exception of one sample that was indeterminate by the TAQman SNP Genotyping Assay, all others showed 100% concordance between the 3 assays. The one indeterminate sample was heterozygous for C282Y by the PCR-RFLP and ABI 7700 real time PCR assays, but there was an insufficient quantity of DNA to perform the TAQman SNP Genotyping Assay. Our study suggests that the ABI 7500 FAST TAQman SNP Genotyping Assay is comparable with the PCR-RFLP and ABI 7700 real time PCR methods in detecting and characterizing these 2 HFE SNPs. Improved software and thermocycling capabilities have resulted in a very robust TAQman assay with the advantage of a much improved turn-around-time and throughput.