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Background Most persons who are homozygous for C282Y, the HFE allele most commonly asssociated with hereditary hemochromatosis, have elevated levels of serum ferritin and transferrin saturation. Diseases related to iron overload develop in some C282Y homozygotes, but the extent of the risk is controversial.
Methods We assessed HFE mutations in 31,192 persons of northern European descent between the ages of 40 and 69 years who participated in the Melbourne Collaborative Cohort Study and were followed for an average of 12 years. In a random sample of 1438 subjects stratified according to HFE genotype, including all 203 C282Y homozygotes (of whom 108 were women and 95 were men), we obtained clinical and biochemical data, including two sets of iron measurements performed 12 years apart. Disease related to iron overload was defined as documented iron overload and one or more of the following conditions: cirrhosis, liver fibrosis, hepatocellular carcinoma, elevated aminotransferase levels, physician-diagnosed symptomatic hemochromatosis, and arthropathy of the second and third metacarpophalangeal joints.
Results The proportion of C282Y homozygotes with documented iron-overload–related disease was 28.4% (95% confidence interval [CI], 18.8 to 40.2) for men and 1.2% (95% CI, 0.03 to 6.5) for women. Only one non-C282Y homozygote (a compound heterozygote) had documented iron-overload–related disease. Male C282Y homozygotes with a serum ferritin level of 1000 µg per liter or more were more likely to report fatigue, use of arthritis medicine, and a history of liver disease than were men who had the wild-type gene.
Conclusions In persons who are homozygous for the C282Y mutation, iron-overload–related disease developed in a substantial proportion of men but in a small proportion of women.
Persons who are homozygous for the C282Y mutation are at increased risk for iron overload. C282Y homozygotes account for 82 to 90% of clinical diagnoses of hereditary hemochromatosis among persons of northern European descent.7 C282Y homozygotes can be characterized by the stage of progression as follows: a genetic predisposition without abnormalities, iron overload without symptoms, iron overload with symptoms (e.g., arthritis and fatigue), and iron overload with organ damage — in particular, cirrhosis.8
The prevalence of C282Y homozygosity is approximately 1 case per 200 persons, with elevated levels of serum ferritin and transferrin saturation occurring in 40 to 60% of female homozygotes and in 75 to 100% of male homozygotes.9,10,11,12,13 An elevated serum ferritin level is a necessary but not sufficient prerequisite for the diagnosis of iron overload, which can be objectively documented either by liver biopsy with determination of quantitative hepatic iron levels or by quantitative phlebotomy.14
The presence of cirrhosis, severe fibrosis, hepatocellular carcinoma, or arthropathy of the second and third metacarpophalangeal joints in the context of documented iron overload and C282Y homozygosity constitutes iron-overload–related disease in patients with hereditary hemochromatosis.15,16,17 It is not known whether other complications that are associated with the disease (e.g., fatigue, abdominal pain, and diabetes) can be attributed to an abnormal HFE genotype, particularly since these conditions are common and nonspecific. Population estimates of both documented disease related to iron overload and conditions associated with hereditary hemochromatosis in C282Y homozygotes have been hindered by the absence of clinical assessment before knowledge of the patient's genetic status or by an inability to account for the long lead time of preclinical iron-overload status.
In this 12-year study involving a cohort of 31,192 subjects, we prospectively assessed the prevalence of iron-overload–related disease, along with morbidity and mortality, for C282Y homozygotes, as compared with three groups of subjects: those with compound heterozygosity for C282Y and the substitution of aspartic acid for histidine at position 63 (H63D), C282Y heterozygotes, and subjects with neither HFE variant.
Methods
Study Population
From 1990 to 1994, a total of 41,528 subjects (including 24,479 women) between the ages of 27 and 75 years (of whom 99% were between the ages of 40 and 69 years) were enrolled in the Melbourne Collaborative Cohort Study,18 which is a prospective longitudinal study of diet and other lifestyle factors and the influence of these factors on the development of common chronic diseases. Subjects were recruited through the Australian Electoral Roll (voting is compulsory in Australia), advertisements, and community announcements in local media. After recruitment, subjects visited a study center, where they were interviewed about a range of lifestyle and dietary factors, underwent physical measurement, and provided a blood sample.
For this study (known as HealthIron), subjects who were born in southern Europe (Italy, Greece, and Malta) were excluded, owing to the low prevalence of the C282Y variant in those populations. This exclusion left 31,192 subjects who reported having been born in Australia, the United Kingdom, Ireland, or New Zealand (i.e., of northern European ancestry), with a mean (±SD) age of 55.3±8.9 years. At baseline, 7270 of 17,951 women (40.5%) reported that they were premenopausal; the menopausal status of 1313 women was unknown. All subjects were included in the analysis of mortality. For analyses of other outcomes, a random sample of 1438 subjects who were stratified according to HFE genotype and included all C282Y homozygotes were invited to undergo a clinical assessment as part of the HealthIron study. Deaths that occurred before December 31, 2004, were identified through linkage to death records of the state of Victoria and the Australian National Death Index.
Preliminary HFE Genotyping
For 23,484 subjects, DNA from stored baseline samples was extracted from Guthrie cards with the use of Chelex reagent. For 7708 subjects, DNA was extracted with the use of buffy-coat CorProtocol 14102 (Corbett Life Science) from frozen blood fractions stored in liquid nitrogen. All samples were genotyped for nucleotide changes that correspond to the amino acid substitution C282Y and H63D in HFE with the use of the Taqman real-time polymerase-chain-reaction (PCR) assay (Applied Biosystems), as described previously.11,19 Owing to initial difficulties with the assay of DNA samples extracted from Guthrie cards, genotyping for the H63D variant was performed only among subjects who were heterozygous for the C282Y variant. Genotyping of 29,676 of the 31,192 samples (95.1%) was successful.
HealthIron Clinics
From 2004 to 2006, letters of invitation to participate in the HealthIron study were sent to a sample of 1438 subjects that included all C282Y homozygotes and a stratified random sample of subjects from the remaining groups with the HFE genotype. Investigators were unaware of subjects' HFE genotype until all clinical assessments had been completed. Before being interviewed and undergoing clinical examination, subjects were asked by the clinic manager not to reveal to the study physicians any previous diagnosis of iron overload or hemochromatosis.
Subjects completed a computer-assisted personal interview and provided a cheek-brush sample for confirmatory HFE genotyping. Blood samples were collected for measurement of iron indexes and liver enzyme levels with the use of automated assays (Roche Diagnostics) and were paired for analysis with the corresponding stored baseline serum samples. Blood samples were collected in the morning at both baseline and follow-up.
Study investigators performed a physical examination of the abdomen and metacarpophalangeal joints. Previously unidentified C282Y homozygotes were referred to a HealthIron follow-up clinic for ongoing treatment and a liver biopsy, if clinically indicated. Results of previous liver biopsy were obtained from physicians of C282Y homozygotes whose condition had been diagnosed before or during the course of the study. The definitions of iron overload and iron overload–related disease are listed in Table 1.
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Statistical Analysis
Prevalences were estimated as observed proportions and compared by calculating prevalence ratios and 95% confidence intervals. Genotype groups were compared by analysis of variance for continuous measures, with serum ferritin analyzed on the natural logarithm scale, or by chi-square tests for proportions. Cox regression with age as the time axis was used to estimate the hazard ratio for the association between the HFE genotype and death. The proportional-hazards assumption was assessed by means of plots of the Nelson–Aalen estimate of the cumulative hazard and formal tests based on Schoenfeld residuals. Statistical analyses were performed with the use of Stata software, version 9.1 (Stata).
Results
Whole-Cohort Analysis
Among 29,676 subjects of northern European ancestry, 203 were homozygous for the C282Y mutation, which corresponded to an estimated prevalence of 1 case per 146 subjects, or 0.68% (95% confidence interval [CI], 0.59 to 0.78). The presence of homozygosity for the C282Y mutation was confirmed in an independent laboratory from either a follow-up cheek swab or a baseline serum sample.
There were 3295 subjects (11.1%) who were heterozygous for the C282Y mutation only. An additional 719 subjects (2.4%) were heterozygous for both the C282Y and H63D mutations (compound heterozygotes). The observed number of both simple (C282Y) and compound (C282Y–H63D) heterozygotes in the whole cohort was lower than expected on the basis of Hardy–Weinberg equilibrium (chi-square=6.25, P=0.01). With the exception of C282Y homozygotes, this baseline genotyping was not repeated with a second, independent method.
Death from Any Cause
During an average of 11.4 years of follow-up, 2488 subjects (8.0%) died, including 19 of 203 C282Y homozygotes (9.4%), 59 of 719 compound heterozygotes (8.2%), and 295 of 3295 C282Y heterozygotes (9.0%). The hazard ratio for death from any cause among the C282Y homozygotes, as compared with subjects who had no C282Y mutation, was 1.04 (95% CI, 0.67 to 1.62; P=0.87).
HealthIron Subjects
Of the 1438 subjects who were invited to participate in the HealthIron study, 1054 (73.3%) completed at least two of the four components: a questionnaire, confirmatory HFE genotyping, blood sampling, and physical examination; 937 subjects (65.2%) completed all components, 58 (4.0%) completed three components (excluding physical examination), and 59 (4.1%) completed only the questionnaire and genotyping. Of the 384 subjects who did not complete any of the four components of the final study, 113 had died, 197 were no longer actively participating in the study, and 74 could not be contacted. A total of 1054 of the 1325 subjects who were still alive participated (79.5%). The male:female ratio and median ages at baseline and follow-up did not differ significantly among the HFE genotype groups (Table 2).
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Serum Ferritin and Transferrin Saturation
Of the 1438 subjects who participated in the HealthIron study, 850 (59.1%) were fasting when the baseline blood sample was obtained, and 941 of 1054 (89.3%) were fasting when the follow-up sample was obtained. At baseline, among C282Y homozygotes, the level of serum ferritin was more than 300 µg per liter in 45 of 55 men (81.8%) and more than 200 µg per liter (>300 µg per liter for postmenopausal subjects) in 36 of 65 women (55.4%). In the same group, transferrin saturation was more than 55% in 40 of 55 men (72.7%) and more than 45% in 45 of 65 women (69.2%). At least one serum ferritin value of 1000 µg per liter or more was recorded in 33 of 74 men (44.6%) and in 7 of 84 women (8.3%) (Table 2).
For men, the mean serum ferritin level decreased during the 12-year period from baseline to follow-up in all HFE genotype groups (Table 2), although there were large changes for only two subgroups of subjects: C282Y homozygotes, who had a mean reduction from 1195 µg per liter to 593 µg per liter, including 30% of subjects who had undergone therapeutic venesection, and compound heterozygotes, who had a mean reduction from 351 µg per liter to 264 µg per liter. In contrast, the mean serum ferritin level increased in women for all genotype groups during the study period (Table 2), except for C282Y homozygotes, of whom 20% had undergone therapeutic venesection. Male C282Y homozygotes (including those who underwent venesection) were the only group in which the mean transferrin saturation changed significantly between baseline and follow-up (a decrease from 73±3% to 63±3%) (P=0.03), although only 64% of male C282Y homozygotes were fasting at the time of the baseline sampling, as compared with 100% at follow-up.
Clinical Features According to HFE Genotype
Male C282Y homozygotes with serum ferritin level of 1000 µg per liter or more at baseline or follow-up had a higher prevalence of reported fatigue, liver disease, and use of arthritis medication than did subjects with the wild-type gene and C282Y heterozygotes combined. In addition, a higher proportion of male C282Y homozygotes had increased levels of alanine aminotransferase or aspartate aminotransferase in the absence of excess alcohol intake (Table 3).
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Iron Overload and Related Disease
For C282Y homozygotes, 21 of 74 men (28.4%; 95% CI, 18.8 to 40.2) and 1 of 84 women (1.2%; 95% CI, 0.03 to 6.5) satisfied the criteria for documented disease related to iron overload (Table 1 and Table 4). Of the 22 C282Y homozygotes with documented iron overload–related disease, 2 had hepatocellular carcinoma, 12 had fibrosis or cirrhosis, 6 had raised levels of alanine aminotransferase or aspartate aminotransferase, 5 had abnormal metacarpophalangeal joints, and 11 had received a previous diagnosis of hereditary hemochromatosis as a result of symptoms that prompted an evaluation.
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Four of the liver biopsies were performed before baseline, with grade 1 fibrosis observed in a 22-year-old subject, grade 0 in a 42-year-old subject, grade 3 in a 64-year-old subject, and grade 0 in a 67-year-old subject. Seven liver biopsies were performed after baseline and before follow-up, with grade 2 fibrosis observed in both a 57-year-old subject and a 58-year-old subject with self-reported alcoholic liver disease, grade 1 in a 58-year-old subject, and grade 0 in a 59-year-old subject; cirrhosis in a 62-year-old subject and a 71-year-old subject; and grade 3 fibrosis in a 76-year-old subject who consumed more than 100 g of alcohol per day. Six biopsies were performed in subjects who were referred to a hepatologist after they had undergone follow-up assessments at the HealthIron study clinic, with grade 0 fibrosis observed in a 54-year-old subject and a 55-year-old subject and grade 1 in three 56-year-old subjects and one 59-year-old subject.
No subjects with biopsy results reported having had viral hepatitis, and only one (as noted above) reported having a high baseline level of alcohol consumption (>40 g per day for women and >60 g per day for men). Only one subject who was not a C282Y homozygote (a compound heterozygote) had documented iron-overload–related disease.
Provisional Iron Overload and Related Disease
Provisional iron overload (defined in Table 1) occurred in six compound heterozygotes, in two C282Y heterozygotes, in one H63D heterozygote, and in two subjects with neither mutation. Only one of the two subjects with neither mutation had evidence of iron-overload–related disease. In comparison, 83 C282Y homozygotes (both men and women) had provisional iron overload; of these subjects, 24% had objective evidence of iron-overload–related disease (Table 4).
Discussion
In a prospective cohort study with more than 12 years of follow-up, for C282Y homozygotes, documented iron-overload–related disease developed in 28.4% of men but only 1.2% of women. C282Y homozygotes with a serum ferritin level of 1000 µg per liter or more were at higher risk for symptoms and disease associated with the HFE gene than were either C282Y homozygotes with a serum ferritin level of 1000 µg per liter or less or subjects with other HFE genotypes. This finding confirms and extends the observation of Guyader and colleagues5 that a serum ferritin level of 1000 µg per liter or more is associated not only with cirrhosis but also with symptomatic hereditary hemochromatosis in C282Y homozygotes. Arthropathy, as defined by clinically abnormal metacarpophalangeal joints, was unrelated to serum ferritin levels in homozygotes, a finding that confirmed results reported previously.17
Investigators who conducted clinical examinations were not aware of the genotype of the study subjects. Our estimate of disease penetrance (the probability of expression of a genotype) is conservative, since we used strictly objective criteria on the basis of liver biopsy, liver-enzyme levels, clinical examination, or physicians' diagnosis in the context of documented iron overload. Subjective criteria such as a history of fatigue or liver disease were excluded from our assessment of disease penetrance. Detailed data on lifestyle factors were collected as part of the study. Because male C282Y homozygotes were not significantly more likely to be obese and did not have greater alcohol intake than male subjects who were not C282Y homozygotes, these factors are unlikely to explain the increased prevalence of elevated levels of alanine aminotransferase or aspartate aminotransferase among male C282Y homozygotes.
The high prevalence of C282Y homozygosity in this study may be attributable to a high rate of British immigration in this age group in Melbourne.20 This high prevalence of C282Y homozygosity makes it unlikely that patients who had received the diagnosis of hereditary hemochromatosis previously were underrepresented in our initial recruitment, and linkage to the National Death Index minimized selective mortality bias. Thirty-seven percent of C282Y homozygotes were identified during follow-up, which may have modified the natural history of disease progression for these subjects and resulted in an underestimation of disease penetrance. Our study did not address the association of HFE mutations with less commonly reported features of hereditary hemochromatosis, such as impotence and cardiomyopathy.
Two other longitudinal studies have attempted to characterize the penetrance of disease related to hereditary hemochromatosis with the use of population-based data.12,21 Andersen and colleagues12 retrospectively assessed 23 C282Y homozygotes (including 16 women) during a 25-year period and found no evidence of liver disease associated with hereditary hemochromatosis. However, the study was potentially compromised by selective mortality bias, owing to the high rate of attrition in the cohort (53%), the fact that 35% of the members of the original cohort were not genotyped for HFE mutations, and the fact that 3 of the 23 C282Y homozygotes died before they could be examined. Olynyk and colleagues,21 who retrospectively assessed 10 C282Y homozygous patients (including 6 women) during a 17-year period, reported that of the 6 patients who underwent liver biopsy, 3 had cirrhosis or fibrosis. However, not all the patients in that study were at an age when symptoms of disease would have been expected. In a systematic review, Whitlock and colleagues14 estimated, after accounting for patients who were lost to follow-up, that disease would eventually develop in 25 to 60% of C282Y homozygotes.
On the basis of a cross-sectional population study of subjects between the ages of 20 and 80 years (an age range that was wider than that of our study and that included a greater proportion of subjects under the age of 50 years), Beutler and colleagues10 suggested that disease attributable to hereditary hemochromatosis occurred in less than 1% of all C282Y homozygotes, regardless of sex. However, Beutler et al. did not perform clinical examinations or liver biopsies, and a quarter of the C282Y homozygotes were excluded on the basis that they had received a previous diagnosis of hereditary hemochromatosis. This exclusion would be expected to reduce the estimate of clinical penetrance of C282Y homozygosity.22,23 Contrary to our results, Beutler et al. found no association between C282Y homozygosity and the presence of fatigue or arthritis, although they reported an association with a history of liver disease. The exclusion of subjects in whom hereditary hemochromatosis had already been diagnosed might account for this finding, since our study showed that subjects with a serum ferritin level of 1000 µg per liter or more were more likely to present with symptoms than were those with a level of less than 1000 µg per liter.
The increased prevalence of iron-overload–related disease in C282Y homozygous men, as compared with that in women, is frequently ascribed to recurrent physiologic blood loss and the resultant slower accumulation of iron in women. However, disparate frequencies of HLA A*03B*07 haplotypes in men and women have been reported in hereditary hemochromatosis probands,24 which may be relevant to sex-specific phenotypic expression of this disease. Studies of iron regulatory pathways in African Americans have also suggested that serum ferritin levels may be genetically determined by sex differences as well as environmental factors.25,26
In conclusion, disease related to iron overload commonly develops in men (but not in women) who are homozygous for the C282Y mutation, especially when serum ferritin levels are 1000 µg per liter or more.
Supported by a grant (1-RO1-DK061885-01A2) from the National Institutes of Health, grants (251668 and 209057) from the National Health and Medical Research Council (NHMRC) of Australia, and VicHealth and the Cancer Council Victoria.
No potential conflict of interest relevant to this article was reported.
We thank Dr. Sue Forrest of the Australian Genome Research Facility, Melbourne, for supervising HFE genotyping; Andrea A. Tesoriero of the University of Melbourne for supervising DNA extraction; Dr. Andy Nydegger of the Royal Children's Hospital, Melbourne, and Dr. Christine van Vliet of the Centre for Molecular, Environmental, Genetic, and Analytic Epidemiology for assisting with clinical examinations; Dr. Geoffrey McColl of the Royal Melbourne Hospital for assisting in the design and interpretation of the metacarpophalangeal examination; Ashley Fletcher for providing assistance with study coordination and sample retrieval; and the many thousands of Melbourne residents who continue to participate in the study.
Source Information
From the Murdoch Children's Research Institute (K.J.A., N.J.O., M.B.D., A.E.N.), the Centre for Molecular, Environmental, Genetic, and Analytic Epidemiology (L.C.G., C.C.C., N.J.O., G.G.G., D.R.E., J.L.H., D.M.G.), and the Departments of Pediatrics (K.J.A., M.B.D., A.E.N.) and Pathology (M.C.S.), the University of Melbourne, Melbourne; the Royal Melbourne Hospital, Melbourne (A.J.N.); the Walter and Eliza Hall Institute, Melbourne (M.B.); the Queensland Institute of Medical Research and the University of Queensland, Brisbane (G.J.A., L.W.P.); the Cancer Council Victoria, Melbourne (G.G.G., D.R.E.); and the University of Western Australia and the Western Australian Institute of Medical Research, Perth (J.K.O.) — all in Australia; the University of California, Irvine, Irvine (C.E.M.); and the University of California, Berkeley, Berkeley (C.D.V.).
Address reprint requests to Dr. Allen at the Department of Gastroenterology, Royal Children's Hospital, Flemington Rd., Parkville 3052, Victoria, Australia, or at katie.allen{at}rch.org.au.
References
A (C282Y) HFE hereditary haemochromatosis mutation in the USA. Lancet 2002;359:211-218. [CrossRef][Web of Science][Medline]
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Related Letters:
Iron-Overload–Related Disease in HFE Hereditary Hemochromatosis
Waalen J., Beutler E., Rienhoff H. Y. Jr., Allen K. J., Gurrin L., Powell L.
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N Engl J Med 2008;
358:2293-2295, May 22, 2008.
Correspondence
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