Utility of genotyping in diagnosis of genetic haemochromatosis
2
Journal of Gastroenterology and Hepatology (1999) 14, 401–402 Australian subjects with iron overload. 12 Eighty-nine per cent of the affected subjects were homozygous for the mutation. It is reassuring that virtually identical propor- tions of affected subjects from different parts of Australia were shown to be homozygous for the C282Y mutation. Thus, it can now be said with some confidence that the Australian data are similar to reports from United States, England and northern Europe. It remains unexplained why fewer patients in Italy and southern France are homozygous for the C282Y mutation. 13,14 A unique feature of the report by Rossi et al. is the observation that one homozygous subject for the C282Y mutation also carried a H63D mutation on one of the affected chro- mosomes. 12 To our knowledge, this is the first report to date of a case where the C282Y and H63D mutations occur on the same chromosome. Clinicians are fortunate that such a high percentage of subjects with iron overload are homozygous for the C282Y mutation, as direct genetic testing will identify the majority of affected subjects.This situation should be contrasted with other diseases of the gastrointestinal tract, such as Wilson disease 15 and familial adenomatous polyposis, 16 where many mutations exist throughout the gene of interest and direct genetic testing can only occur in centres where a major research interest exists. Fur- thermore, the method of genetic testing for the mutation is relatively straightforward because the G to A base mutation creates a restriction site for commercially avail- able restriction enzymes.Thus, many hospital and private laboratories are capable of testing for the mutation. The phenotype of the C282Y mutation is character- ized principally by a transferrin saturation above 45%. 17 If this occurs in association with elevated serum ferritin levels, genetic testing for the C282Y mutation is appro- priate. If the patient is homozygous for the C282Y muta- tion, the diagnosis of hereditary haemochromatosis is established.The next decision point for the clinician is to consider liver biopsy. Liver biopsy is important to estab- lish whether the patient has cirrhosis, as haemochro- matosis patients with cirrhosis are at risk of developing hepatocellular carcinoma 18 and should probably enter a See article on page 427 The dust is settling on the discovery of the underlying genetic abnormality responsible for the majority of cases of haemochromatosis. Thus, it is timely to reflect on the contribution that the cloning of the haemochro- matosis gene has made to the clinical strategy underly- ing the diagnosis and management of this common disorder of iron metabolism. In 1996, Feder et al. reported that 83% of a population of American haemochromatosis patients were homozy- gous for a mutation in a novel gene (now known as HFE) located on chromosome 6p. 1 This mutation results in an amino acid substitution of cysteine to tyrosine at position 282 (C282Y) of the gene product. The gene product is an human leucocyte antigen (HLA)-class I like mole- cule, which is cell membrane-bound and has been reported to interact with the transferrin receptor to reg- ulate cellular iron uptake. 2–5 While the frequency of this mutation in haemochromatosis patients varies, there is general agreement that 80–100% of affected subjects are homozygous for the C282Y mutation. 6–8 Indeed the fre- quency of the mutation in the original American cohort is higher than initially thought because some of the sub- jects have subsequently been shown to have other under- lying causes of iron overload. 9 A second mis-sense mutation resulting in a substitution of histidine to aspar- tic acid at codon 63 (H63D) has also been described by Feder et al. 1 A number of reports have shown that some H63D homozygous and compound heterozygous patients develop iron overload. 10,11 The initial report by Feder et al. was quickly followed by an analysis of well-documented Australian families in whom more than one family member was affected. 6 All of the affected individuals were homozygous for the C282Y mutation. However, when the analysis was expanded to include all subjects with iron overload, irrespective of the presence of a family history, 83% were shown to be homozygous for the mutation. 11 In this issue of the journal, Rossi et al. report the preva- lence of the C282Y mutation in a cohort of Western EDITORIAL Utility of genotyping in diagnosis of genetic haemochromatosis DARRELL HG CRAWFORD AND LINDA M FLETCHER Department of Gastroenterology and Hepatology, Princess Alexandra Hospital, Brisbane, Queensland, Australia Correspondence: Darrell HG Crawford, Department of Gastroenterology and Hepatology, Princess Alexandra Hospital, Ipswich Road,Woolloongabba, Qld 4102, Australia. Email: <[email protected]> Accepted for publication 4 January 1999. © 1999 Blackwell Science Asia Pty Ltd.
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Transcript of Utility of genotyping in diagnosis of genetic haemochromatosis
Journal of Gastroenterology and Hepatology (1999) 14, 401–402
Australian subjects with iron overload.12 Eighty-nine percent of the affected subjects were homozygous for themutation. It is reassuring that virtually identical propor-tions of affected subjects from different parts of Australiawere shown to be homozygous for the C282Y mutation.Thus, it can now be said with some confidence that theAustralian data are similar to reports from United States,England and northern Europe. It remains unexplainedwhy fewer patients in Italy and southern France arehomozygous for the C282Y mutation.13,14 A uniquefeature of the report by Rossi et al. is the observation thatone homozygous subject for the C282Y mutation alsocarried a H63D mutation on one of the affected chro-mosomes.12 To our knowledge, this is the first report todate of a case where the C282Y and H63D mutationsoccur on the same chromosome.
Clinicians are fortunate that such a high percentage ofsubjects with iron overload are homozygous for theC282Y mutation, as direct genetic testing will identifythe majority of affected subjects.This situation should becontrasted with other diseases of the gastrointestinaltract, such as Wilson disease15 and familial adenomatouspolyposis,16 where many mutations exist throughout thegene of interest and direct genetic testing can only occurin centres where a major research interest exists. Fur-thermore, the method of genetic testing for the mutationis relatively straightforward because the G to A basemutation creates a restriction site for commercially avail-able restriction enzymes.Thus, many hospital and privatelaboratories are capable of testing for the mutation.
The phenotype of the C282Y mutation is character-ized principally by a transferrin saturation above 45%.17
If this occurs in association with elevated serum ferritinlevels, genetic testing for the C282Y mutation is appro-priate. If the patient is homozygous for the C282Y muta-tion, the diagnosis of hereditary haemochromatosis isestablished.The next decision point for the clinician is toconsider liver biopsy. Liver biopsy is important to estab-lish whether the patient has cirrhosis, as haemochro-matosis patients with cirrhosis are at risk of developinghepatocellular carcinoma18 and should probably enter a
See article on page 427
The dust is settling on the discovery of the underlyinggenetic abnormality responsible for the majority ofcases of haemochromatosis. Thus, it is timely to reflecton the contribution that the cloning of the haemochro-matosis gene has made to the clinical strategy underly-ing the diagnosis and management of this commondisorder of iron metabolism.
In 1996, Feder et al. reported that 83% of a populationof American haemochromatosis patients were homozy-gous for a mutation in a novel gene (now known as HFE)located on chromosome 6p.1 This mutation results in anamino acid substitution of cysteine to tyrosine at position282 (C282Y) of the gene product. The gene product isan human leucocyte antigen (HLA)-class I like mole-cule, which is cell membrane-bound and has beenreported to interact with the transferrin receptor to reg-ulate cellular iron uptake.2–5 While the frequency of thismutation in haemochromatosis patients varies, there isgeneral agreement that 80–100% of affected subjects arehomozygous for the C282Y mutation.6–8 Indeed the fre-quency of the mutation in the original American cohortis higher than initially thought because some of the sub-jects have subsequently been shown to have other under-lying causes of iron overload.9 A second mis-sensemutation resulting in a substitution of histidine to aspar-tic acid at codon 63 (H63D) has also been described byFeder et al.1 A number of reports have shown that someH63D homozygous and compound heterozygouspatients develop iron overload.10,11
The initial report by Feder et al. was quickly followedby an analysis of well-documented Australian familiesin whom more than one family member was affected.6
All of the affected individuals were homozygous for theC282Y mutation. However, when the analysis wasexpanded to include all subjects with iron overload,irrespective of the presence of a family history, 83%were shown to be homozygous for the mutation.11
In this issue of the journal, Rossi et al. report the preva-lence of the C282Y mutation in a cohort of Western
EDITORIAL
Utility of genotyping in diagnosis of genetic haemochromatosis
DARRELL HG CRAWFORD AND LINDA M FLETCHER
Department of Gastroenterology and Hepatology, Princess Alexandra Hospital, Brisbane, Queensland,Australia
Correspondence: Darrell HG Crawford, Department of Gastroenterology and Hepatology, Princess Alexandra Hospital,Ipswich Road, Woolloongabba, Qld 4102, Australia. Email: <[email protected]>
Accepted for publication 4 January 1999.© 1999 Blackwell Science Asia Pty Ltd.
surveillance programme of regular ultrasound andalpha-fetoprotein measurements. A recent French andCanadian study has provided strong evidence that cir-rhosis is most unlikely if all three of the following criteriaare met: (i) serum ferritin concentration less than 1000mg/L; (ii) normal alanine aminotransferase levels; and(iii) normal liver size.19 A preliminary analysis of Aus-tralian patients has confirmed the importance of a serumferritin of less than 1000 mg/L, but also suggested thatage (less than 35 years) and alcohol intake less than60–80 g per day were important independent predictorsof the absence of severe fibrosis.20 At present, it could berecommended that liver biopsy does not need to be per-formed if all of the above five criteria are met as thechance of underlying cirrhosis is minimal.
The situation for subjects who are not homozygous forthe C282Y mutation is less clear. Some subjects who areheterozygous for C282Y or compound heterozygous(i.e. carry one copy of the C282Y mutation and one copyof the H63D mutation) develop phenotypic evidence ofiron overload.The degree of iron overload is often modestand usually not associated with the levels associated withcirrhosis.11There is little reason to doubt that the five cri-teria listed above regarding the risk of cirrhosis would bedifferent between C282Y homozygous, C282Y het-erozygous and compound (C282Y/H63D) heterozygoussubjects. However, a substantial proportion of heterozy-gotes will have other causes of increased serum ferritin,such as steatosis, and liver biopsy is often needed in thesesubjects to clarify their clinical diagnosis.
A final issue worthy of consideration is non-expressionof the underlying genetic defect. Reports indicate that6.7% of homozygous men and 32.7% of homozygouswomen do not meet conventional diagnostic criteria forgenetic haemochromatosis.11 The majority of thesewomen are in their reproductive years. If identified, thesesubjects should be questioned closely about physiologi-cal and pathological blood loss and clinicians shouldhave a low threshold for further investigation. However,there are many reports of men and postmenopausalwomen homozygous for the C282Y mutation withoutpathological blood loss who do not express the disease.Non-expression of an underlying genetic defect may haveimportant implications for insurance and vocationalmatters and has obvious implications for widespreadpopulation screening based on genetic testing alone.
In summary, the report by Rossi et al. highlights thatthe majority of Australian haemochromatosis patientsare homozygous for the C282Y mutation in HFE andfurther illustrates that the cloning of HFE and the iden-tification of the C282Y mutation has been one of themost significant advances in the discipline of hepatol-ogy in the 1990s. Indeed it is unusual that major sci-entific breakthroughs affect clinical practice as rapidlyas the discovery of HFE.
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402 DHG Crawford and LM Fletcher
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