Which genes are implicated in hereditary hemochromatosis and what is the recommended genetic testing strategy?

Genes Implicated in Hereditary Hemochromatosis and Genetic Testing Strategy

The HFE gene on chromosome 6p21.3 is the primary gene implicated in hereditary hemochromatosis, with the C282Y mutation accounting for >90% of clinically diagnosed cases, and genetic testing should begin with HFE C282Y analysis after confirming transferrin saturation ≥45%. 1

Primary Gene: HFE

• The C282Y mutation in the HFE gene is transmitted in an autosomal recessive pattern and represents the most common genetic cause of hereditary hemochromatosis. 1
• C282Y homozygosity (C282Y/C282Y) is found in more than 90% of patients with clinically overt hemochromatosis and in 80.6% of all clinically recognized hemochromatosis patients of European ancestry. 1, 2
• The C282Y mutation has an allelic frequency of approximately 6.2% in populations of European ancestry, with homozygosity occurring in about 0.44-0.5% of individuals of northern European descent. 1
• Compound heterozygosity (C282Y/H63D) accounts for only 3-5% of hemochromatosis cases and represents a low penetrance genotype, with only 10.1% developing documented iron overload over 10 years. 1, 3

Secondary HFE Variant: H63D

• The H63D mutation has an average allelic frequency of 14% in European populations but H63D heterozygosity alone does not cause clinically significant iron overload. 1
• H63D homozygosity or compound heterozygosity (C282Y/H63D) is insufficient to cause hemochromatosis without additional genetic or environmental risk factors such as alcohol use, viral hepatitis, or fatty liver disease. 1
• Only 5.3% of C282Y/H63D compound heterozygotes exhibit iron overload-related disease on the background of documented iron overload, confirming low penetrance. 3

Non-HFE Hemochromatosis Genes (Types 2-4)

• Mutations in TFR2, SLC40A1, HAMP, and HJV genes cause non-HFE hereditary hemochromatosis, which should be considered when iron overload persists despite negative or non-diagnostic HFE testing. 1, 4
• Juvenile hemochromatosis (Type 2) is caused by mutations in HJV (hemojuvelin) or HAMP (hepcidin) genes and presents with severe iron overload before age 30. 4, 5
• Type 3 hemochromatosis is caused by TFR2 (transferrin receptor 2) mutations and presents similarly to HFE-related disease but is much rarer. 4, 6
• Type 4 hemochromatosis (ferroportin disease) is caused by SLC40A1 mutations and is inherited in an autosomal dominant pattern, unlike other forms. 4, 5

Recommended Genetic Testing Strategy

Step 1: Confirm Biochemical Iron Overload

• Measure fasting transferrin saturation (TS) and serum ferritin simultaneously as the essential initial laboratory tests. 7
• A transferrin saturation ≥45% is the primary screening threshold that triggers genetic evaluation, offering high sensitivity for detecting C282Y homozygotes. 7
• Diagnostic thresholds requiring genetic testing are: males with TS >50% and/or ferritin >300 μg/L, and females with TS >45% and/or ferritin >200 μg/L. 7

Step 2: First-Line HFE Testing

• Test for C282Y first in patients with biochemical evidence of iron overload (TS ≥45%), as this is the primary genotype requiring clinical action. 1, 7
• Include H63D testing simultaneously with C282Y, but recognize that H63D testing alone should not guide treatment decisions. 1
• If C282Y homozygosity is confirmed, this establishes the diagnosis of HFE-related hemochromatosis and therapeutic phlebotomy can be initiated. 7

Step 3: Interpretation of HFE Results

• C282Y homozygotes (C282Y/C282Y) confirm HFE hemochromatosis and require therapeutic phlebotomy. 7
• Compound heterozygotes (C282Y/H63D) or H63D homozygotes with confirmed iron overload require investigation for other causes of iron overload, as these genotypes alone rarely cause significant disease. 7, 3
• Simple heterozygotes (C282Y/wild-type or H63D/wild-type) do not develop hereditary hemochromatosis and do not require phlebotomy or monitoring. 1

Step 4: Non-HFE Gene Testing (When Indicated)

• Consider testing for non-HFE genes (TFR2, SLC40A1, HAMP, HJV) if iron overload persists with TS ≥45% but C282Y homozygosity is excluded. 1, 4
• Next-generation sequencing (NGS)-based panels targeting all five hemochromatosis genes (HFE, HAMP, HJV, TFR2, SLC40A1) represent a useful second-level genetic test for unexplained iron overload. 6
• Non-HFE testing is particularly indicated in patients with early-onset disease (age <30 years), family history inconsistent with autosomal recessive inheritance, or geographic regions where C282Y is rare (Mediterranean, Asian, African populations). 4, 6, 2

Family Screening Recommendations

• First-degree relatives of C282Y homozygous patients should be tested specifically for the C282Y variant, as penetrance is higher in family members than general population screening. 1
• Perform both HFE genetic testing and phenotypic screening (transferrin saturation and ferritin) simultaneously in first-degree relatives of confirmed cases. 7
• The primary significance of carrier status is reproductive risk: 25% chance of homozygous offspring if both parents are carriers, and 50% chance if one parent is a carrier and the other is homozygous. 1

Critical Pitfalls to Avoid

• Never order HFE genetic testing when transferrin saturation is <45%, as iron overload is unlikely and over 90% of elevated ferritin cases are due to secondary causes. 7, 8
• Do not assume all C282Y homozygotes will develop clinical disease—penetrance is variable, with only 58-70% developing progressive tissue iron overload and 28.4% of male homozygotes and 1.2% of female homozygotes developing iron-overload-related disease. 1
• Do not rely on H63D variants to explain iron overload—actively search for cofactors such as alcohol use, viral hepatitis, fatty liver disease, or other secondary causes. 1
• Exclude common causes of hyperferritinemia before genetic testing, including chronic alcohol consumption, inflammatory conditions, cell necrosis, malignancy, and metabolic syndrome. 7, 8