6p22.2

HFE1,HH,HLA-H,MVCD7,TFQTL2

Iron Overload

Mutations in HFE increase body iron levels and homozygosity or compound heterozygosity may cause iron overload. The penetrance is low. Dietary iron intake, alcohol consumption and blood loss are environmental modifiers. The importance of iron overload is that it increases the risk for cancer development presumably due to its potential to cause oxidative DNA damage.

Hereditary Hemochromatosis

Hereditary hemochromatosis (HH; OMIM 235200) is a recessive iron storage disorder resulting from defects in HFE. HH (type 1) is the most common autosomal recessive disease in Caucasians adults. Most patients (about 90%) are homozygous for the C282Y mutation and another 4% are compound heterozygotes (C282Y, H63D). Different forms of non-HFE hemochromatosis are caused by other iron-related genes: type 2 (mutations in HFE2), type 3 (mutations in TFR2) and type 4 (mutations in SLC40A1 ferroportin). HH is characterized by abnormal intestinal iron absorption and elevated total body iron levels. Iron overload results in clinical complications including cirrhosis, cardiopathy, endocrine dysfunctions including diabetes, arthropathy and susceptibility to liver cancer. The penetrance is higher in males due to regular blood loss in premenopausal women. Disease complications can be prevented by regular phlebotomy. The effect of HFE on disease phenotype can be modified by other iron-related genes including hepcidin (HAMP), transferrin (TF), transferrin receptor (TFRC), haptoglobin (HP) and ceruloplasmin (CP).

Porphyria variegata

Defects in HFE also cause porphyria variegata (OMIM 176200). Porphyrias are inherited defects in the biosynthesis of heme, resulting in the accumulation and increased excretion of porphyrins or porphyrin precursors. Porphyria variegata is the prevalent form in South Africa. It is characterized by skin hyperpigmentation and hypertrichosis, abdominal pain, tachycardia, hypertension and neuromuscular disturbances. Iron overload is the hallmark of the disease.

Leukemias

HFE mutations do not cause cancer and HFE mutations are not detected preferentially in cancer cells as somatic mutations. Both C282Y and H63D mutations, however, have been implicated in susceptibility to leukemias and other cancers. In South Wales (U.K.), C282Y mutation is associated with increased risk to childhood acute lymphoblastic leukemia in boys only. This association has not been noted in other studies in Finland, Spain and Mexico. In Italy, adult leukemia shows an association with H63D mutation.

Breast Cancer

Studies in USA, Russia and Turkey have found risk associations with HFE mutations C282Y and/or H63D with breast cancer. A Swedish study found a risk association only in women homozygous for the TFRC variant S142G.

Other cancers

In Sweden, combination of HFE mutation C282Y and/or H63D and homozygosity for the TFRC variant S142G increase susceptibility to multiple myeloma, hepatocellular carcinoma and colon cancer (besides breast cancer). An interaction of HFE mutations with dietary intake of excessive iron also increases the risk for colorectal cancer. Various studies have reported increased frequency of HFE mutations in hepatocellular carcinoma secondary to hepatic iron overload but not in HCV-induced hepatocellular carcinoma. There appears to be an interaction between HFE and alcohol in the induction of iron overload, cirrhosis and subsequent hepatocellular carcinoma. For each genetic association report between HFE and any cancer, there is also one or more negative association report. It appears that only large and comprehensive studies taking into account gene x gene and gene x environment interactions may conclude this issue.