Hereditary Hemochromatosis via the HFE Gene

Hereditary Hemochromatosis (HH) is an inherited iron overload disorder which, if untreated, may lead to progressive and potentially fatal organ dysfunction. Chronic iron deposition can result in advanced fibrotic liver disease, cirrhosis, hepatocellular carcinoma, atherosclerosis, arthritis, fatigue, diabetes, hypogonadism, cardiomyopathy, and diffuse skin pigmentation. Fatigue, impotence, and arthritis are the most commonly found symptoms (Vujic 2014; Leitman 2013). About 5 in every 1,000 western Whites have HH, although the majority are asymptomatic. Disease penetrance is variable with environmental factors such as nonalcoholic fatty liver disease, excessive alcohol consumption, and viral hepatitis hastening symptom onset. Symptom onset is typically after age 40 with men being more susceptible than women. There are six types of hemochromatosis each due to a different genetic cause: type 1- HFE, type 2- HAMP or HJV, type 3- TRF2, type 4- SLC40A1, type 5- FTH1, and type 6- FTL. Type 1 hemochromatosis is the most prevalent form of the disease and represents >90% of cases (Vujic 2014). Genetic testing can be helpful in differential diagnosis of HH from other liver function disorders and for determining the underlying cause of hemochromatosis (Zarrilli et al. 2013). Phlebotomy is standard treatment to reduce serum iron levels and prevent progressive liver damage.

HH is inherited in an autosomal recessive manner through mutation in the HFE gene. Hemochromatosis may also been inherited in an autosomal recessive mode through mutations in the HAMP, HJV, or TRF2 genes or an autosomal dominant pattern through mutations in the SLC40A1, FTH1, or FTL genes. In patients identified with HH via transferrin saturation analysis, 60% of patients are homozygous for c.845G>A (p.Cys282Tyr), 8% homozygous for c.187C>G (p.His63Asp), and 7% are compound heterozygous for the two variants (de Villiers et al. 1999; Stuhrmann et al. 2010). The p.Cys282Tyr mutation disrupts an internal disulfide bond affecting the tertiary protein structure leading to intracellular degradation (Feder et al. 1996). The c.187C>G mutation (p.His63Asp) is a low penetrant mutation with homozygous individuals largely being asymptomatic (Sham et al. 2000). Other missense and nonsense mutations have been identified in the HFE gene in compound heterozygotes with the p.Cys282Tyr mutation (Piperno et al. 2000). Deletion of the entire HFE gene has been reported in two cases (Le Gac et al. 2008; Pelucchi et al. 2009). The HFE gene encodes the HFE protein which is sequestered by transferrin receptor 1 at the hepatocyte cell membrane to act as an iron sensing complex. In cases of iron overload, the HFE/transferrin complex will upregulate herpcidin expression which functions to regulate iron transport from the gut (Vujic 2014).

Variability in disease penetrance, criteria for diagnosis, and environmental factors contribute to difficulties in determining clinical sensitivity. In patients where HH is clearly defined and other causes of iron overload have been ruled out, clinical sensitivity is >90% in a study of Northern Europeans (Bryant et al. 2008). Analytical sensitivity for detection of causative variants in the HFE gene is >95% as gross gene deletions have been reported in very few cases (Le Gac et al. 2008).

This test provides full coverage of all coding exons of the HFE gene, plus ~10 bases of flanking noncoding DNA. We define full coverage as >20X NGS reads or Sanger sequencing.