NSG/521 Week 1: Genetics and Disease: Hereditary Hemochromatosis, sample paper

Two Copies of One Mutation: The Pathophysiology of HFE Hereditary Hemochromatosis in a 52-Year-Old Man With Aching Knuckles and a High Ferritin

[Student Name]

University of Phoenix

NSG/521: Advanced Pathophysiology

Week 1 Assignment

[Instructor Name]

[Date]

Composite patient written as a model document. No real patient is described.

A composite 52-year-old man of Irish descent saw a nurse practitioner for fatigue and aching in his hands. On examination, the second and third metacarpophalangeal joints of both hands were tender and slightly swollen. Laboratory tests showed an alanine aminotransferase of 68 U/L, a fasting transferrin saturation of 78% and a serum ferritin of 1,450 ng/mL. Genetic testing found him homozygous for the C282Y variant of the HFE gene. One altered amino acid in a protein most people have never heard of had been quietly loading his liver, joints and pancreas with iron for decades. This paper explains the pathophysiology of HFE hereditary hemochromatosis as it applies to his presentation.

Normal Iron Regulation

The body has no active way to excrete iron. It loses only small amounts through shed cells and bleeding, so iron balance is controlled almost entirely at the point of absorption in the duodenum. The key regulator is hepcidin, a peptide hormone made by the liver. Hepcidin binds ferroportin, the only known cellular iron exporter, on duodenal enterocytes and macrophages, causing ferroportin to be internalized and degraded. When hepcidin is high, iron stays trapped in enterocytes and is lost when they shed; when hepcidin is low, ferroportin exports iron into the blood (Pietrangelo, 2010).

The liver adjusts hepcidin production to iron stores. HFE protein, expressed on hepatocytes, interacts with transferrin receptors and participates in the signaling that tells the liver how much iron is circulating, so that rising iron increases hepcidin and limits further absorption.

From Gene to Disease

The C282Y variant replaces cysteine with tyrosine at position 282 of the HFE protein. The substitution disrupts a disulfide bond needed for the protein to fold correctly and reach the cell surface, so the mutant HFE is largely retained inside the cell. Without functional HFE at the cell surface, hepatocytes underestimate body iron and produce inappropriately little hepcidin (Pietrangelo, 2010).

Low hepcidin leaves ferroportin active on duodenal enterocytes and macrophages. Absorption continues at a high rate regardless of body stores, and macrophages release recycled iron freely. The result is a slow, lifelong positive iron balance, typically a few milligrams a day more than needed, which accumulates to many grams over decades.

As transferrin, the plasma iron carrier, becomes saturated, iron circulates in forms not bound to transferrin. This non-transferrin-bound iron is taken up avidly by parenchymal cells of the liver, heart, pancreas and endocrine glands. Inside cells, excess free iron catalyzes the formation of hydroxyl radicals through the Fenton reaction, damaging lipids, proteins and DNA. The injury triggers inflammation and, in the liver, activation of stellate cells, which lay down collagen and drive fibrosis and eventually cirrhosis.

Linking the Mechanism to the Findings

Each of the patient's findings follows from this chain. His transferrin saturation of 78% reflects the unregulated absorption and release of iron into plasma; a fasting saturation above 45% is the usual threshold for suspecting iron overload (Kowdley et al., 2019). His ferritin of 1,450 ng/mL reflects large tissue stores. His elevated alanine aminotransferase reflects hepatocellular injury from iron deposition, and a ferritin above 1,000 ng/mL raises the risk of advanced fibrosis enough that guidelines recommend assessing for it (Kowdley et al., 2019). His fatigue is a common, nonspecific symptom. His arthropathy of the second and third metacarpophalangeal joints is characteristic of hemochromatosis and reflects iron-associated cartilage damage and associated crystal deposition.

Other organs are at risk as the disease progresses: iron in pancreatic beta cells can cause diabetes, iron in the heart can cause cardiomyopathy and arrhythmias, iron in the pituitary can cause hypogonadism and iron with increased melanin can darken the skin. The patient's fasting glucose and cardiac examination should therefore be checked.

Ruling Out Other Causes of a High Ferritin

A high ferritin does not always mean iron overload. Ferritin is also an acute phase reactant and rises with inflammation, infection, alcohol use, metabolic dysfunction-associated fatty liver disease and cell injury of many kinds. In those conditions, ferritin rises while transferrin saturation usually stays normal, because the problem is iron trapped in cells or ferritin released from damaged tissue, not excess iron entering the plasma. This patient's high transferrin saturation alongside the high ferritin is what points to true iron overload, and his genotype explains it. Secondary iron overload, from repeated transfusions or ineffective red cell production in conditions such as thalassemia, can produce a similar pattern, so his blood count and transfusion history were reviewed and were normal. Distinguishing these causes matters because phlebotomy treats genetic iron overload but would harm a patient whose high ferritin reflects inflammation or anemia.

Inheritance and Penetrance

HFE hemochromatosis is inherited in an autosomal recessive pattern. The patient inherited one C282Y allele from each parent. Each of his siblings had a 25% chance of being homozygous, a 50% chance of being a carrier and a 25% chance of inheriting neither variant. His children are obligate carriers and will be homozygous only if their other parent also carries a variant.

Penetrance is incomplete and differs by sex. Allen et al. (2008), in a large prospective population cohort, found that iron-overload-related disease occurred in about 28% of male C282Y homozygotes but in only about 1% of female homozygotes. Women are protected in part by iron losses through menstruation and pregnancy. Alcohol use, obesity and other liver disease increase the risk of liver damage in homozygotes.

Genetic Testing and the Family

Genetic testing confirmed the diagnosis in this patient, but its larger value is for his family. Guidelines recommend testing first-degree relatives of people with HFE hemochromatosis, including siblings, with iron studies and HFE genotyping (Kowdley et al., 2019). The patient has two brothers and a sister. Because of sex differences in penetrance, his brothers are at particular risk if they are homozygous, and early detection would allow them to begin treatment before organ damage occurs. Carrier status in his children has little health significance but could be relevant to their own future children.

Implications for Treatment

The mechanism explains the treatment. Because the body cannot excrete iron, iron must be removed by therapeutic phlebotomy; each unit of blood removes iron in hemoglobin, and the body mobilizes stored iron to make new red cells. Guidelines recommend weekly phlebotomy until ferritin falls to a target range, followed by maintenance phlebotomy (Kowdley et al., 2019). Treatment started before cirrhosis or diabetes develops can prevent them, which is why recognizing the disease early, and testing relatives, matters.

Conclusion

A single amino acid substitution in the HFE protein disrupts the liver's sensing of iron, lowers hepcidin, leaves ferroportin active and allows iron to accumulate for decades. Non-transferrin-bound iron enters the liver, joints, pancreas and heart, where oxidative injury produces the fibrosis, arthropathy and endocrine damage that define the disease. In this 52-year-old man, the mechanism explains each finding and the treatment, and the genetics turn his diagnosis into an opportunity to protect his siblings.

References

Allen, K. J., Gurrin, L. C., Constantine, C. C., Osborne, N. J., Delatycki, M. B., Nicoll, A. J., McLaren, C. E., Bahlo, M., Nisselle, A. E., Vulpe, C. D., Anderson, G. J., Southey, M. C., Giles, G. G., English, D. R., Hopper, J. L., Olynyk, J. K., Powell, L. W., & Gertig, D. M. (2008). Iron-overload-related disease in HFE hereditary hemochromatosis. New England Journal of Medicine, 358(3), 221-230. https://doi.org/10.1056/NEJMoa073286

Kowdley, K. V., Brown, K. E., Ahn, J., & Sundaram, V. (2019). ACG clinical guideline: Hereditary hemochromatosis. American Journal of Gastroenterology, 114(8), 1202-1218. https://doi.org/10.14309/ajg.0000000000000315

Pietrangelo, A. (2010). Hereditary hemochromatosis: Pathogenesis, diagnosis, and treatment. Gastroenterology, 139(2), 393-408. https://doi.org/10.1053/j.gastro.2010.06.013