Hereditary hemochromatosis (HH) workshop week 3 Flashcards
What are the most affected tissues of iron overload?
Clinical manifestations of iron overload are related to excessive iron deposition in tissues, especially the liver, heart, pancreas, and pituitary
liver: elevated aminostransferases, alk phaos
pituitary: decreased FSH and LH
pancreas: diabetes
(all seen in the case study pt)
What serum iron studies may be performed to diagnose iron overload?
What other tests may be performed?
• The methods used to establish the presence of iron overload include:
– serum iron studies
• Plasma (or serum) iron concentration — normal 60 to 150 µg/dL
• Transferrin concentration (plasma total iron binding capacity) — normal 300 to 360 µg/dL. The ratio of plasma iron to transferrin permits calculation of the transferrin saturation — normal 20 to 50%. transferrin binds 2 iron molecules at once
• Plasma ferritin — normal 40 to 200 ng/mL
– various radiologic techniques
– liver biopsy
– assessment of the response to phlebotomy or chlelation therapy.
• Genetic testing for HH (as almost exclusive cause for iron overload) in the absence of transfusional iron overload. The genetic testing for HH should be performed in all patients suspected of having this disease.
Is transferrin or ferritin more sensitive in detection of iron overload? Why?
Increased iron also stimulates the hepatic production and release of ferritin. An elevated plasma ferritin is generally less sensitive than the transferrin saturation in screening for HH because a greater degree of iron overload is required to raise the ferritin concentration.
In what group of people is HH most common in?
- Hereditary hemochromatosis (HH) is the prototypical disease of iron overload
- The most common Mendelian (autosomal recessive) genetic disorder in Caucasians, HH occurs in 1:200; cases are concentrated in Northern Europe.
T or F: In pts with HH, increased iron absorption occurs when iron intake is normal.
True.
The diagnosis of HH is established based on transferrin saturation (TS) which is defined as what?
- The diagnosis of HH is established based on transferrin saturation (TS), defined as serum iron divided by total iron binding capacity (TIBC).
- Measurement of fasting TS is recommended as a first screen to detect iron overload (serum iron and ferritin levels lack specificity for diagnosis when used alone).
- TS is the best indirect biochemical marker of iron stores; TS greater than 45% will indicate phenotypic HH.
- Once serum TS >45% and the serum Ferritin elevated, a PCR-based gene test for HH is recommended.
What are the 2 mutations consistent with HH? In what protein are these mutations?
2 genotypic profiles in HFE gene are consistent with the diagnosis of HH and are the cause of increased intestinal iron absorption:
Homozygosity for the C282Y mutation
Heterozygosity for C282Y/H63D mutation
Human hemochromatosis protein is encoded by the HFE gene (HFE for High Iron Fe).
What protein does the HFE protein interact with? Why is this protein important?
What are the domains of the HFE protein?
In wh5at domain are the two most common mutations (C282Y and H63D) found in?
Which mutation is more common?
• It codes a protein that requires interaction with β2 microglobulin for normal presentation to the cell surface. If it cannot interact with this protein, HFE will not go to the membrane.
• The protein contains 343 aa
– 22 aa signal peptide
– Large extracellular domain
– Single transmembrane domain
– Short cytoplasmic tail
• Extracellular domain includes three loops (D1, D2, and D3) with intramolecular disulfide bonds within the second and third loops.
- The two most common mutations, C282Y and H63D, are in the extracellular domain.
- The C282Y mutation represents a change from Cysteine to Tyrosine at aa 282.
- The H63D mutation is a substitution of Aspartate for Histidine at aa 63.
- 83-100% of patients with HH are homozygous for C282Y.
- 10-15% of Caucasians of European ancestries are heterozygous for the C282Y mutation.
Pts with what genotype are candidates for having symptomatic iron load? Why is this?
Why do women become symptomatic later in life?
Patients with a C282Y/C282Y genotype are HOMOZYGOUS for HH, and are candidates for having symptomatic iron overload. In these patients, the absorption of heme iron is not regulated by the content of iron stores, as measured by the serum ferritin. Subjects with HH may absorb 2 to 4 mg of iron per day from heme and non-heme iron sources, rather than the normal value of 1 mg/day needed in males to balance iron loss from sloughed skin and minor gastrointestinal blood loss. Absorption of 4 mg/day (3 mg/day in excess of needs) would lead to net iron accumulation of approximately 1 g per year, or more than 20 g of iron from the end of the adolescent growth spurt in males to age 40 or 50. Subjects with this degree of iron overload are poised to develop the clinical features of HH. Women become symptomatic later in life, because of the extra iron losses associated with menses, delivery, and lactation.
Pts that are heterozygotes for the C282Y/H63D mutations are at what risk of iron loading?
Patients with a C282Y/H63D genotype (ie, one allele with each mutation) have a COMPOUND HETEROZYGOTE genotype, which occurs in 4-7% of patients with HH.
Approximately 60% of compound heterozygotes have an intermediate degree of iron loading, and 35 percent have normal iron burdens.
What protein does HFE protein interact with to regulate iron stores? What does HFE binding to this protein do?
What does the C282Y mutation do as it pertains to interaction with this protein?
Iron balance in the normal state is maintained through mechanisms that control absorption. Iron in the circulation is bound to the protein, called “transferrin”, which maintains it in a non-toxic state. Cells contain receptors for transferrin on their plasma membranes which mediate cellular iron uptake. Transferrin receptors bind iron-transferrin complexes which are taken into endosomes. Iron is separated from transferrin in the endosome, and is shuttled into the interior of the cell. The iron-free transferrin (apotransferrin) is recycled into the circulation and is free to bind and transport additional iron atoms.
The HFE protein acts as a major regulator of iron absorption by binding to the transferrin receptor thus decreasing the affinity of the transferrin receptor for iron-loaded transferrin. HFE protein production is regulated in response to iron stores.
- The HFE protein interacts with other proteins on the cell surface to detect the amount of iron in the body.
- The HFE protein interacts with TfR1 at the HFE-Beta 2 microglobulin complex.
- By abolishing a disulfide bond in the D3 loop of the HFE’s transmembrane domain, the C282Y mutation interferes with this interaction.
- HFE binds TfR1 with an affinity similar to that of transferrin.
- Binding to TfR1 is required for HFE to be transported to transferrin-positive endosomes for regulation of intracellular iron homeostasis.
see figures on pg 142-143 of course notes
What protein does HFE regulate the production of? What is the role of this protein?
- The HFE protein regulates the production of another protein called hepcidin, which is considered the “master” iron regulatory hormone.
- Hepcidin determines how much iron is absorbed from the diet and released from storage sites in the body
• Circulating hepcidin may play a role in inhibiting iron absorption in the small bowel, and that this effect is suppressed in patients with HFE mutations
Hepcidin inhibits ferroportin, a protein responsible for the transport of iron from enterocytes, macrophages and hepatocytes into the blood.
see figure on pg 145 of course notes
What is the cellular mechanism of HFE regulation of hepcidin?
A multiprotein complex consisting of HFE, TfR2 (transferrin receptor 2), HJV. and BMP exists in hepatocyte cell membranes.
BMP is activated by increased iron levels. HFE binds to TfR2 and interacts with this complex to decrease the ubiquitination of BMP. BMP, through the SMAD pathway, upregulates hepcidin transcription.
Note that there may be mutations of other proteins in this complex but HFE mutation is more common.
Why is the liver particularly susceptible to iron overload?
What does excess iron cause in the liver?
• Without a mechanism for its excretion, iron accumulates in vital organs
• Because the liver binds both circulating non-transferrin and transferrin-bound iron, the liver is at particular risk for iron overload
• Excess iron causes:
– damage to hepatocytes through induction of oxidative stress.
– damage to hepatocellular organelles:
• Mitochondria shows increased fragility, volume, pH, lipid peroxidation; decreased fluidity;
• Lysosomes show increased fragility and release of hydrolytic enzymes directly into the cytoplasm, further promoting cellular damage.
– fibrosis of the liver (excessive accumulation of the extracellular matrix), direct effect
– hepatocellular necrosis, direct effect on hepatocytes
– fibrogenesis acting as a cofactor for other hepatotoxins (alcohol, viruses), indirect effect
What is the therapy for iron overload? What sx may this therapy reverse?
What diseases will this therapy likely not reverse?
What is the major cause of death in pts with noncirrhotic HH?
What 2 levels should continue to be monitored in these pts?
How long should this therapy be continued?
• Once iron overload is established, current recommendations are to proceed with therapeutic phlebotomy, which ideally should be initiated before the onset of clinically significant disease
• Phlebotomy:
– Could reverse the malaise, fatigue, skin pigmentation, abdominal pain
– Has less chance to effect hemochromatosis associated arthropathy, hypogonadism, cirrhosis
– Can prevent diabetes mellitus, if initiated prior to pancreatic damage. While phlebotomy may reduce insulin requirements, hemochromatosis-associated diabetes with onset prior to initiation of phlebotomy will likely continue
• Diabetes mellitus remains a major cause of death in patients with noncirrhotic HH, occurring 7-times more frequently when compared with normal controls
• Routine therapeutic phlebotomy, with a goal of the removal of 500 ml of whole blood (~200-250 mg of iron) weekly or biweekly, should be continued until iron-limited erythropoiesis develops
• Transferrin saturation and ferritin levels should be also monitored
