Liver Disease in Iron Overload


Abbreviations

A1AT

alpha-1 antitrypsin

DIOS

dysmetabolic iron overload syndrome

DMT1

divalent metal transporter 1

FPN

ferroportin

HH

hereditary hemochromatosis

HIC

hepatic iron concentration

HII

hepatic iron index

HJV

hemojuvelin

IRIDA

iron-refractory iron-deficient anemia

MHC

major histocompatibility complex

NAFLD

nonalcoholic fatty liver disease

NASH

nonalcoholic steatohepatitis

NTBI

nontransferrin-bound iron

RES

reticuloendothelial system

ROS

reactive oxygen species

TF

transferrin

TFR1

transferrin receptor 1

TFR2

transferrin receptor 2

In 1996, the genetic mutation responsible for the vast majority of cases of hereditary hemochromatosis was identified by Feder and colleagues, who described two missense mutations in the HFE gene located on chromosome 6p. However, the path to this milestone in the history of hereditary hemochromatosis was long, beginning with von Recklinghausen, who in 1889 described excess tissue iron at autopsy thought to be derived from the bloodstream; he termed the condition hemochromatose . In 1935, Sheldon proposed that iron overload was secondary to increased iron absorption. Almost two decades later, in 1952, Davis and Arrowsmith reported benefits from treating these patients with phlebotomy. In 1962, the diagnosis of hemochromatosis in liver samples along with a grading system for iron was developed by Scheuer and associates. A human leukocyte antigen locus associated with the condition was recognized in 1975, which, along with blood tests for serum iron, iron-binding capacity, and ferritin, further facilitated the diagnosis of hemochromatosis. The discovery of the HFE gene mutation in 1996 led to a new genetic era that has witnessed the identification of several molecules involved in iron homeostasis along with their related genes.

Iron accumulation in the liver is not an uncommon histologic finding. In addition to liver biopsies performed for investigation of clinically suspected iron overload, iron accumulation is often seen as an unexpected finding in liver biopsies performed for unrelated conditions. Hepatic siderosis occurs in a wide array of conditions including hereditary iron overload disorders, chronic liver diseases, and cirrhosis of various causes, as well as hematologic diseases, which result in hemolysis or require transfusions. The terms genetic hemochromatosis and hereditary hemochromatosis (HH) refer to disorders of iron overload transmitted as autosomal recessive traits; iron overload, due to mutations in the HFE gene, is sometimes called HFE hemochromatosis, whereas that caused by mutations in genes other than the HFE gene may be referred to as non-HFE hemochromatosis. Classification of hereditary hemochromatosis and its characteristics are outlined in Table 11.1 .

Table 11.1
Classification of Hereditary Hemochromatosis
Type Genetics Inheritance Age at Onset Salient Clinical Features
1 HFE gene mutations (chromosome 6):
C282Y homozygous
C282Y/H63D
Recessive Fifth decade, with men presenting earlier than women Symptoms related to effects of iron deposition in various organs: cardiomyopathy, diabetes mellitus, complications of cirrhosis (eg, portal hypertension, hepatocellular carcinoma, skin pigmentation, hypogonadism, arthritis)
2A Hemojuvelin gene mutation (chromosome 1) Recessive Adolescents or adults before the age of 30 Prominent cardiac iron deposition with risk of heart failure and potential hypogonadism
2B Hepcidin gene mutation (chromosome 19) Recessive Adolescents or adults before the age of 30 Phenotypically similar to hereditary hemochromatosis type 1
3 Transferrin receptor 2 gene mutation (chromosome 7) Recessive Same as hereditary hemochromatosis type 1 Mimics hereditary hemochromatosis type 1
4 SLC40A1 gene mutation (chromosome 2) encodes ferroportin Dominant Biochemical abnormalities as early as 1st decade, disease manifestation fourth to fifth decade Variable: In most cases, iron deposits found within macrophages, no significant fibrosis, classical asymptomatic form with elevated hyperferritinemia contrasting with normal or mildly increased transferrin saturation
Rarely, iron predominantly located within parenchymal cells, either severe fibrosis or cirrhosis in some cases. Transferrin saturation usually markedly elevated

The purpose of this chapter is to review the histologic patterns of iron deposition and to provide a pattern recognition approach to the pathology of iron overload in the liver. A knowledge of iron homeostasis and the genes involved in it is necessary for understanding the disease states that result from disrupted homeostasis or genetic alterations and that generate the iron overload patterns encountered in liver tissue.

Iron Homeostasis

Approximately 1 to 2 mg of iron are absorbed daily in the duodenum, and the body has an additional 0.5 g of iron stored in the liver. Once absorbed, the body possesses no physiologic mechanisms to eliminate iron; therefore, control of iron homeostasis occurs solely by modulating duodenal iron absorption in the duodenum. Following the discovery of the HFE gene, several additional molecules involved in iron homeostasis have been identified ; principal among these are divalent metal transporter 1 (DMT1), ferroportin (FPN), hepcidin, transferrin receptor 2 (TFR2), and HFE. Mutations in genes encoding for all these molecules have been associated with HH, except for DMT1, the transmembrane transporter of ferrous ions from the intestinal lumen into the duodenal absorptive cells.

Upon absorption into plasma, iron is quickly bound to the iron transporting protein transferrin (TF) and delivered to cells throughout body cells. Target cells take up iron bound to transferrin by binding with and endocytosis of transferrin receptor 1 (TFR1). Within individual cells, iron is either used for metabolic processes or stored within ferritin. When the plasma iron concentration is elevated, transferrin becomes saturated and excess iron binds to low-molecular-weight molecules present in plasma such as citrate and acetate, or to albumin known, when it is referred to as nontransferrin-bound iron (NTBI).

HFE

HFE gene mutations characterize type 1 HH and contribute to the majority of cases of clinical HH. The gene encodes for a ubiquitously expressed major histocompatibility complex (MHC) class I–like mole, which, like other molecules in this class, is complexed with β-2 microglobulin on cellular membranes, but differs from them by the absence of a peptide-binding groove necessary for antigen presentation. The HFE/β-2 microglobulin complex is localized adjacent to and binds with TFR2, which facilitates cellular uptake of iron. Expression of the HFE protein is particularly strong on hepatocytes and cells in the deeper portions of duodenal crypts; however, because neither site is involved in absorption of iron, these localizations fail to explain a simple cause-and-effect relationship between HFE and the increased iron absorption that occurs in HH. The control of iron absorption by HFE is therefore indirect and is now known to occur through modulation of hepcidin, a suppressor of iron absorption produced by the liver. The abundance of HFE on hepatocyte membranes and the production of hepcidin by the liver is in keeping with the age-old observation that the liver plays a central role in iron homeostasis and the development of HH.

Hepcidin and Ferroportin

Hepcidin, a protein encoded by the hepatic antimicrobial protein gene, was initially discovered as an antimicrobial peptide produced by hepatocytes in response to inflammatory stimuli. It has subsequently been recognized as a negative regulator of iron absorption, being a central player in many forms of hemochromatosis. Secreted by hepatocytes into the circulation, it travels via the bloodstream to exert its effect on distant cells, thus behaving like an “iron hormone” and earning itself the moniker of “The Hemochromatosis Hormone.” The existence of a circulating substance was long suspected as the only plausible mechanism for control of iron homeostasis through duodenal absorption, a site distant from the main sites of iron use. Similar to all hormones, hepcidin is controlled by a feedback loop, and serum hepcidin levels are inversely related to serum iron levels. Hepcidin production is regulated by iron levels in the blood and iron stores in the liver, and by erythropoietic activity, through the hormone erythroferrone. When iron levels are high, hepcidin production is increased to inhibit dietary iron absorption, and release iron from the stores. When iron levels are low or there is increased demand on iron for cellular processes; hepcidin production is decreased to allow iron to enter the plasma to meet the iron demand. Hepcidin production is decreased when iron is needed for hemoglobin synthesis.

FPN is a molecule located on cell membranes that facilitates export of iron out of cells. It is expressed on the basolateral surface of duodenal enterocytes, where it facilitates export of absorbed iron into the circulation, as well as on macrophages and hepatocytes, where it allows release of stored iron into the circulation when serum iron levels are low ( Fig. 11.1 ).

Figure 11.1, Iron is absorbed ( black arrows ) from the intestinal lumen through the brush border of the duodenal crypt cells. It exits the basolateral membrane of this cell via ferroportin into the bloodstream to reach various sites of utilization ( white arrows ). When required, stored iron is also released from macrophages into the bloodstream through ferroportin. When body stores are adequate, hepcidin is released from the liver ( red arrows ), and it binds with ferroportin. Binding causes internalization of ferroportin, decreasing both release from macrophages as well as absorption through modulation of “upstream” events.

Hepcidin exerts its biologic effect by binding FPN, which causes the latter to be internalized and degraded, thus halting iron export into the circulation. When there is excess iron in the circulation, hepcidin is released and it binds FPN, inhibiting further release of iron into the circulation (see Fig. 11.1 ). Although the inverse relationship between iron levels and hepcidin is well established, the exact mechanism that senses iron levels and controls hepcidin release remains to be completely elucidated; it perhaps involves the HFE molecule, which, through its biologic interaction with TFR2, could well serve as a sensor of iron levels. The function of HFE is to stimulate hepcidin expression in proportion to the concentrations of iron transferrin, by associating with and stabilizing other hepcidin regulators—TFR2 and the BMP receptor Alk3. This sensor function is presumably interrupted by mutations in the HFE gene (type 1 HH) or in the TFR2 gene (type 3 HH), leading to inadequate release of hepcidin and continued iron absorption in the face of increasing iron accumulation. Both HFE and TFR2 are particularly abundant on the surface of hepatocytes, the cells that produce and secrete hepcidin.

The major role of hepcidin in regulation of iron homeostasis is borne out by several clinical observations. Hepcidin levels are greatly reduced in all forms of HH; the variation in the degree of iron accumulation among individuals may be explained by the presence or absence of additional mutations either in the hepcidin gene itself or in other molecules that modulate its secretion. Thus, patients who have mutations in the hepcidin gene in addition to the HFE gene develop severe iron accumulation. Patients with mutations in the hepcidin gene itself have a severe and early onset form of hemochromatosis (type 2A hemochromatosis, juvenile hemochromatosis). Severe clinical disease is also seen with mutations of hemojuvelin (HJV) (type 2B hemochromatosis). In vitro evidence suggests that HJV is a transcriptional regulator of hepcidin. In one study, cellular HJV was found to positively regulate hepcidin mRNA expression, and recombinant soluble HJV suppressed hepcidin mRNA expression in a primary culture of human hepatocytes. Levels of hepcidin are depressed in individuals with HJV mutations and in HJV knockout mice. The less severe iron accumulation in patients with types 1 and 3 HH suggests that HFE and TFR2 may be independent but complementary modulators of hepcidin; thus, mutation in one gene may be compensated by the other in the presence of normal hepcidin.

In addition, hepcidin exerts an antimicrobial role by limiting the amount of iron available to invading microbes. Hepcidin levels are elevated in infections. Chronic infections and prolonged inflammatory diseases lead to sustained low hepcidin levels, accounting for accumulation of iron within enterocytes and macrophages with resultant reduced availability for erythropoiesis and so-called “anemia of chronic disease.” In contrast, hepcidin expression is decreased by iron deficiency, erythropoiesis, and hypoxia, resulting in increased iron absorption and enhanced iron release from macrophages.

The interplay between these iron regulating genes, and their modulating effects on iron homeostasis, and those of their mutations on each other, can be conceptualized along 2 axes: (a) hepcidin deficiency/hepcidin resistance and (b) hepcidin excess/ferroportin deficiency.

Hepcidin Deficiency/Hepcidin Resistance

As mentioned earlier in the discussion of hereditary hemochromatosis, there is deficient hepcidin production and increased expression of ferroportin on cell membranes, resulting in hyperabsorption of dietary iron by duodenal enterocytes, as well as increased release of iron from macrophages. This leads to increased iron concentration and transferrin saturation in plasma. Saturation of transferrin in turn leads to presence of NTBI in plasma. Because there is no regulated mechanism for excretion of iron in humans, excess iron is deposited in tissues that express transporters for NTBI (predominantly the liver, but also heart, pancreas, and other endocrine glands). The inadequate hepcidin production in response to iron loading is most commonly caused by mutations in genes encoding iron sensors or signaling pathways that regulate hepcidin production. Autosomal recessive mutations in the hemochromatosis gene (HFE) are the most frequent cause of genetic iron overload (type 1 hereditary hemochromatosis), particularly homozygous HFE C282Y mutations and occasionally compound heterozygote C282Y/H63D mutations. However, the genetic disease is incompletely penetrant and iron overload usually occurs only in the presence of modifying factors that further decrease hepcidin production, such as alcohol consumption or additional genetic mutations; that is, in the GNPAT gene.

Mutations in HJV (type 2A hereditary hemochromatosis), hepcidin gene itself (type 2B hereditary hemochromatosis), or TFR 2 (type 3 hereditary hemochromatosis) cause more penetrant and severe forms of hereditary hemochromatosis. Mutations in hemojuvelin and hepcidin result in juvenile hemochromatosis, an early onset and severe form of genetic iron overload. Hemojuvelin functions as a BMP coreceptor and loss of its function results in severe decrease in hepcidin expression, which is unresponsive to iron loading. The specific mechanism by which TFR2 affects hepcidin expression is not well understood. Autosomal dominant mutations in the hepcidin receptor, ferroportin (type 4 hereditary hemochromatosis) such as C326S lead to resistance of ferroportin to the effects of hepcidin so that iron is exported to the circulation even in the presence of adequate levels of circulating hepcidin.

Hepcidin Excess/Ferroportin Deficiency

Elevated levels of hepcidin result in decreased availability of iron for erythropoiesis leading to different types of anemia. The underlying causes of elevated hepcidin levels in iron-restricted anemias may be genetic or nongenetic. Familial iron-refractory iron-deficient anemia (IRIDA) is a genetic cause of elevated levels of hepcidin in plasma. IRIDA is an autosomal recessive disorder caused by a mutation in matriptase-2 (also known as TMPRSS6), a negative regulator of hepcidin expression. Patients with IRIDA cannot suppress hepcidin production even when iron-deficient, resulting in abnormally increased hepcidin production. Inflammation is a nongenetic cause of hepcidin overproduction. In chronic inflammatory disorders such as inflammatory bowel disease, patients have elevated hepcidin and low iron plasma levels in the presence of anemia (“anemia of chronic disease”).

Microscopic Pathology

Iron Pigment

Iron may be deposited in cells in soluble or nonsoluble forms. Ferritin, the soluble form of iron, cannot be seen on a hematoxylin and eosin stain; a Prussian blue reaction demonstrates a nongranular, pale blue cytoplasmic “blush” in hepatocytes and/or macrophages ( Fig. 11.2 ). Hemosiderin, the insoluble form of iron sequestered within cells in membrane-bound lysosomes, appears as golden-brown, refractile granules on a hematoxylin and eosin stain ( Fig. 11.3 ) and as coarse, dark blue particles and granules on a Perls’ stain (see Fig. 11.2 ). Iron deposits may be present in hepatocytes, bile duct epithelium, endothelial cells lining portal or hepatic veins, sinusoidal endothelial cells, Kupffer cells, and tissue macrophages within portal tracts ( eSlide 11.1 ). Performing iron stains routinely on liver biopsy specimens allows for detection of iron overload in unsuspected cases or when accumulated amounts are small.

Figure 11.2, Hepatocytes showing faint blue cytoplasmic blush representing ferritin ( asterisk ), the soluble form of iron. Discrete granules of hemosiderin ( arrowhead ), the insoluble form of iron, are also seen in a hepatocyte (Prussian blue stain).

Figure 11.3, Hemosiderin appears as golden-brown, refractile granules in hepatocytes. Periportal sideronecrosis is seen as a focus of aggregated acidophilic hepatocytes ( between arrows ) closely associated with heavily iron-laden Kupffer cell aggregates. PT , Portal tract.

Patterns of Iron Deposition

Before identification of the HFE gene, the pattern of iron distribution was used as a significant parameter in the differentiation of HH from secondary causes of iron deposition, then called primary and secondary hemosiderosis , respectively. In this paradigm, HH showed a parenchymal pattern of iron deposition, whereas deposition in Kupffer cells was a feature of secondary hemosiderosis. Deposition in bile ducts was thought to occur in primary but not secondary hemosiderosis.

It has subsequently become apparent that there is considerable overlap between these patterns ( Figs. 11.4 and 11.5 ), as detailed later. Iron deposition in the liver assumes one of three patterns: parenchymal iron deposition, mesenchymal iron deposition, or mixed parenchymal-mesenchymal deposition ( Table 11.2 ).

Figure 11.4, Hemosiderin deposition in hepatocytes and in bile duct epithelium ( arrows ), in HFE-related hemochromatosis with severe accumulation (Prussian blue stain). Notice similarity to hemosiderin deposition in ducts in Fig. 11.5 in a case of nongenetic iron overload due to dyserythropoiesis (see also eSlide 11.1 , eSlide 11.5 ).

Figure 11.5, High-power view of liver biopsy shown in Fig. 11.9 . Hemosiderin granule deposition in hepatocytes ( asterisk ), in the wall of an artery in a portal tract ( arrowhead ), and in bile duct epithelium ( arrow ) in severe iron accumulation in myelodysplastic syndrome (Prussian blue stain). Notice the similarity to hemosiderin deposition in a duct in genetic HFE-related hemochromatosis in Fig. 11.4 (see also eSlide 11.1 , eSlide 11.3 ).

Table 11.2
Patterns of Iron Deposition in Various Iron Overload Disorders
Histologic Pattern of Iron Overload
Disorders Parenchymal Mesenchymal Mixed Mutations (if applicable)
Genetic/Hereditary Disorders
Hereditary hemochromatosis, type 1 (see eSlide 11.1 and 11.5 ) Yes Yes, late HFE gene
C282Y/C282Y
C282Y/H63D
Hereditary hemochromatosis, type 2A (juvenile hemochromatosis) Yes Yes, late Gene encoding hepcidin
Hereditary hemochromatosis, type 2B (juvenile hemochromatosis) Yes Yes, late Gene encoding hemojuvelin
Hereditary hemochromatosis, type 3 Yes Yes, late Gene encoding transferrin receptor 2
Hereditary hemochromatosis, type 4 Yes Yes, late Gain-of-function mutations in SLC40A1 gene encoding ferroportin
Hereditary hemochromatosis, type 4 (ferroportin disease) Yes Yes Loss-of-function mutations in SLC40A1 gene encoding ferroportin
African iron overload Yes Genetic defect suspected but not yet identified
Porphyria cutanea tarda Yes Gene encoding uroporphyrinogen III decarboxylase in 20%
Nongenetic Disorders
Dyserythropoietic syndromes (see eSlide 11.3 ) Yes Yes, late No
Transfusion Yes Yes, late No
Hemolysis Yes Yes, late No
Sickle cell disease (see eSlide 11.4 ) Yes Yes, late No
Hemodialysis (see eSlide 11.2 ) Yes Yes, late No
Dysmetabolic iron overload syndrome Yes No
Alcoholic liver disease (see eSlide 11.6 ) Yes No
Nonalcoholic steatohepatitis Yes No
Viral hepatitis Yes No

The parenchymal pattern of deposition is associated with iron overload resulting from increased absorption of ingested iron. Absorbed iron is delivered to the liver through the portal vein; therefore, deposition begins in, and is heavier in, periportal hepatocytes in zone 3. As accumulation continues, hepatocytes in zone 2 and zone 3 become involved. Thus, parenchymal accumulation demonstrates a decreasing gradient from periportal zone 1 to centrilobular zone 3 hepatocytes ( Fig. 11.6 ); this gradient is maintained even in severe iron overload. Hemosiderin deposition in hepatocytes occurs along the biliary pole of the cell. Continued, severe parenchymal iron deposition may lead to deposition in mesenchymal cells, producing a mixed pattern of accumulation.

Figure 11.6, Parenchymal pattern of deposition in a case of HFE-related hemochromatosis: hemosiderin deposition preferentially involving hepatocytes in zone 1 with a decreasing gradient toward zones 2 and 3 around the hepatic vein (Prussian blue stain). HV , Hepatic vein; PT , portal tract.

The mesenchymal pattern of deposition is associated with hemolysis and multiple transfusions and is characterized by deposition of hemosiderin within Kupffer cells ( Fig. 11.7 ) and/or portal macrophages (see eSlide 11.2 ). Hemosiderin-laden macrophages are present individually or in clusters in the lobule without any zonal preference. Iron may be seen to a lesser extent in hepatocytes. When present, hepatocellular iron deposition is sparse and usually located within cells close to iron-loaded macrophages.

Figure 11.7, Mesenchymal pattern of deposition, with discrete blue granules of hemosiderin within Kupffer cells ( arrowheads ) (Prussian blue stain) (also see eSlide 11.2 ).

The mixed parenchymal-mesenchymal pattern of deposition incorporates the histologic features of the two previous patterns and is usually seen in complex conditions or in severe iron loading (see eSlide 11.1, eSlide 11.3, eSlide 11.4 ) ( Fig. 11.8 ).

Figure 11.8, Mixed pattern of deposition, with hemosiderin granules in hepatocytes as well as in macrophages in HFE-related hemochromatosis. The characteristic parenchymal pattern of HFE evolves into a mixed pattern as accumulation progresses and hemosiderin “spills” into macrophages (Prussian blue stain). HV , Hepatic vein (also see eSlide 11.1 ).

Parenchymal Iron Overload

Hereditary Hemochromatosis Types 1, 2A, 2B, and 3

Of the four types of hereditary hemochromatosis, types 1, 2, and 3 are transmitted as autosomal recessive traits; types 1 and 3 present as late onset (adult type), and types 2A and 2B present as early onset (juvenile type) hemochromatosis. The genetics and clinical manifestations of these disorders are shown in Table 11.1 . The histopathology of iron accumulation and liver injury is very similar in these disorders, reflecting the common pathophysiologic mechanism of impaired hepcidin production as the cause of iron accumulation; the degree of severity modulating the severity of iron burden.

HFE mutations account for the vast majority of clinical cases of hemochromatosis. Two common mutations in the HFE gene are known: the first, C282Y, results from the substitution of cysteine by tyrosine at amino acid 282, and the second, H63D, results from the substitution of histidine by aspartate at amino acid 63. These mutations are common in people of Nordic-Celtic descent and are uncommon in southern and eastern Europe, Africa, Central and South America, and Asia. At least one copy of the mutated C282Y gene is found in 10% to 20% of Caucasian individuals of North European descent, and a copy of the mutated H63D gene is present in 15% to 40% of these individuals. However, single copies do not lead to iron overload, and the H63D genotype does not cause iron overload in the absence of C282Y. The two genotypes commonly associated with iron overload are C282Y/C282Y (C282Y homozygotes) and C282/H63D (double heterozygotes). Furthermore, even with these two genotypes, only a minority of individuals demonstrate iron overload or experience clinical symptoms; thus the penetrance of the HFE genetic mutations is very low. Only about 1% of C282Y homozygotes develop classical HH; this variability in expression may be explained by the presence or lack of, as yet undefined, compensatory molecular mechanisms and pathways. However, C282Y homozygotes and C282Y/H63D double heterozygotes are responsible for the majority of clinical cases of HH in Caucasians; approximately 90% of those with clinically classic HH are homozygous for C282Y, and 1 to 5% are compound heterozygotes for C282Y and H63D

Microscopic Pathology

Liver injury correlates directly with the duration and amount of iron overload. Reactive oxygen species (ROS) in conjunction with intracellular iron accumulation result in formation of hydroxyl radicals, which lead to cellular damage. Free iron generates ROS through the Fenton and Haber-Weiss reactions. The superoxide radical (O2 •−) reduces ferric iron to ferrous iron, which reacts with hydrogen peroxide (H 2 O 2 ) to generate highly reactive hydroxyl radicals (OH•) Hydroxyl radicals in turn cause peroxidation of phospholipids of cellular and organellar membranes, oxidation of amino acid side-chains, DNA strand breaks, and protein fragmentation. Although the exact mechanism by which high intracellular iron leads to liver fibrosis is unclear, iron-induced cellular damage has been shown to directly increase hepatocyte cell death and activate Kupffer and stellate cells.

In early stages, iron remains localized within hepatocytes along the pericanalicular axis of the cell. Iron is distributed within the hepatic lobule in a decreasing gradient from periportal to centrilobular regions, iron deposits being heavier in the periportal hepatocytes and decreasing toward the perivenular, zone 3 hepatocytes. This gradient is maintained even with severe overload. It was noted decades ago in a seminal description of HH that iron granules may also be identified in the reticuloendothelial system (RES) (see Fig. 11.8 , eSlide 11.1 ). As hepatocytic iron load increases with time, periportal sideronecrosis occurs (see Fig. 11.3 ). Foci of acidophilic or lytic hepatocytes develop, usually closely associated with heavily iron-laden Kupffer cell aggregates (referred to as siderotic nodules ). Lobular chronic inflammation may be seen in approximately 50% of cases. Sideronecrosis is responsible for macrophage activation, which leads to both development of fibrosis and redistribution of iron toward nonparenchymal cells. Unlike most liver diseases, hepatic injury leading to fibrosis in HFE-related HH may develop in the absence of significant necrosis or marked inflammation (ie, even when there is no sideronecrosis). Progressive iron overload in HFE-HH without therapeutic phlebotomy leads to fibrosis and ultimately cirrhosis ( eSlide 11.5 ). Fibrosis is portal-based, beginning in and around portal tracts with the heaviest iron deposition. With increasing iron accumulation, there is progression to portal-portal bridging fibrosis and eventually to cirrhosis. The portal-central relationships are usually preserved; thus, the terminal hepatic venule is not incorporated into the fibrous septa. The fibrous septa in HH are wide, and fibrosis assumes a “holly leaf” configuration.

The threshold of hepatic iron concentration (HIC) associated with the development of cirrhosis has been studied by several authors ; in Powell’s series, all patients with cirrhosis had HIC values greater than 200 μmol/g dry weight. Thus, substantial hepatocyte and Kupffer cell iron loading is required before fibrosis becomes evident. It should be noted that the previously mentioned pattern of iron deposition, along with the features that are recognized as “standard” in the progression of HH, including iron accumulation in cells other than hepatocytes such as bile duct epithelium (see Fig. 11.4 ) and macrophages (see Fig. 11.8 ), were initially described before genetic testing and discovery of non-HFE iron overload genes.

A retrospective blinded biopsy review by Brunt and coworkers reinforced the concept that although a histologic pattern of iron deposition in RES is a strong negative predictor of HH, the presence of hepatocytic iron deposition does not necessarily indicate HH. In this study, none of the biopsy specimens with predominantly RES iron were C282Y homozygous. On the other hand, of the 72 biopsies with predominantly hepatocellular, zone 1 granular iron deposition, only 42 were C282Y homozygous; the remaining 30 were divided among varying combinations of heterozygosity for C282Y and/or H63D (14 biopsies) and other chronic liver diseases without HFE mutations (16 biopsies). Although the pattern of HH is one of preferential zone 1 hepatocellular iron deposition, this is neither specific nor exclusive to HH. A non-HH pattern of iron distribution reliably predicted the absence of homozygosity or compound heterozygosity for the C282Y mutation, with a negative predictive value of 100%. However, the HH pattern of iron deposition may be seen in other forms of liver disease and is not predictive of C282Y homozygosity or compound heterozygosity.

Hereditary Hemochromatosis Type 4 (Ferroportin Gain-of-Function Mutations)

Pathogenic mutations of FPN, which characterize type 4 HH, are of two types: loss-of-function mutations and gain-of-function mutations. Both are transmitted as autosomal dominant traits. FPN allows export of iron out of cells, and it is present on the basolateral surface of duodenal enterocytes, which export absorbed iron into blood, as well as on macrophages and other storage cells, which export stored iron into the circulation. Loss-of-function mutations reduce cell surface localization of FPN, reducing the cells’ ability to export iron. These mutations show a mixed pattern of iron accumulation and are discussed later. Gain-of-function mutations do not alter cell surface expression of FPN but rather abolish hepcidin-induced internalization and degradation of FPN. Thus, iron continues to be absorbed regardless of need and is stored in a parenchymal distribution similar to HFE-HH.

Hereditary Aceruloplasminemia

Hereditary aceruloplasminemia is transmitted as an autosomal recessive trait and is characterized by complete absence of ceruloplasmin in serum. Ceruloplasmin is a ferroxidase, which catalyzes the oxidation of ferrous iron to ferric iron, and has a role in uptake of ferric iron by transferrin. The essential role of ceruloplasmin in iron metabolism is demonstrated in copper-deficient animals whose anemia does not respond to administration of iron but is correctable by ceruloplasmin. Microscopically, iron is found predominantly in parenchymal cells. No case of liver cirrhosis has been described even in the most iron-overloaded cases.

Dyserythropoietic Syndromes

These include the hereditary thalassemias, sideroblastic anemias, anemias of ineffective erythropoiesis (myelodysplastic syndromes) ( eSlide 11.3 ), and congenital dyserythropoietic anemias. Impaired incorporation of iron into red cell precursors leads to increased intestinal iron absorption. Iron deposition in these conditions therefore resembles hereditary hemochromatosis ( Fig. 11.9 ; see also Fig. 11.5 ) with a predominantly parenchymal/hepatocellular pattern of accumulation. Iron deposition in hepatocytes is pericanalicular and initially involves zone 1 hepatocytes. Portal-based fibrosis and cirrhosis may occur. Once iron overload becomes severe or transfusions are required, iron may deposit in both parenchymal and mesenchymal cells, resulting in a mixed pattern.

Figure 11.9, Liver biopsy specimen from a patient with myelodysplastic syndrome. Deposition in hepatocytes in a parenchymal pattern mimics hereditary hemochromatosis, including deposition in bile duct epithelium (Prussian blue stain) (also see eSlide 11.3 ).

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