Hepatic endoplasmic reticulum storage diseases

Genetic and immunological characterization of fibrinogen inclusion bodies in patients with hepatic fibrinogen storage and liver disease

Fibrinogen storage in liver cells can occur under three different morphological inclusions. Type I contain all three fibrinogen chains (A alpha, B beta, and gamma) as well as D and E fragments, whereas type II and III lack B beta as well as D and E fragments. Patients with type I inclusions carry a point mutation (gamma 284 Gly-Arg). The mutation is not present in patients with type II and III inclusions. These results appear to suggest that the three various phenotypic expressions (i.e., morphological variants) reflect different genetical abnormalities of fibrinogen.

Molecular mechanisms of hypo- and afibrinogenemia

Point mutations responsible for hypo- and afibrinogenemia are yielding new insights into amino acid side chains involved in the molecular processing, assembly, secretion, and domain stability of fibrinogen. Reverse phase chromatography, isoelectric focussing, electrospray mass spectrometry, and tryptic peptide mass mapping have shown that chains with heterozygous mutations of gamma 284 Gly-->Arg, B beta 316 Asp-->Tyr and gamma 371 Thr-->Ile are absent from plasma fibrinogen. The nonexpression of these mutations appears to result from perturbation of the five-stranded beta sheet of the D domain. We propose that this is due to retention of the variant in the endoplasmic reticulum and that in turn this leads to hypofibrinogenemia. Other mutations effect intracellular proteolysis and chain assembly. For example the mutation, A alpha 20 Val-->Asp, makes the protein a substrate for furin, which removes the first 19 residues of the A alpha chain as the mature molecule transits the trans golgi complex. Transient expression of gamma 153 Cys-->Arg chains together with A alpha and B beta chains suggests this mutation might perturb chain assembly, and the incorporation of mutations of B beta 353 Leu-->Arg or B beta 400 Gly-->Asp into intracellular fibrinogen precludes its subsequent export from host cells expressing fibrinogen genes. The graded severity of the hypo- and afibrinogenemias associated with homozygous A alpha chain truncations suggest the absolute minimal requirement for molecular assembly is the formation of the C terminal disulfide ring of the coiled coil.

Cytoplasmic inclusion bodies and minimal hepatitis: fibrinogen storage without hypofibrinogenemia

A 12-year-old Japanese boy had chronic elevation and fluctuation of serum transaminase levels since infancy, with no signs or symptoms of liver failure. Usual infections or metabolic disorders were eliminated from consideration. No coagulopathy or abnormality in plasma concentrations of clotting factors was found. Light microscopy of liver biopsy specimens obtained at ages 2, 5, and 7 years showed slight hepatocyte disarray and minimal mononuclear-leukocyte lobular inflammation, with eosinophilic inclusion bodies in the cytoplasm of hepatocytes throughout the lobule. These bodies stained with the periodic acid-Schiff (PAS) technique; the PAS-positive material was partly diastase digestible and on immunostaining marked for fibrinogen but not for alpha 1-antitrypsin. On transmission electron microscopy, the bodies were represented by finely granular material contained within membranes and were interpreted as tentatively endoplasmic reticulum. Fibrinogen storage may be manifest as minimal hepatitis without coagulopathy.

Traffic jam: a compendium of human diseases that affect intracellular transport processes

As sequencing of the human genome nears completion, the genes that cause many human diseases are being identified and functionally described. This has revealed that many human diseases are due to defects of intracellular trafficking. This 'Toolbox' catalogs and briefly describes these diseases.

Fibrinogen brescia: hepatic endoplasmic reticulum storage and hypofibrinogenemia because of a gamma284 Gly-->Arg mutation

The proposita suffered from liver cirrhosis and biopsy showed type 1 membrane-bound fiberglass inclusions. The hepatic inclusion bodies were weakly periodic acid-Schiff diastase-positive, and on immunoperoxidase staining reacted specifically with anti-fibrinogen antisera. Coagulation investigations revealed low functional and antigenic fibrinogen together with a prolonged thrombin time of 37 seconds (normal, 17 to 22 seconds) suggestive of a hypodysfibrinogenemia. DNA sequencing of all three fibrinogen genes showed a single heterozygous mutation of GGG (Gly)-->CGG (Arg) at codon 284 of the gamma-chain gene. However, examination of purified fibrinogen chains by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, reverse-phase high-performance liquid chromatography, ion-exchange high-performance liquid chromatography, and isoelectric focusing, failed to show any evidence of the mutant gamma(Br) chain in plasma fibrinogen. This finding was substantiated by electrospray ionization mass spectrometry, which showed only a normal gamma (and Bbeta) chain mass, but a large increase in the portion of their disialo isoforms. We speculate that misfolding of the variant protein causes hepatic retention and the subsequent hypofibrinogenemia, and that the functional defect (dysfibrinogenemia) results from hypersialylation of otherwise normal Bbeta and gamma chains consequent to the liver cirrhosis. These conclusions were supported by studies on six other family members with hypofibrinogenemia, and essentially normal clotting times, who were heterozygous for the gamma284 Gly-->Arg mutation.

Endoplasmic reticulum retention is a common defect associated with tyrosinase-negative albinism

Tyrosinase is a melanocyte-specific enzyme critical for the synthesis of melanin, a process normally restricted to a post-Golgi compartment termed the melanosome. Loss-of-function mutations in tyrosinase are the cause of oculocutaneous albinism, demonstrating the importance of the enzyme in pigmentation. In the present study, we explored the possibility that trafficking of albino tyrosinase from the endoplasmic reticulum (ER) to the Golgi apparatus and beyond is disrupted. Toward this end, we analyzed the common albino mouse mutation Tyr(C85S), the frequent human albino substitution TYR(T373K), and the temperature-sensitive tyrosinase TYR(R402Q)/Tyr(H402A) found in humans and mice, respectively. Intracellular localization was monitored in albino melanocytes carrying the native mutation, as well as in melanocytes ectopically expressing green fluorescent protein-tagged tyrosinase. Enzymatic characterization of complex glycans and immunofluorescence colocalization with organelle-specific resident proteins established that all four mutations produced defective proteins that were retained in the ER. TYR(R402Q)/Tyr(H402A) Golgi processing and transport to melanosomes were promoted at the permissive temperature of 32 degrees C, but not at the nonpermissive 37 degrees C temperature. Furthermore, evidence of protein misfolding was demonstrated by the prolonged association of tyrosinase mutants with calnexin and calreticulin, known ER chaperones that play a key role in the quality-control processes of the secretory pathway. From these results we concluded that albinism, at least in part, is an ER retention disease.

Pancreatic cancer and fibrinogen storage disease

BACKGROUND

Ductal adenocarcinoma is the most common type of pancreatic carcinoma while squamous, carcinosarcoma, sarcoma, giant cell carcinoma, and clear cell types are all rare. Hepatocellular fibrinogen storage disease is also an uncommon disorder which may be associated with hepatocellular carcinoma. Two cases of pancreatic carcinoma were encountered in a family with fibrinogen storage disease, further raising the possibility of a predilection to malignancy in this unusual disorder. The tumour in one case was of the rare clear cell type. These two cases are the basis for this report.

METHODS

Sections were cut from retrieved paraffin embedded tissue and stained for routine histology. Immunohistochemistry using the avidin-biotin technique was applied for the expression of the markers p53 (D07), carcinoembryonic antigen (CEA), c-erbB-2, epithelial membrane antigen (EMA), and alpha-fetoprotein (AFP).

RESULTS

Both cases were adenocarcinoma of pancreatic ductal origin. The tumour in one case showed features of a clear cell carcinoma. The tumour cells expressed p53, CEA, and EMA immunoreactivity and were negative for c-erbB-2 and AFP.

CONCLUSIONS

Hepatocellular fibrinogen storage disease is rare and has been described in association with chronic hepatitis, cirrhosis, and rarely with hepatocellular carcinoma. This represents the first report of its association with carcinoma outside of the liver.

Congenital hypothyroid goiter with deficient thyroglobulin. Identification of an endoplasmic reticulum storage disease with induction of molecular chaperones

Recent advances in understanding the molecular pathogenesis of congenital hypothyroid goiter in cog/cog mice, have raised important questions concerning the maturation of thyroglobulin (the thyroid prohormone) in certain human kindreds with congenital goiter. We have now examined affected siblings from two unrelated families that synthesize an apparently normally glycosylated, > 300 kD immunoreactive thyroglobulin, yet have a reduced quantity of intraglandular thyroglobulin and that secreted into the circulation. From thyroid tissues of the four patients, light microscopic approaches demonstrated presence of intracellular thyroglobulin despite its absence in thyroid follicle lumina, while electron microscopy indicated abnormal distention of the endoplasmic reticulum (ER). We have confirmed biochemically that most intrathyroidal thyroglobulin fails to reach the (Golgi) compartment where complex carbohydrate modification takes place. Moreover, the disease in the affected patients is associated with massive induction of specific ER molecular chaperones including the hsp90 homolog, GRP94, and the hsp70 homolog, BiP. The data suggest that these patients synthesize a mutant thyroglobulin which is defective for folding/assembly, leading to a markedly reduced ability to export the protein from the ER. Thus, these kindreds suffer from a thyroid ER storage disease, a cell biological defect phenotypically indistinguishable from that found in cog/cog mice.

An endoplasmic reticulum storage disease causing congenital goiter with hypothyroidism

In humans, deficient thyroglobulin (Tg, the thyroid prohormone) is an important cause of congenital hypothyroid goiter; further, homozygous mice expressing two cog/cog alleles (linked to the Tg locus) exhibit the same phenotype. Tg mutations might affect multiple different steps in thyroid hormone synthesis; however, the microscopic and biochemical phenotype tends to involve enlargement of the thyroid ER and accumulation of protein bands of M(r) < 100. To explore further the cell biology of this autosomal recessive illness, we have examined the folding and intracellular transport of newly synthesized Tg in cog/cog thyroid tissue. We find that mutant mice synthesize a full-length Tg, which appears to undergo normal N-linked glycosylation and glucose trimming. Nevertheless, in the mutant, Tg is deficient in the folding that leads to homodimerization, and there is a deficiency in the quantity of intracellular Tg transported to the distal portion of the secretory pathway. Indeed, we find that the underlying disorder in cog/cog mice is a thyroid ER storage disease, in which a temperature-sensitive Tg folding defect, in conjunction with normal ER quality control mechanisms, leads to defective Tg export. In relation to quality control, we find that the physiological response in this illness includes the specific induction of five molecular chaperones in the thyroid ER. Based on the pattern of chaperone binding, different potential roles for individual chaperones are suggested in glycoprotein folding, retention, and degradation in this ER storage disease.

Hepatic accumulation of alpha-1-antitrypsin in chronic liver disease in the dog

Alpha-1-antitrypsin deficiency has long been known to cause liver cirrhosis in man, but whether it does so in the dog is uncertain. To investigate this point 57 dogs with clinically and histopathologically diagnosed chronic liver disease were examined. Isoelectric focusing of blood serum from these dogs and from 25 clinically healthy dogs revealed three different types of alpha-1 antitrypsin, designated F(fast), I(intermediate) and S(slow). They appeared in both homozygous and heterozygous forms, the F type being seen most frequently. The I type was more common in cocker spaniels than in other breeds. Immunostaining for alpha-1 antitrypsin revealed that 37 diseased dogs had alpha-1 antitrypsin in the cytoplasm of their hepatocytes. Of these, 21 dogs had globular alpha-1 antitrypsin inclusions in the endoplasmic reticulum, indicating aggregated protein. Accumulated alpha-1 antitrypsin was found most frequently in dogs having the I and S types of alpha-1 antitrypsin, either homozygously or heterozygously. With a few exceptions, F-homozygotic dogs had no hepatocellular alpha-1 antitrypsin accumulation. As alpha-1 antitrypsin aggregation is lethal to hepatocytes and as cell death attracts mononuclear blood cells whose cytokines induce continued alpha-1 antitrypsin synthesis with subsequent risk of further alpha-1 antitrypsin accumulation, liver disease may thus be maintained. Whether alpha-1 antitrypsin aggregates actually initiate liver disease in dogs, as in man, remains to be elucidated by further biochemical investigation of the three canine alpha-1 antitrypsin types found.

The interaction of hydrophobic bile acids with the alpha 1-proteinase inhibitor

An in vitro complex formation between cholesterol and human alpha 1-proteinase inhibitor (alpha 1-antitrypsin, alpha 1-Pi) has been described. Hydrophobic bile acids were studied for a similar interaction using lithocholic acid (LC) as a prototype of a hydrophobic acid. At a molar ratio of 5:1, LC induced conformational changes of alpha 1-Pi reflected in an abnormal gel-electrophoretic appearance, loss of anodal immunoreactivity on crossed immunoelectrophoresis, exposition of new antigenic determinant(s) on immunodiffusion, and loss of antiproteinase activity. After 6 h incubation, LC and alpha 1-Pi form a complex of approximately 200 kDa molecular mass seen following gel-filtration. After prolonged (24 h) interaction a series of large alpha 1-Pi polymers were seen on SDS-PAGE under reducing conditions followed by Western blotting. Glycolitho-, sulfolitho-, deoxycholic and 3-beta-hydroxy-5-cholenoic acids induced similar but less pronounced changes of alpha 1-Pi, whereas transferrin remained unaffected. Hydrophilic acids lacked effect on alpha 1-Pi. The results are compatible with a specific, irreversible interaction of alpha 1-Pi with hydrophobic bile acids affecting its physical and proteinase inhibitory properties. The cholestatic potency of the hydrophobic acids studied and their ability to induce alpha 1-Pi polymerization may be important in cholestatic conditions.

Association of folding intermediates of glycoproteins with calnexin during protein maturation

Calnexin, an endoplasmic reticulum transmembrane protein, represents a new type of molecular chaperone that selectively associates in a transient fashion with newly synthesized monomeric glycoproteins in HepG2 cells. Calnexin only recognizes glycoproteins when they are incompletely folded. Dissociation of glycoproteins from calnexin occurs at different rates and is related to the time taken for their folding, which may then initiate their differential transport rates from the endoplasmic reticulum.