Mitochondrial DNA depletion: mutations in thymidine kinase gene with myopathy and SMA.

BACKGROUND: The mitochondrial DNA (mtDNA) depletion syndrome (MDS) is an autosomal recessive disorder of early childhood characterized by decreased mtDNA copy number in affected tissues. Recently, MDS has been linked to mutations in two genes involved in deoxyribonucleotide (dNTP) metabolism: thymidine kinase 2 (TK2) and deoxy-guanosine kinase (dGK). Mutations in TK2 have been associated with the myopathic form of MDS, and mutations in dGK with the hepatoencephalopathic form. OBJECTIVES: To further characterize the frequency and clinical spectrum of these mutations, the authors screened 20 patients with myopathic MDS. RESULTS: No patient had dGK gene mutations, but four patients from two families had TK2 mutations. Two siblings were compound heterozygous for a previously reported H90N mutation and a novel T77M mutation. The other siblings harbored a homozygous I22M mutation, and one of them had evidence of lower motor neuron disease. The pathogenicity of these mutations was confirmed by reduced TK2 activity in muscle (28% to 37% of controls). CONCLUSIONS: These results show that the clinical expression of TK2 mutations is not limited to myopathy and that the myopathic form of MDS is genetically heterogeneous.

Mitochondrial neurogastrointestinal encephalomyopathy syndrome maps to chromosome 22q13.32-qter.

Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) syndrome is a rare, multisystem disorder characterized clinically by ptosis, progressive external ophthalmoplegia, gastrointestinal dysmotility, leukoencephalopathy, thin body habitus, and myopathy. Laboratory studies reveal defects of oxidative-phosphorylation and multiple mtDNA deletions frequently in skeletal muscle. We studied four ethnically distinct families affected with this apparently autosomal recessive disorder. Probands from each family were shown, by Southern blot, to have multiple mtDNA deletions in skeletal muscle. We mapped the MNGIE locus to 22q13.32-qter, distal to D22S1161, with a maximum two-point LOD score of 6.80 at locus D22S526. Cosegregation of MNGIE with a single chromosomal region in families with diverse ethnic backgrounds suggests that we have mapped an important locus for this disorder. We found no evidence to implicate three candidate genes in this region, by using direct sequence analysis for DNA helicase II and by assaying enzyme activities for arylsulfatase A and carnitine palmitoyltransferase.

Protective action of 2(3)-tert-butyl-4-hydroxyanisole (BHA) on acetaminophen-induced liver necrosis in male A/J mice.

Groups of male A/J mice were given either a regular diet or a regular diet supplemented with 1% BHA. After 10 days of feeding acetaminophen was administered intraperitoneally in amounts known to induce liver necrosis (500 mg/kg). Liver slices from the central lobe, were examined histologically for the extent of necrosis. Eight of the 17 mice treated with BHA showed no necrosis. In contrast 12 of 16 specimens from the mice treated with acetaminophen alone showed a severe centrilobular necrosis. The microsomal-mediated covalent binding of acetaminophen metabolites in-vitro was lower for microsomes from BHA pretreated mice than those from control mice. These results suggest that the ability of BHA to modify the activity of the drug metabolizing enzymes or to act as a scavenger may account for its protective action against acetaminophen-induced liver necrosis.

Phagocytosis by rat peritoneal mast cells: independence of IgG Fc-mediated and C3-mediated signals.

Phagocytosis of sheep erythrocytes (E) bearing rabbit immunoglobulin and rat C3 by rat peritoneal mast cells was quantitated by using 51Cr-labeled E and was confirmed by electron microscopy. The relative importance of IgG Fc receptor interaction in C3-mediated phagocytosis was assessed. Removal of any traces of IgG antibody by absorption of IgM antibody with protein A-Sepharose and absorption of the other reagents with sheep E had no effect on the phagocytosis of C3b- and C3bi-coated cells. IgG antibody enhanced the phagocytosis of intermediates prepared with IgM, purified components and rat C3 (EaIgMClgp4hu oxy2hu3brat) with a graded dose response, but was only additive and not synergistic with C3. The independence of the C3- and Fc-mediated signals was confirmed by using chemically produced polymers of rabbit IgG to inhibit phagocytosis. These polymers, especially tetramers and higher aggregates, completely blocked ingestion of EAIgG, but not that of EAIgMC1423b or -3bi. When IgG was substituted for IgM in the C3b intermediate, the IgG polymers inhibited about 50% of the phagocytosis. Cumulatively, the data demonstrated that in the case of rat mast cells, the stimuli to phagocytosis induced by C3 and IgG are independent; either is sufficient by itself.

An in vitro model of immune complex-mediated basement membrane zone separation caused by pemphigoid antibodies, leukocytes, and complement.

In this study, an in vitro model of immune complex-mediated basement membrane zone separation caused by periphigoid antibodies, serum complement, and peripheral blood leukocytes is described. When cryostat sections of fresh-frozen normal human skin were treated with either of 4 bullous pemiphigoid sera containing complement-activating anti-basement membrane zone antibodies and subsequently incubated at 37 degrees C with normal human peripheral blood leukocytes and fresh human serum, leukocytes attached to 96% of the basement membrane zone in 100% of sections. Sixty-seven percent of the sections developed focal areas of basement membrane zone separation resembling dermal-epidermal separation described in early pemphigoid lesions. In control sections in which either leukocytes, pemphigoid antibody or fresh human serum were omitted, significantly less leukocyte attachment and basement membrane zone separation occurred. Evidence that leukocytes caused separation was supported by an absolute requirement for viable leukocytes during incubation, a high correlation between leukocyte attachment and separation and experiments showing that leukocytes attached to the basement membrane zone were activated. This study provides the first in vitro evidence directly supporting a functional role for immune-complex mediated inflammation in the pathogenesis of basement membrane zone separation and blisters in bullous pemphigoid.

Functional evidence for complement-activating immune complexes in the skin of patients with bullous pemphigoid.

Previous immunofluorescent studies showing deposits of immunoglobulin and complement at the cutaneous basement membrane zone have provided evidence supporting a role for immune complexes in the pathogenesis of bullous pemphigoid. In this study the functional activity of the deposits has been examined using leukocyte attachment, a method for detecting and quantitating the biological activity of complement-activating immune complexes in tissues. When peripheral blood leukocytes suspended in serum complement were incubated with cryostat sections of lesional and adjacent normal-appearing skin from 9 patients with pemphigoid, skin from 11 normal controls and lesional skin from 14 nonpemphigoid disease controls there was significantly greater attachment of leukocytes to the basement membrane zone of lesional bullous pemphigoid skin compared to normal-appearing pemphigoid skin and skin of both control groups. A significant reduction in attachment in the absence of serum complement suggested the reaction was dependent on activation of complement by tissue-deposited complexes. Although leukocyte attachment was greater in lesional than normal-appearing pemphigoid skin, a comparison of the incidence and intensity of cutaneous IgG and complement immunofluorescence between the 2 groups showed no significant differences. Furthermore, no correlation between leukocyte attachment and serum titers of immunoglobulin G or complement-binding anti-basement membrane zone antibodies was observed. These results suggest that immune reactants in lesional pemphigoid skin are functional complement-activating immune complexes, that differences exist between the activity of complexes in lesional and normal-appearing pemphigoid skin and may explain why lesions develop at some sites and not others.

Cleavage of membrane bound C3bi, an intermediate of the third component of complement, to C3c and C3d-like fragments by crude leucocyte lysosomal lysates and purified leucocyte elastase.

Partial degradation of the biologically-active major fragment of the third complement component (C3b) to C3bi is catalysed by the endopeptidase C3b inactivator (I) and its co-factor, beta 1H globulin (H). Complete degradation to the fragments C3c and C3d requires an additional protease, which can be simulated in vitro by trypsin. This study was designed to identify the in vivo correlate of trypsin. Purified and 125I-labelled C3b bound to sheep erythrocytes was used as substrate. Release of label into the supernate served as an index of proteolysis. The chain structure of the peptides in the supernate or remaining bound to the erythrocytes was assessed by SDS polyacrylamide gel electrophoresis. Significant cleavage of cell-bound C3b was obtained by treatment with I, H and extracts from human peripheral blood leucocyte azurophil granules. Purified elastase also removed label in the presence of I and H. The peptide remaining on the cell had the characteristic 33K molecular weight of C3d. The activity of elastase in cleaving was blocked by alpha-1-anti-trypsin, the chloromethyl ketone, MeO-Suc-Ala-Pro-Val-Ch2Cl and by rabbit antibody to elastase. Thus, elastase purified from azurophil granules of human polymorphonuclear neutrophils (PMN) is a potent catalyst of the cleavage of C3bi to C3c- and C3d-like fragments and may contribute in vivo to the control of complement-mediated inflammation.

Nonantibody binding of serum proteins to 5S anti-Rh fragments produced by chymotrypsin.

Chymotrypsin hydrolysis of the IgG anti-Rh antibodies Ri results in both bivalent and univalent antibody fragments. The bivalent fragments coated on Rh-positive erythrocytes are agglutinable by albumin and other serum proteins in 3% polyethylene glycol. The bivalent structure of the 5S fragment is essential for expression of this site since 5S fragments produced by trypsin and pepsin are also agglutinable, while univalent fragments produced by papain and subtilisin are not. The agglutination by albumin of the 5S fragments is not caused by residual enzyme. The reaction appears to be irreversible in that once albumin has reacted with the 5S fragment, either in the fluid phase or at the cell surface, fresh addition of albumin and PEG will not result in agglutination. The nonantibody reaction of albumin and the other serum proteins with these 5S IgG fragments is believed to be caused by hydrophobic bonding involving the intrachain disulfide in the 5S fragment and hydrophobic areas of other proteins.

Localization of the receptors for activated complement on the visceral epithelial cells by the human renal glomerulus by immunoenzymatic microscopy.

Horseradish peroxidase-labeled IgG-anti-IgG immune complexes bearing activated third component of complement, C3, were used as indicator particles for the localization of the complement receptor in the human renal glomerulus. The HRP-labeled IgG-anti-IgG complexes were coated with activated C3 by exposing them to fresh human serum as a source of complement. This represents a more physiologic indicator particle for complement receptor-bearing structures than the synthetic immune complexes previously used. By light microscopy, the deposition of the complexes was restricted to the glomeruli. By transmission electron microscopy, the complexes were selectively associated with the surface of the visceral epithelial cells (podocytes). Complexes exposed to buffer alone or to heat-inactivated human serum failed to adhere to any of the renal structures examined.

Pemphigoid antibody mediated attachment of peripheral blood leukocytes at the dermal-epidermal junction of human skin.

It has been proposed that cutaneous inflammation and blister formation in bullous pemphigoid is caused by antibodies to the cutaneous basement membrane zone which active complement, thereby, attracting leukocytes to the dermal-epidermal junction. There is, however, no functional evidence which supports a role for pemphigoid antibodies in complement activation or leukocyte activity in skin. This study describes the in vitro attachment of human peripheral blood leukocytes to the dermal-epidermal junction of cryostat skin sections treated with 9/13 pemphigoid sera containing antibodies to the cutaneous basement membrane zone. A requirement for complement in the reaction was supported by the findings that only complement-fixing pemphigoid sera mediated the leukocyte response, a strong correlation existed between complement-fixation titers and leukocyte attachment titers and only leukocytes suspended in fresh serum but not buffer or heat inactivated serum attached at the junction. A requirement for antibody was supported by the observation that IgG fractions of 4 pemphigoid sera were as effective as whole sera in mediating leukocyte attachment. The leukocyte response was shown to be specific for complement-fixing pemphigoid sera since it was not observed with noncomplement-fixing sera or sera from 15 normal human and 22 nonpemphigoid disease controls. This study offers functional evidence for an interaction between pemphigoid antibody, complement and leukocytes in the immunopathogenesis of bullous pemphigoid and demonstrates that complement-fixing antibasement membrane zone antibodies may be important in initiating the cellular inflammatory events observed near the dermal-epidermal junction in vivo.

Demonstration of the complement regulating protein, beta 1H, in skin biopsies from patients with bullous pemphigoid.

beta 1H-globulin is a recently characterized plasma protein which regulates the biologic activities of the major fragment of the third complement component, C3b. The major function of this protein is to act as a co-factor for C3b Inactivator (C3bINA) in the cleavage of C3b to an intermediate molecule, C3b', consisting of an intact beta-chain covalently bound by disulfide bridges to 2 alpha-chain fragments of 40,000 and 67,000 daltons. Final cleavage of C3b' to the C3c and C3d fragments requires an additional protease such as plasmin or elastase. Additionally, beta 1H interferes with the activity of the alternative pathway convertases, C3bBb and C3bBbP, by displacing or competing with the binding of factor B. In this study, perilesional skin biopsies from 10 patients with active bullous pemphigoid were examined for the presence of beta 1H at the dermal-epidermal junction by immunofluorescent methods. The protein was found in 8 of 9 biopsies in which C3 also was deposited. In a single case where C3 was not found, beta 1H was not seen. These findings suggest that beta 1H plays a role in the in vivo control of C3b and provides additional evidence for the participation of the complement system in the pathogenesis of bullous pemphigoid.

Complement receptor binding of C3b-coated cells treated with C3b inactivator, beta 1H globulin and trypsin.

Treatment of 125I-C3b bound to EAC1423b with C3b inactivator (C3bINA) and beta 1H globulin (beta 1H) cleaved the alpha-chain of C3b into 65,000- and 42,000-dalton fragments, both of which remained disulfide-bonded to the intact beta-chain (C3bi). Subsequent treatment with trypsin (0.1 microgram/ml) released 125I into the supernatant and yielded cells coated with a 33,000-dalton fragment of alpha-chain, presumably C3d. These results are in agreement with those obtained by others using fluid phase C3b. C3b-coated cells (EAC1423b) adhered to complement (C) receptors on human erythrocytes, glomeruli, and monocytes. C3bi-coated cells adhered to the receptors on glomeruli and monocytes, but not to those on human erythrocytes. C3d-coated cells adhered only to the monocyte receptors. The findings suggest that the glomerular C receptor recognizes portions of the C3 molecule different from those recognized by either the erythrocyte or monocyte receptors.

C3b receptors in normal human tissues.

Normal human kidney, lung, liver, heart, skin, thymus, spleen, lymph node, pancreas, and choroid plexus were reacted with a C3b-coated particle (fluoresceinated Salmonella typhi) to determine if these tissues contained C3b receptor cells. Clusters of these cells were identified in the spleen, lymph nodes, and in the renal glomeruli. All other studied tissues demonstrated a minimal homogeneous deposition of the indicator bacteria throughout the entire aspect of the tissue. Deposition of the indicator bacteria on the tissues was abrogated when the bacteria were prepared with heat inactivated serum as a source of complement.

Deposition of beta 1H globulin in kidneys of patients with immune renal disease.

Renal biopsies from patients with a variety of immune type renal diseases were examined by immunofluorescence for the presence of C3 and its control protein, beta 1H globulin. Deposits of beta 1H were found in every instance (21 of 21 biopsies) in which C3 deposits were observed, irrespective of the underlying disease resulting in the C3 deposits, including lupus nephritis, acute poststreptococcal glomerulonephritis, idiopathic membranous glomerulonephritis, and Goodpasture's syndrome. In no instance was beta 1H found independently of C3. As previously shown in in vitro systems, beta 1H also binds to C3, presumably C3b, during activation of the complement system in immunologically induced renal disease.

Demonstration of beta 1H globulin together with C3 in the dermal-epidermal junction of patients with systemic lupus erythematosus.

Skin lesions and clinically normal skin from 10 patients with systemic lupus erythematosus (SLE) were examined by immunofluorescence for the presence of C3 and its control protein, beta 1H globulin. beta 1Hwas always found in association with deposited C3; in the instance where C3 deposits were not found beta 1H was also not found. Granular deposits of C3 and beta 1H were found in the dermal-epidermal junction (DEJ) in all of the 5 nonlesional skins studied. In the lesional skin, C3 was found in 4 of 5; in 2 of these 4, beta 1H was also found. As previously demonstrated for in vitro systems, beta 1H also binds in vivo to fragments of C3, presumably C3b, generated during activation of the complement system.

Interaction of beta1H globulin with cell-bound C3b: quantitative analysis of binding and influence of alternative pathway components on binding.

Purified beta1H globulin (beta1H) was shown to bind to C3b coated cells by both immunofluorescent and radioactive tracer techniques. With EAC43, the amount of beta1H bound was directly proportional to the amount of C3 used to prepare the cells; EA, EAC14 and EAC14oxy2 bound very small amounts of beta1H. The C3b binding site on beta1H was labile in that not all of the purified 125I-beta1H was capable of binding to C3b, even when an excess of cell-bound C3b was present. Scatchard analysis of binding of beta1H to C3b-coated cells indicated an equilibrium constant of 10(9) L/M. Deviations from linearity were regularly found on Scatchard analyses. This was consistent with the hypothesis that the beta1H binding sites exhibit negative cooperativity in that as more sites become occupied, it becomes more difficult to fill the remaining sites. The stoichiometry of the reaction between C3b and beta1H was examined using EAC14oxy23 prepared with 131I-C3 and beta1H labeled with 125I. Between 0.5--0.8 beta1H molecules were bound per C3b molecule. Other alternative pathway components influenced the binding of 125I-beta1H to cell bound C3b. Both C3b and native C3 inhibited binding of labeled beta1H at an efficiency approximately 1/1,000 that of unlabeled beta1H. Factor B inhibited binding with 1/280 the efficiency of unlabeled beta1H. Properdin caused a dose-dependent increase in the binding of beta1H; this enhancement was abrogated if B was also present in the reaction mixture. Scatchard analysis indicated that the enhancement of beta1H binding by P resulted in an increased number of available binding sites rather than an increase in the affinity of binding.

The nature of the receptor for complement (C3b) in the human renal glomerulus.

The physicochemical nature of the human glomerular complement receptor was studied. Receptor activity was measured by determining the avidity of glomeruli of normal human renal tissue for fluorescein-labeled bacteria (S.typhi) coated with C3b. Maximal binding of C3b-coated bacteria to normal human glomeruli took place in phosphate-saline buffers of pH 6.5 and 0.08 to 0.15 mu ionic strength. Pretreatment of renal tissue with neuraminidase enhanced receptor activity. On the other hand, binding of C3b-coated bacteria to the glomeruli was diminished by pretreatment of the tissue with proteolytic enzymes, phospholipase C and certain lipid solvents. The binding of C3b-coated bacteria to the glomeruli was also diminished by pretreatment of the tissue with fluid-phase C3b, or by pretreatment of the bacteria with C3b inactivator. Normal human serum and purified fluid-phase C3 or the absence of magnesium and calcium ions had little effect on glomerular complement receptor activity.

Localization of receptors for activated complement on visceral epithelial cells of the human renal glomerulus.

Receptors for activated C3 have recently been demonstrated to be present in glomeruli of normal human kidneys. In the present communication, the precise location of these receptors within the renal corpuscle was studied with scanning electron microscopy. By this technique, the glomerular complement receptor (GCR) was found to be located on the visceral epithelial cell of the renal corpusle. This epithelial cell location was confirmed by comparison of in vitro GCR activity with the location of immunoglobulin and C3 deposited in vivo. Renal tissues in which Ig and C3 had been deposited in vivo diffusely in subepithelial loci had no in vitro GCR activity. Renal biopsies not showing Ig or C3 deposition or biopsies with Ig and C3 in the mesangium retained GCR activity. These results further confirm an important role for GCR in the trapping and deposition of C3 in some forms of immunologically mediated renal disease.