| Lysyl oxidases: Cloning and characterization of the fourth and the fifth human lysyl oxidase isoenzymes, and the consequences of a targeted inactivation of the first described lysyl oxidase isoenzyme in mice | ||
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Reduced lysyl oxidase activities are found in two X-linked recessively inherited disorders, Menkes disease and occipital horn syndrome (OHS), and in nutritional copper deficiency and lathyrism (see Kagan 1986, Steinmann et al. 1993, Smith-Mungo & Kagan 1998, Kaler 1998 for reviews). In nutritional copper deficiency, as well as the Menkes disease and OHS, reduced availability of copper for lysyl oxidase leads to diminished number cross-links in collagens and elastin, which is manifested by several connective tissue defects. However, several other copper-dependent enzymes are also affected, as will be discussed in Section 2.5.2. In contrast, lathyrism is caused by β -aminopropionitrile, a potent irreversible inhibitor of lysyl oxidase (see Section 2.3.2.3), and therefore connective tissue abnormalities in this disease are due to direct effects of decreased lysyl oxidase activity (Kagan 1986, Steinmann et al. 1993). Abnormalities in copper availability and administration of β -aminopropionitrile may in principle decrease activities of all known lysyl oxidase isoenzymes.
Copper may be an essential metal cofactor of all lysyl oxidase isoenzymes. The functional activity of lysyl oxidase has been shown to vary up to 10-fold in response to variations in the availability of dietary copper (Harris 1976, Rayton & Harris 1979, Harris 1986, Opsahl et al. 1982). Reduced lysyl oxidase activity due to copper deficiency has been detected in the chick aorta (Harris 1976), tendon (Opsahl et al. 1982, Rucker et al. 1999), and bone (Opsahl et al. 1982), as well as in rat skin (Romero-Chapman et al. 1991, Rucker et al. 1996). In these situations, reduced lysyl oxidase activity leads to a reduction in the cross-link formation in vivo, and therefore to connective tissue defects in growing animals, including osteoporosis, joint deformaties, bone fragility, internal hemorrhages, and aortic aneurysms (Rucker et al. 1975, Harris 1976, Rayton & Harris 1979, Opsahl et al. 1982).
Lysyl oxidase activity is reduced in two X-linked recessively inherited disorders, the Menkes disease and its less severe variant occipital horn syndrome (OHS, also known as X-linked cutis laxa) (see Kagan 1986, Smith-Mungo & Kagan 1998, Kaler 1998 for reviews). Both diseases are caused by mutations in the ATP7A gene that encodes a copper-transporting P-type ATPase. As a result, copper accumulates in connective tissue cells and does not enter secretory vesicles (Chelly et al. 1993, Mercer et al. 1993, Vulpe et al. 1993, Vulpe & Packman 1995). Therefore, the reduced lysyl oxidase activity is secondary to the copper deficiency, as are activities of at least 12 different copper-dependent enzymes (Kaler 1998).
The findings in both diseases include abnormal neural development, connective tissue abnormalities, and often premature death. Manifestations in OHS include bladder diverticula with spontaneus ruptures, inguinal hernias, slight skin laxity and hyperextensibility, skeletal changes, such as occipital horn-like exostoses, and vascular tortuoisity. Manifestations in Menkes disease also include bladder diverticula, hyperextensibility and laxity of skin, and skeletal abnormalities. Unlike OHS, Menkes disease further involves neurological degeneration, mental retardation, and generalized arterial disease with grossly abnormal elastic lamellaes. These complications are often so severe that the disease leads to premature death by the age of 3 years (see Kivirikko & Kuivaniemi 1987, Danks 1993, Kaler 1998 for reviews).
The reduced lysyl oxidase activity level may significantly reduce the strength of connective tissues in many tissues, and therefore probably has major clinical consequences in both diseases. The vascular tortuoisity, bladder diverticula, and castric polyps are all likely to be results of a decreased lysyl oxidase activity (Royce et al. 1980, Harcke et al. 1977, Daly & Rabinovitch 1981, Kaler et al. 1993). Because OHS patients exhibit mainly connective tissue manifestations with only mild neurological changes, lysyl oxidase may be one of the most sensitive copper-dependent enzymes to defects in the ATP7A gene product (Das et al. 1995, Levinson et al. 1996). It has been hypothesized that OHS results from the presence of low levels of a functional ATP7A, whereas no normal ATP7A activity exists in Menkes disease (Das et al. 1995). Møller et al. (2000) recently demonstrated that ATP7A-transcript levels as low as 2-5% of normal are sufficient to result in the less severe OHS, thus supporting the earlier hypothesis.
In Menkes and OHS patients, a reduction in lysyl oxidase activity level has been demonstrated in skin specimens (Byers et al. 1980, Peltonen et al. 1983) and in the medium of cultured fibroblasts (Byers et al. 1980, Peltonen et al. 1983, Royce et al. 1980, Kuivaniemi et al. 1982, Kuivaniemi et al. 1985, Kemppainen et al. 1996). Cultured skin fibroblasts from Menkes and OHS patients have also been found to contain and secrete reduced amounts of the LOX protein, indicating that transcription or translation may be impaired (Kuivaniemi et al. 1985, Gacheru et al. 1993). Kemppainen et al. (1996) studied potential transcriptional abnormalities in fibroblasts from Menkes and OHS fibroblasts by using quantitative PCR. Their work demonstrated a decreased amount of LOX mRNA in cultured fibroblasts from patients with Menkes and OHS diseases when compared to control cell lines.
Mouse mutants involving the X-linked locus mottled (Mo) serve as animal models of the Menkes disease and OHS (Danks 1993, Mercer 1998). This locus represents a murine homologue of the human ATP7A locus. In mouse, various Atp7a gene mutations lead to phenotypes similar to those found in patients with the Menkes disease and OHS (Mercer 1998). The mottled brindle (Mobr) mouse is phenotypically and biochemically similar to patients with Menkes disease, exhibiting severe neurological impairment and death at an early age (Danks 1993). Likewise, the mottled blotchy (Moblo) mouse represents a phenotype similar to OHS with predominant connective tissue manifestations (Rowe et al. 1977, Mercer 1998). Interestingly, Moblo males and Moblo/+ females have a significantly higher incidence of spontaneous aortic aneurysms when compared to the other mouse lines. Histologically, the Moblo aortas exhibit disrupted elastic lamellae and thickening of the interlamellar spaces (Andrews et al. 1975, Brophy et al. 1988). Similar findings have also been made in other mottled mice (Fry et al. 1967, Rowe et al. 1974). However, in Moblo males, the incidence of aneurysms increased progressively, reaching 100% by 6 months of age (Brophy et al. 1988).
Lathyrism is caused by chronic ingestion of the sweet pea Lathyrys odoratus, which contains β -(γ -glutamyl)aminopropionitrile, a potent irreversible inhibitor of lysyl oxidase when metabolized to β -aminopropionitrile (β APN), as discussed in Section 2.3.2.3 (see Kagan 1986, 1994, Smith-Mungo & Kagan 1998 for reviews). Lysyl oxidase inhibition leads to the diminished cross-linking of collagens and elastin, and consequently, to connective tissue defects (Kagan 1986, Steinmann et al. 1993). The connective tissue manifestations include kyphoscoliosis, bone deformities, weakening of tendons and ligament attachments, dislocation of joints, weakening of skin and cartilage, hernias, and dissecting or saccular aneurysms of the aorta (Steinmann et al. 1993). The connective tissue manifestations seen in lathyrism resemble closely those observed in the Menkes and OHS diseases and also manifestations observed in dietary copper-deficient animals discussed in previous sections. This fact supports the suggestion that abnormalities in the functioning of LOX or its isoenzymes have a central role in the connective tissue defects seen in these conditions.