- Email: [email protected]
The genetics of dominant osteopetrosis
Vol. 2, No. 4 2005
Drug Discovery Today: Disease Mechanisms
DRUG DISCOVERY
TODAY
Editors-in-Chief Toren Finkel – National Heart, Lung and Blood Institute, National Institutes of Health, USA Charles Lowenstein – The John Hopkins School of Medicine, Baltimore, USA
DISEASE Dominant inheritance disorders MECHANISMS
The genetics of dominant osteopetrosis Annalisa Frattini, Paolo Vezzoni, Anna Villa* Department of Human Genome, Istituto di Tecnologie Biomediche, CNR, Via Fratelli Cervi 93, 20090 Segrate, Milan, Italy
Autosomal dominant osteopetroses (ADO) are classically divided into two types, ADOI and ADOII, which are differentiated according to the main sites of osteosclerosis localization. For ADOI this is the cranial vault and, for ADOII, the spine, pelvis and skull base. ADOII
Section Editors: Fabio Candotti – National Human Genome Research Institute, NIH, Bethesda, USA Luigi Notarangelo – Angelo Nocivelli Institute for Molecular Medicine, University of Brescia, Italy
patients are heterozygous for a mutation in the ClCN7 gene, coding for a putative chloride channel; ADOI patients are heterozygous for mutations in the LRP5 gene, a Wnt coreceptor, which affects osteoblast function. This second finding challenges the old assumption that osteopetroses are always due to an osteoclast defect in bone resorption.
Introduction Bone remodeling is a tightly regulated process that requires synthesis of bone matrix by osteoblasts and bone resorption by osteoclasts. These cells have different origins: osteoblasts derive from mesenchymal stem cells, whereas osteoclasts, which are multinucleated cells, arise from hematopoietic monocyte/macrophage precursors [1]. Interference with this delicate balance can lead to a variety of skeletal abnormalities that are characterized by decreased (osteoporosis) or increased (osteopetrosis) bone mass [2]. Several interacting factors play crucial roles in maintaining this delicate dynamic balance and explain the heterogeneity shown by bone disorders. The purpose of this paper is to review the autosomal dominant osteopetrosis (ADO), a heterogeneous group of bone diseases, whose molecular basis has recently been identified.
*Corresponding author: A. Villa ([email protected]) 1740-6765/$ ß 2005 Elsevier Ltd. All rights reserved.
DOI: 10.1016/j.ddmec.2005.10.002
The main components of bone remodeling: osteoblasts and osteoclasts Osteoblasts are cells of mesenchymal origin whose differentiation is driven by various molecules [2]. Here, we will focus our attention on the Wnt co-receptor, LRP5 (GenBank accession no. NM_002335) whose abnormalities are involved in several pathologies characterized by different clinical pictures. Wnt molecules are lipid-modified glycoproteins capable of activating signal transduction pathways involved in a variety of cellular activities [3,4]. Wnts activate three distinct intracellular signaling cascades: the Wnt/b-catenin pathway (canonical pathway, shown in Fig. 1), the Wnt/Ca2+ pathway and the Wnt/planar polarity pathway. We discuss here only a few items relevant to the field of ADO and refer to previous excellent reviews for further details [5,6]. The canonical pathway affects cellular functions by regulating b-catenin levels, which start to accumulate after Wnt binding to LRP5/6 heterodimer and Frizzled (Fzd) transmembrane proteins. The accumulation of b-catenin in the cytoplasm is crucial because when the b-catenin molecules are nonphosphorylated, they enter the nucleus where they recruit transcriptional factors activating the expression of many genes, including c-myc and cyclin D1 [5]. Several extracellular and intracellular proteins negatively regulate the Wnt/b-catenin pathway; among these are Dickkopfs (Dkks) and secreted Fzd-related proteins (sFRPs). Apparently, Dkks bind to LRP5/6, inhibiting Wnt access to the receptor, whereas sFRPs bind to Wnt ligands preventing their association to LRP and Fzd receptors. www.drugdiscoverytoday.com
503
Drug Discovery Today: Disease Mechanisms | Dominant inheritance disorders
Vol. 2, No. 4 2005
Figure 1. Canonical Wnt pathway in osteoblasts. The canonical Wnt signaling pathway acts by regulating levels of b-catenin. (a) In the absence of Wnts, b-catenin molecules are ubiquitinated and degraded by the 26S proteosome. The degradation of excess of b-catenins is mediated by a multiprotein complex containing casein kinase (CK), glycogen synthase kinases (GSK3b) and scaffolding proteins (Axin, APC and Dsh). (b) b-Catenins which are not phosphorylated enter into the nucleus where they recruit the transcriptional factor TCF (T-cell factor) to stimulate expression of many genes. Wnts signaling leads to the accumulation of b-catenin molecules following the binding to the receptor complex consisting of LRP5/6 and Fzd transmembrane proteins. (c) Dickkopfs (Dkks) and secreted frizzled related proteins (sFRPs) play an antagonist role to Wnt activity: Dkks sequester LRP5/6 and promote their internalization to lysosomes; whereas sFRPs bind directly to Wnt inhibiting its binding to LRP5. Mutations in LRP5 causing high bone mass (G171V) impair its interaction with Dkk and the consequent lysosome degradation. The Wnt Gene Homepage can be found at: http://www.stanford.edu/rnusse/wntwindow.html.
The observation of LRP5 mutations in bone defects highlights the role of this gene as a regulator of bone density. Its activation by Wnt causes proliferation of osteoblasts, where it is expressed, yet it is absent in osteoclasts [7–9]. Surprisingly, mutations in LRP5 lead to both an increase and a decrease in bone mineral density [10]. Osteoclasts are giant cells originating from monocyte/ macrophage precursors capable of differentiating in vivo and in vitro after exposure to several endogenous or exogenous factors (see [11–13] for review). It has long been known that osteoclast bone resorption is linked to acidification of the bone/cell interface by the vacuolar proton pump (VPP, H+-ATPase). The acidification is achieved by the export of hydrogen ions generated by the dissociation of H2CO3 by carbonic anhydrase II (GenBank accession no. NM_000067) 504
www.drugdiscoverytoday.com
(Fig. 2); a defect in the function of this enzyme is responsible for the recessive autosomal osteopetrosis associated with renal tubular acidosis (MIM 259730) [14]. The protons are transported into the resorption lacuna through the ruffled border by the VPP [15], which is assembled from 13 different types of subunits coded by separate genes, one of which (TCIRG1, GenBank accession no. NM_006019) is responsible for the severe recessive form of osteopetrosis both in mouse and in human [16–18]. To counteract proton extrusion by the VPP, a mechanism is needed to dissipate the membrane potential generated and it has been suggested that a chloride channel could provide this function. Disruption of the ClCN7 gene (GenBank accession no. NM_001287) in mice resulted in osteopetrosis, suggesting that ClC-7 could be the channel involved in this
Vol. 2, No. 4 2005
Drug Discovery Today: Disease Mechanisms | Dominant inheritance disorders
Figure 2. Schematic representation of osteoclast physiology. The polarized osteoclast develops three functionally distinct domains, the resorptive surface, the basolateral membrane and the secretory domain. The resorption (or Howship’s) lacuna is defined by the sealing zone of osteoclasts on bone. To create a wide area of bone adhesion, osteoclasts form the so-called ruffled membrane, which is the cell resorptive organelle containing filamentous actin and the avb3 integrin. Integrins are heterodimeric transmembrane proteins, crucial not only for mediating cell–cell and cell–matrix interactions but also to act as signaling receptors. Integrins associate with the matrix proteins osteopontin and vitronectin forming a tight seal of attachment, the clear zone, which is rich in filaments. In the Howship’s lacuna, resorption occurs through acidification allowing dissolution of the mineral matrix and secretion of enzymes digesting the organic component. Through this coordinated process, osteoclasts erode the underlying bone.
process [19]. However, although the role of ClC-7 in osteoclast function is fundamental, the precise mechanism of its action is still being debated.
Autosomal dominant osteopetrosis The term osteopetrosis is used to describe bone diseases characterized by an increase in bone mass. The spectrum of the disease includes a severe form, called recessive malignant osteopetrosis (ARO), typical at neonatal age and leading to death unless treated by bone marrow transplantation. In addition to the already mentioned osteopetrosis with renal tubular acidosis (MIM 259730), three genes have been found to be responsible for a portion of ARO: the TCIRG1/ATP6i gene coding for the alpha3 subunit of the vacuolar proton pump (MIM 604592); the ClCN7 gene encoding an osteoclastspecific chloride channel (MIM 602727); and the Grey-Lethal gene (MIM 607649), whose function is still unknown [18–20]. In contrast to what happens in the recessive form, the dominant form generally presents with a milder phenotype and for this reason it has also been known as the ‘benign’ type. This form usually manifests itself in late childhood or adulthood and presents with a clinical spectrum which varies even within the same family. On the basis of radiological and clinical differences, two types of benign osteopetrosis compatible with a normal life span have classically been recognized [21,22]. Both have
generalized osteosclerosis, but the two forms could be radiologically differentiated, at least to some extent, because in ADOI the sclerosis is more pronounced at the cranial vault, whereas in ADOII the osteosclerosis is more evident at the base of the skull, spine and pelvis.
ADOI Despite the higher bone mass, this form of osteopetrosis is not associated with an increased fracture rate, and bone pain is a rare symptom. Genetically, this defect is fully penetrant and linkage studies in two Danish families with ADOI mapped the disease to the chromosome 11q12–13 region, between markers D11S1765 and D11S4113 encompassing the LRP5 gene [23] which was soon found to be responsible for the disease. The low-density lipoprotein (LDL) receptor related protein (LRP5) gene encodes a transmembrane protein of 1615 amino acids that is a member of the LDL receptor-related family [24]. So far, three bone defects and a hereditary eye disorder have been ascribed to different mutations in the LRP5 gene: recessive osteoporosis–pseudoglioma syndrome (OPPG, MIM 259770), dominantly inherited high bone mass trait (HBM, MIM 601884), autosomal dominant osteopetrosis type I (ADOI, MIM 1666600) and familial exudative vitreoretinopathy (FEVR, MIM 133780). At least some of these pathologies reflect mutations affecting distinct domains of the protein (Fig. 3). www.drugdiscoverytoday.com
505
Drug Discovery Today: Disease Mechanisms | Dominant inheritance disorders
Vol. 2, No. 4 2005
Figure 3. LRP5 mutations in relation to specific protein domains. Schematic representation of LRP5 mutations (arrows) associated with high bone mass and ADOI (red) [9,26,28,29], osteoporosis–pseudoglioma (blue) [7] and familial exudative vitreoretinopathy (green) [42,43] diseases. Wnt and DKK1 below the bar indicates the regions of interaction with these two proteins.
In particular, severe null mutations (frameshift or nonsense) resulting in a diminished LRP5 function are responsible for the OPPG form. This phenotype is reproduced in mouse by LRP5 inactivation [8]. Thus, a block in LRP5 activity results in an impaired response to Wnt signaling with consequent altered bone formation and osteoporosis. It is worth mentioning that the parents of the patients bearing the mutation on only one allele show a reduced bone mineral density (BMD), suggesting a co-dominant effect [7]. On the contrary, the High Bone Mass syndrome is the result of a G171V mutation in the gene [9,25,26]. In particular, experimental data showed that the normal inhibition of Wnt signaling provided by Dkk1 was altered, owing to the inability of the antagonistic protein Dkk1 to bind the mutated LRP5, resulting in an unopposed Wnt activity [26,27]. Owing to the clinical similarities between HBM and ADOI, dominant mutations in LRP5 are probably responsible for a continuum in clinical manifestations and the clinical picture can be variable. It appears that screening for LRP5 abnormalities is of clinical value in the identification of ADOI because the right diagnosis cannot easily be performed on clinical grounds alone. Van Hul’s group has identified six novel mutations causing an imbalance of bone formation in ten unrelated patients, some of which were initially classified as endosteal hyperostosis, Van Buchem disease or autosomal dominant osteosclerosis [28]. All the missense mutations reported by the authors (D111Y, G171R, A214T, A214V, A242T and T253I) involve amino acids within the first b-propeller of the protein and are located at the amino-terminal part of the protein. 506
www.drugdiscoverytoday.com
Interestingly, G171R affects the same codon mutated in HBM. More recently, the A242T mutation was also found in several members of an additional large family showing craniosynostosis and dysmorphic features in addition to an HBM phenotype [29]. Therefore, single amino acid changes in the N-terminal portion lead to conditions that, although initially classified under different diagnoses, have as a common feature an increased thickness of the skull and of the cortices of the long bones and that, retrospectively, can now be classified as LRP5-dependent ADO, type I. Interestingly, if the current interpretation of LRP5 function is correct (no function in osteoclasts), the old tenet that osteopetrosis are due to a defect in osteoclasts does not apply to the ADOI subset, in which it seems that an increased bone deposition and not a decrease in resorption is responsible for the phenotype.
ADOII The frequency of ADOII (Albers–Schonberg disease) is 0.2/ 100,000 in Brazil and 5.5/100,000 in Denmark [30], however owing to its relative benign clinical picture the real prevalence of ADO is probably underestimated. The radiological hallmarks are characterized by diffuse osteosclerosis, especially, in the vertebral endplate thickening, sandwich vertebra appearance (Fig. 4) and iliac wings with the classical appearance of bone within bone and thickening of the skull base [21,31]. A large study on 42 patients demonstrated a radiological penetrance of 90%, whereas only 81% were clinically symptomatic [32]. The clinical manifestations include spontaneous or after minimal trauma fractures, and pain due to osteoarthritis and osteomyelitis. This
Vol. 2, No. 4 2005
Figure 4. Skeletal radiographs of bones of patients affected by ADOI and ADOII. Plain radiograph of the lumbar spine of a patient affected by ADOI. There is a generalized increased radio opacity of the vertebral bodies and the posterior elements of the spine. In ADOII, lateral X-ray of the lower spine reveals the classic ‘sandwich’ vertebrae of osteopetrosis and of the bone adjacent to the endplates (figure courtesy of Professor W. Van Hul).
form is usually diagnosed radiographically, although the patients have elevated serum levels of tartrate resistant acid phosphatase (TRAP) and the BB isoenzyme of creatine kinase (CK-BB) [33]. Although this disease was initially mapped to chromosome 1p21 [34], more recent studies have definitively shown that the gene responsible for the large majority of ADOII is the ClCN7 gene on chromosome 16p13.3 [30,35], coding for a putative chloride-channel protein highly expressed in osteoclasts, which had already been shown to cause a rare form of human recessive osteopetrosis [19]. In 2001, the group of Van Hul found heterozygous mutations in the ClCN7 gene in 12 out of 12 families affected by the ADOII form [35]. These results and other data from various groups demonstrated that ADOII is allelic with ClCN7-dependent ARO and that different mutations within the same gene, ClCN7, lead to different phenotypes of osteopetrosis [35–39]. When tested in vitro, osteoclasts from ADOII patients bearing the common G215R mutation maintain 10– 20% of normal resorption activity [40]. To explain the different picture, Cleiren and co-workers [35] hypothesized that the recessive form is the result of a complete loss of function of the chloride channel (null mutations), whereas the dominant form could be owing to
Drug Discovery Today: Disease Mechanisms | Dominant inheritance disorders
the presence of an abnormal molecule which might interfere, to various extents, with the normal channel. The intermediate form would therefore derive from the combination of two (recessive) hypomorphic mutations maintaining a partial chloride channel function. However, further studies on patients with ClCN7 mutations complicate the overall picture. First, Frattini and coworkers [36] have investigated a large number of osteopetrotic patients, both recessive and dominant, demonstrating that, although the presence of two recessive mutations is almost always linked to a very severe course of the disease, heterozygous ClCN7 mutations are also responsible for the intermediate forms as well as for severe clinical pictures. Although it is felt that the severity of the disease is somehow linked to the specific mutation, a precise correlation between molecular changes and phenotype is precluded by our current lack of data about the function and the structure of the channel. Second, even within the same family, patients bearing the same mutation can present with phenotypes ranging from asymptomatic to severely affected [36,41]. Although penetrance and genetic background have been invoked, this essentially masks our complete ignorance about the factor(s) modifying the phenotypes.
Conclusions The disruption of the delicate equilibrium between boneforming osteoblast cells and bone-resorbing osteoclast cells leads to a variety of pathological states (Fig. 5). Autosomal dominant osteopetrosis represents a heterogeneous group of inherited disorders whose molecular basis has recently been unraveled. These studies have shown that mutations mapping to the N-terminal portion of LRP5 lead to an increase of bone mineral density, known as ADOI, characterized by increased thickness of the skull and of the cortices of long bones. On the contrary, diffuse osteosclerosis and classical ‘bone within a bone’ appearance most commonly noted in the vertebrae, pelvis and the ends of long bones are the hallmarks of ADOII due to mutation in the ClCN7. Although the identification of disease-causing mutations in these two genes has enhanced our understanding of the dominant osteopetrotic forms, the therapeutic options are very limited (Table 1). In addition, many pathophysiological aspects still remain obscure. In particular, the exact function of the ClCN7 gene is unclear, as are the biochemical consequences of missense mutations. The reasons why the same mutation causes different phenotypes even in the same family and the extent to which the genetic and epigenetic background influences the manifestations of the disease are also unknown. Finally, a few patients still exist with a clinical picture compatible with an ADO diagnosis where no mutation has been found (A. Frattini, unpublished). These rare patients, who could be classified as ADOIII, are awaiting definition of their molecular defect. www.drugdiscoverytoday.com
507
Drug Discovery Today: Disease Mechanisms | Dominant inheritance disorders
Vol. 2, No. 4 2005
Figure 5. Pathophysiology of osteopetrosis. Panel a: Loss of function mutations of LRP5 lead to an impaired response to Wnt signaling with reduced osteoblastogenesis and consequent decreased bone mineralization. This leads a decreased bone formation and insurgence of osteoporosis–pseudoglioma syndrome (OPPG). Panel b: In contrast to the situation in OPPG, mutations in LRP5 impairing its interaction with Dkk1 result in activation of the Wnt signaling (Fig. 1) with a consequent increase in bone formation. This altered pathway is at the basis of ADOI and high bone mass. Panel c: Mutations in the ClCN7 gene are responsible for decreased osteoclast activity in bone resorption. Osteoclast cells are present but unable to resorb bone matrix. This condition leads to an increased bone density typical of ADOII.
Table 1. Various forms of osteopetrosis and related therapies Target
Strategic approach to target
Expected outcome
Who is working on target
Refs
ADOI and ADOII ADOI and ADOII ADOI and ADOII ADOI and ADOII ADOII and ARO ARO ARO and ADOII
Calcitrol (1,25-dihydroxy Vitamin D) Interferon gamma-1b Prednisone (Steroids) Parathyroid hormone Bone marrow transplantation Ventriculo-peritoneal shunt Decompression of the optic nerves
Stimulation of osteoclasts differentiation Increased bone resorption, hematopoiesis Increased hematopoiesis and bone resorption Stimulation of osteoclasts Increased hematopoiesis and bone resorption Treatment of hydrocephalus Treatment of visual impairment
Carolino et al. Key et al. Kocher et al.
[44] [45] [46]
Multiple investigators
[47]
Vanier et al.
[48]
Acknowledgements Partially funded by grants from FIRB-MIUR to A.V. and P.V. (RBNE019J9W) and from Progetto Nazionale Cellule Staminali (No. CS3 to P.V.). This is manuscript no. 87 of the Genoma 2000/ITB Project funded by Cariplo.
References 1 Ash, P. et al. (1980) Osteoclasts derived from haematopoietic stem cells. Nature 283, 669–670
508
www.drugdiscoverytoday.com
2 Karsenty, G. and Wagner, E.F. (2002) Reaching a genetic and molecular understanding of skeletal development. Dev. Cell 2, 389–406 3 Moon, R.T. et al. (2002) The promise and perils of Wnt signalling through beta-catenin. Science 296, 1644–1646 4 Glass, D.A., II et al. (2005) Canonical Wnt signalling in differentiated osteoblasts controls osteoclast differentiation. Dev. Cell 8, 751–764 5 Westendorf, J.J. et al. (2004) Wnt signaling in osteoblasts and bone diseases. Gene 341, 19–39 6 Logan, C.Y. and Nusse, R. (2004) The Wnt signalling pathway in development and disease. Annu. Rev. Cell Dev. Biol. 20, 781–810
Vol. 2, No. 4 2005
7 Gong, Y. et al. (2001) LDL receptor related protein 5(LRP5) affects bone accrual and eye development. Cell 107, 513–523 8 Kato, M. et al. (2002) Cbfa1-independent decrease in osteoblast proliferation, ostepenia and persistent embryonic eye vascularization in mice deficient in Lrp5 a Wnt coreceptor. J. Cell Biol. 157, 303–314 9 Little, R.D. et al. (2002) A mutation in the LDL receptor related protein 5 gene results in the autosomal dominant high bone mass trait. Am. J. Hum. Genet. 70, 11–19 10 Koay, M.A. and Brown, M.A. (2005) Genetic disorders of the LRP5-Wnt signalling pathway affecting the skeleton. Trends Mol. Med. 11, 129–137 11 Boley, W.J. et al. (2003) Osteoclast differentiation and activation. Nature 423, 337–342 12 Blair, H.C. and Athanasou, N.A. (2004) Recent advances in osteoclast biology and pathological bone resorption. Histol. Histopathol. 19, 189–199 13 Teitelbaum, S.L. and Ross, F.P. (2003) Genetic regulation of osteoclast development and function. Nat. Rev. Genet. 4, 638–649 14 Sly, W.S. et al. (1983) Carbonic anhydrase II deficiency identified as the primary defect in the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification. Proc. Natl. Acad. Sci. U S A 80, 2752–2756 15 Vaananen, K.H. et al. (2000) The cell biology of osteoclast function. J. Cell Sci. 113, 377–381 16 Ly, J.P. et al. (1999) Atp6i-deficient mice exhibit severe osteopetrosis due to loss of osteoclast-mediated extracellular acidification. Nat. Genet. 23, 447– 451 17 Scimeca, J.C. et al. (2000) The gene encoding the mouse homologue of the human osteoclast-specific 116-kDa V-ATPase subunit bears a deletion in osteosclerotic (oc/oc) mutants. Bone 26, 207–213 18 Frattini, A. et al. (2000) Defects in TCIRG1 subunit of the vacuolar proton pump are responsible for a subset of human autosomal recessive osteopetrosis. Nat. Genet. 25, 343–346 19 Kornak, U. et al. (2001) Loss of the ClC-7 chloride channel leads to osteopetrosis in mice and man. Cell 104, 205–215 20 Chalhoub, N. et al. (2003) Grey-lethal mutation induces severe malignant autosomal recessive osteopetrosis in mouse and human. Nat. Med. 9, 399– 406 21 Andersen, P.E. and Bollerslev, J. (1987) Heterogeneity of autosomal dominant osteopetrosis. Radiology 164, 223–225 22 Bollerslev, J. (1989) Autosomal dominant osteopetrosis: bone metabolism and epidemiological, clinical and hormonal aspects. Endocr. Rev. 10, 45–67 23 Van Hul, E. et al. (2002) Localization of the gene causing autosomal dominant osteopetrosis type I to chromosome 11q12–13. J. Bone Miner. Res. 17, 1111–1117 24 Hey, P.J. et al. (1998) Cloning of a novel member of the low density lipoprotein receptor family. Gene 216, 103–111 25 Johnson, M.L. et al. (1997) Linkage of a gene causing high bone mass to human chromosome 11 (11q12–13). Am. J. Hum. Genet. 60, 1326–1332 26 Boyden, L.M. et al. (2002) High bone density due to a mutation in LDLreceptor-related protein 5. N. Engl. J. Med. 346, 1513–1521 27 Ai, M. et al. (2005) Reduced affinity to and inhibition by DKK1 form a common mechanism by which high bone mass-associated missense mutations in LRP5 affect canonical Wnt signaling. Mol. Cell. Biol. 25, 4946–4955 28 Van Wesenbeeck, L. et al. (2003) Six novel missense mutations in the LDL receptor related protein 5 (LRP5) gene in different conditions with an increased bone density. Am. J. Hum. Genet. 72, 763–771
Drug Discovery Today: Disease Mechanisms | Dominant inheritance disorders
29
30
31 32
33
34
35
36
37
38
39
40
41
42
43
44 45 46 47
48
Kwee, M.L. et al. (2005) An autosomal dominant high bone mass phenotype in association with craniosynostosis in an extended family is caused by an LRP5 missense mutation. J. Bone Miner. Res. 20, 1254–1260 Benichou, O. et al. (2001) Mapping of autosomal dominant osteopetrosis type II (Albers–Schonberg disease) to chromosome 16p13.3. Am. J. Hum. Genet. 69, 647–654 El-Tawil, T. and Stoker, D.J. (1993) Benign osteopetrosis: a review of 42 cases showing two different patterns. Skeletal Radiol. 22, 587–593 Benichou, O.D. et al. (2000) Type II autosomal dominant osteopetrosis (Albers–Schonberg disease): clinical and radiological manifestations in 42 patients. Bone 26, 87–93 Waguespack, S. et al. (2003) Chloride channel (ClCN7) gene mutations and autosomal dominant osteopetrosis, type II. J. Bone Miner. Res. 18, 1513–1518 Van Hul, W. et al. (1997) Localization of a gene for autosomal dominant osteopetrosis (Albers–Schonberg disease) to chromosome 1p21. Am. J. Hum. Genet. 61, 363–369 Cleiren, E. et al. (2001) Albers–Schonberg disease (autosomal dominant osteopetrosis, type II) results from mutations in the ClCN7 chloride channel gene. Hum. Mol. Gen. 10, 2861–2867 Frattini, A. et al. (2003) Chloride channel ClCN7 mutations are responsible for severe recessive, dominant, and intermediate osteopetrosis. J. Bone Miner. Res. 18, 1740–1747 Campos-Xavier, A.B. et al. (2003) Chloride channel 7 (ClCN7) gene mutations in intermediate autosomal recessive osteopetrosis. Hum. Genet. 112, 186–189 Waguespack, S. et al. (2002) Measurement of tartrate resistant acid phosphatase and the brain isoenzyme of creatine kinase accurately diagnoses type II autosomal dominant osteopetrosis but does not identify gene carriers. J. Clin. Endocrinol. Metab. 87, 2212–2217 Letizia, C. et al. (2004) Type II benign osteopetrosis (Albers–Schonberg disease) caused by a novel mutation in ClCN7 presenting with unusual clinical manifestations. Calcif. Tissue Int. 74, 42–46 Henriksen, K. et al. (2004) Characterization of osteoclasts from patients harboring a G215R mutation in ClC-7 causing autosomal dominant osteopetrosis type II. Am. J. Pathol. 164, 1537–1545 Campos-Xavier, A.B. et al. (2005) Intrafamilial phenotypic variability of osteopetrosis due to chloride channel 7 (CLCN7) mutations. Am. J. Med. Genet. 133, 216–218 Toomes, C. et al. (2004) Mutations in LRP5 or FZD4 underlie the common familial exudative vitreoretinopathy locus on chromosome 11q. Am. J. Hum. Genet. 74, 721–730 Jiao, X. et al. (2004) Autosomal recessive familial exudative vitreoretinopathy is associated with mutations in LRP5. Am. J. Hum. Genet. 75, 878–884 Carolino, J. et al. (1998) Osteopetrosis. Am. Fam. Physician 57, 1293–1296 Key, L.L., Jr et al. (1995) Long-term treatment of osteopetrosis with recombinant human interferon gamma. N. Engl. J. Med. 15, 1594–1599 Kocher, M.S. and Kasser, J.R. (2003) Osteopetrosis. Am. J. Orthop. 32, 222– 228 Driessen, G.J. et al. (2003) Long-term outcome of haematopoietic stem cell transplantation in autosomal recessive osteopetrosis: an EBMT report. Bone Marrow Transplant. 32, 657–663 Vanier, V. et al. (2000) Bilateral visual improvement after unilateral optic canal decompression and cranial vault expansion in a patient with osteopetrosis, narrowed optic canals, and increased intracranial pressure. J. Neurol. Neurosurg. Psychiatry 69, 405–406
www.drugdiscoverytoday.com
509
Drug Discovery Today: Disease Mechanisms
DRUG DISCOVERY
TODAY
Editors-in-Chief Toren Finkel – National Heart, Lung and Blood Institute, National Institutes of Health, USA Charles Lowenstein – The John Hopkins School of Medicine, Baltimore, USA
DISEASE Dominant inheritance disorders MECHANISMS
The genetics of dominant osteopetrosis Annalisa Frattini, Paolo Vezzoni, Anna Villa* Department of Human Genome, Istituto di Tecnologie Biomediche, CNR, Via Fratelli Cervi 93, 20090 Segrate, Milan, Italy
Autosomal dominant osteopetroses (ADO) are classically divided into two types, ADOI and ADOII, which are differentiated according to the main sites of osteosclerosis localization. For ADOI this is the cranial vault and, for ADOII, the spine, pelvis and skull base. ADOII
Section Editors: Fabio Candotti – National Human Genome Research Institute, NIH, Bethesda, USA Luigi Notarangelo – Angelo Nocivelli Institute for Molecular Medicine, University of Brescia, Italy
patients are heterozygous for a mutation in the ClCN7 gene, coding for a putative chloride channel; ADOI patients are heterozygous for mutations in the LRP5 gene, a Wnt coreceptor, which affects osteoblast function. This second finding challenges the old assumption that osteopetroses are always due to an osteoclast defect in bone resorption.
Introduction Bone remodeling is a tightly regulated process that requires synthesis of bone matrix by osteoblasts and bone resorption by osteoclasts. These cells have different origins: osteoblasts derive from mesenchymal stem cells, whereas osteoclasts, which are multinucleated cells, arise from hematopoietic monocyte/macrophage precursors [1]. Interference with this delicate balance can lead to a variety of skeletal abnormalities that are characterized by decreased (osteoporosis) or increased (osteopetrosis) bone mass [2]. Several interacting factors play crucial roles in maintaining this delicate dynamic balance and explain the heterogeneity shown by bone disorders. The purpose of this paper is to review the autosomal dominant osteopetrosis (ADO), a heterogeneous group of bone diseases, whose molecular basis has recently been identified.
*Corresponding author: A. Villa ([email protected]) 1740-6765/$ ß 2005 Elsevier Ltd. All rights reserved.
DOI: 10.1016/j.ddmec.2005.10.002
The main components of bone remodeling: osteoblasts and osteoclasts Osteoblasts are cells of mesenchymal origin whose differentiation is driven by various molecules [2]. Here, we will focus our attention on the Wnt co-receptor, LRP5 (GenBank accession no. NM_002335) whose abnormalities are involved in several pathologies characterized by different clinical pictures. Wnt molecules are lipid-modified glycoproteins capable of activating signal transduction pathways involved in a variety of cellular activities [3,4]. Wnts activate three distinct intracellular signaling cascades: the Wnt/b-catenin pathway (canonical pathway, shown in Fig. 1), the Wnt/Ca2+ pathway and the Wnt/planar polarity pathway. We discuss here only a few items relevant to the field of ADO and refer to previous excellent reviews for further details [5,6]. The canonical pathway affects cellular functions by regulating b-catenin levels, which start to accumulate after Wnt binding to LRP5/6 heterodimer and Frizzled (Fzd) transmembrane proteins. The accumulation of b-catenin in the cytoplasm is crucial because when the b-catenin molecules are nonphosphorylated, they enter the nucleus where they recruit transcriptional factors activating the expression of many genes, including c-myc and cyclin D1 [5]. Several extracellular and intracellular proteins negatively regulate the Wnt/b-catenin pathway; among these are Dickkopfs (Dkks) and secreted Fzd-related proteins (sFRPs). Apparently, Dkks bind to LRP5/6, inhibiting Wnt access to the receptor, whereas sFRPs bind to Wnt ligands preventing their association to LRP and Fzd receptors. www.drugdiscoverytoday.com
503
Drug Discovery Today: Disease Mechanisms | Dominant inheritance disorders
Vol. 2, No. 4 2005
Figure 1. Canonical Wnt pathway in osteoblasts. The canonical Wnt signaling pathway acts by regulating levels of b-catenin. (a) In the absence of Wnts, b-catenin molecules are ubiquitinated and degraded by the 26S proteosome. The degradation of excess of b-catenins is mediated by a multiprotein complex containing casein kinase (CK), glycogen synthase kinases (GSK3b) and scaffolding proteins (Axin, APC and Dsh). (b) b-Catenins which are not phosphorylated enter into the nucleus where they recruit the transcriptional factor TCF (T-cell factor) to stimulate expression of many genes. Wnts signaling leads to the accumulation of b-catenin molecules following the binding to the receptor complex consisting of LRP5/6 and Fzd transmembrane proteins. (c) Dickkopfs (Dkks) and secreted frizzled related proteins (sFRPs) play an antagonist role to Wnt activity: Dkks sequester LRP5/6 and promote their internalization to lysosomes; whereas sFRPs bind directly to Wnt inhibiting its binding to LRP5. Mutations in LRP5 causing high bone mass (G171V) impair its interaction with Dkk and the consequent lysosome degradation. The Wnt Gene Homepage can be found at: http://www.stanford.edu/rnusse/wntwindow.html.
The observation of LRP5 mutations in bone defects highlights the role of this gene as a regulator of bone density. Its activation by Wnt causes proliferation of osteoblasts, where it is expressed, yet it is absent in osteoclasts [7–9]. Surprisingly, mutations in LRP5 lead to both an increase and a decrease in bone mineral density [10]. Osteoclasts are giant cells originating from monocyte/ macrophage precursors capable of differentiating in vivo and in vitro after exposure to several endogenous or exogenous factors (see [11–13] for review). It has long been known that osteoclast bone resorption is linked to acidification of the bone/cell interface by the vacuolar proton pump (VPP, H+-ATPase). The acidification is achieved by the export of hydrogen ions generated by the dissociation of H2CO3 by carbonic anhydrase II (GenBank accession no. NM_000067) 504
www.drugdiscoverytoday.com
(Fig. 2); a defect in the function of this enzyme is responsible for the recessive autosomal osteopetrosis associated with renal tubular acidosis (MIM 259730) [14]. The protons are transported into the resorption lacuna through the ruffled border by the VPP [15], which is assembled from 13 different types of subunits coded by separate genes, one of which (TCIRG1, GenBank accession no. NM_006019) is responsible for the severe recessive form of osteopetrosis both in mouse and in human [16–18]. To counteract proton extrusion by the VPP, a mechanism is needed to dissipate the membrane potential generated and it has been suggested that a chloride channel could provide this function. Disruption of the ClCN7 gene (GenBank accession no. NM_001287) in mice resulted in osteopetrosis, suggesting that ClC-7 could be the channel involved in this
Vol. 2, No. 4 2005
Drug Discovery Today: Disease Mechanisms | Dominant inheritance disorders
Figure 2. Schematic representation of osteoclast physiology. The polarized osteoclast develops three functionally distinct domains, the resorptive surface, the basolateral membrane and the secretory domain. The resorption (or Howship’s) lacuna is defined by the sealing zone of osteoclasts on bone. To create a wide area of bone adhesion, osteoclasts form the so-called ruffled membrane, which is the cell resorptive organelle containing filamentous actin and the avb3 integrin. Integrins are heterodimeric transmembrane proteins, crucial not only for mediating cell–cell and cell–matrix interactions but also to act as signaling receptors. Integrins associate with the matrix proteins osteopontin and vitronectin forming a tight seal of attachment, the clear zone, which is rich in filaments. In the Howship’s lacuna, resorption occurs through acidification allowing dissolution of the mineral matrix and secretion of enzymes digesting the organic component. Through this coordinated process, osteoclasts erode the underlying bone.
process [19]. However, although the role of ClC-7 in osteoclast function is fundamental, the precise mechanism of its action is still being debated.
Autosomal dominant osteopetrosis The term osteopetrosis is used to describe bone diseases characterized by an increase in bone mass. The spectrum of the disease includes a severe form, called recessive malignant osteopetrosis (ARO), typical at neonatal age and leading to death unless treated by bone marrow transplantation. In addition to the already mentioned osteopetrosis with renal tubular acidosis (MIM 259730), three genes have been found to be responsible for a portion of ARO: the TCIRG1/ATP6i gene coding for the alpha3 subunit of the vacuolar proton pump (MIM 604592); the ClCN7 gene encoding an osteoclastspecific chloride channel (MIM 602727); and the Grey-Lethal gene (MIM 607649), whose function is still unknown [18–20]. In contrast to what happens in the recessive form, the dominant form generally presents with a milder phenotype and for this reason it has also been known as the ‘benign’ type. This form usually manifests itself in late childhood or adulthood and presents with a clinical spectrum which varies even within the same family. On the basis of radiological and clinical differences, two types of benign osteopetrosis compatible with a normal life span have classically been recognized [21,22]. Both have
generalized osteosclerosis, but the two forms could be radiologically differentiated, at least to some extent, because in ADOI the sclerosis is more pronounced at the cranial vault, whereas in ADOII the osteosclerosis is more evident at the base of the skull, spine and pelvis.
ADOI Despite the higher bone mass, this form of osteopetrosis is not associated with an increased fracture rate, and bone pain is a rare symptom. Genetically, this defect is fully penetrant and linkage studies in two Danish families with ADOI mapped the disease to the chromosome 11q12–13 region, between markers D11S1765 and D11S4113 encompassing the LRP5 gene [23] which was soon found to be responsible for the disease. The low-density lipoprotein (LDL) receptor related protein (LRP5) gene encodes a transmembrane protein of 1615 amino acids that is a member of the LDL receptor-related family [24]. So far, three bone defects and a hereditary eye disorder have been ascribed to different mutations in the LRP5 gene: recessive osteoporosis–pseudoglioma syndrome (OPPG, MIM 259770), dominantly inherited high bone mass trait (HBM, MIM 601884), autosomal dominant osteopetrosis type I (ADOI, MIM 1666600) and familial exudative vitreoretinopathy (FEVR, MIM 133780). At least some of these pathologies reflect mutations affecting distinct domains of the protein (Fig. 3). www.drugdiscoverytoday.com
505
Drug Discovery Today: Disease Mechanisms | Dominant inheritance disorders
Vol. 2, No. 4 2005
Figure 3. LRP5 mutations in relation to specific protein domains. Schematic representation of LRP5 mutations (arrows) associated with high bone mass and ADOI (red) [9,26,28,29], osteoporosis–pseudoglioma (blue) [7] and familial exudative vitreoretinopathy (green) [42,43] diseases. Wnt and DKK1 below the bar indicates the regions of interaction with these two proteins.
In particular, severe null mutations (frameshift or nonsense) resulting in a diminished LRP5 function are responsible for the OPPG form. This phenotype is reproduced in mouse by LRP5 inactivation [8]. Thus, a block in LRP5 activity results in an impaired response to Wnt signaling with consequent altered bone formation and osteoporosis. It is worth mentioning that the parents of the patients bearing the mutation on only one allele show a reduced bone mineral density (BMD), suggesting a co-dominant effect [7]. On the contrary, the High Bone Mass syndrome is the result of a G171V mutation in the gene [9,25,26]. In particular, experimental data showed that the normal inhibition of Wnt signaling provided by Dkk1 was altered, owing to the inability of the antagonistic protein Dkk1 to bind the mutated LRP5, resulting in an unopposed Wnt activity [26,27]. Owing to the clinical similarities between HBM and ADOI, dominant mutations in LRP5 are probably responsible for a continuum in clinical manifestations and the clinical picture can be variable. It appears that screening for LRP5 abnormalities is of clinical value in the identification of ADOI because the right diagnosis cannot easily be performed on clinical grounds alone. Van Hul’s group has identified six novel mutations causing an imbalance of bone formation in ten unrelated patients, some of which were initially classified as endosteal hyperostosis, Van Buchem disease or autosomal dominant osteosclerosis [28]. All the missense mutations reported by the authors (D111Y, G171R, A214T, A214V, A242T and T253I) involve amino acids within the first b-propeller of the protein and are located at the amino-terminal part of the protein. 506
www.drugdiscoverytoday.com
Interestingly, G171R affects the same codon mutated in HBM. More recently, the A242T mutation was also found in several members of an additional large family showing craniosynostosis and dysmorphic features in addition to an HBM phenotype [29]. Therefore, single amino acid changes in the N-terminal portion lead to conditions that, although initially classified under different diagnoses, have as a common feature an increased thickness of the skull and of the cortices of the long bones and that, retrospectively, can now be classified as LRP5-dependent ADO, type I. Interestingly, if the current interpretation of LRP5 function is correct (no function in osteoclasts), the old tenet that osteopetrosis are due to a defect in osteoclasts does not apply to the ADOI subset, in which it seems that an increased bone deposition and not a decrease in resorption is responsible for the phenotype.
ADOII The frequency of ADOII (Albers–Schonberg disease) is 0.2/ 100,000 in Brazil and 5.5/100,000 in Denmark [30], however owing to its relative benign clinical picture the real prevalence of ADO is probably underestimated. The radiological hallmarks are characterized by diffuse osteosclerosis, especially, in the vertebral endplate thickening, sandwich vertebra appearance (Fig. 4) and iliac wings with the classical appearance of bone within bone and thickening of the skull base [21,31]. A large study on 42 patients demonstrated a radiological penetrance of 90%, whereas only 81% were clinically symptomatic [32]. The clinical manifestations include spontaneous or after minimal trauma fractures, and pain due to osteoarthritis and osteomyelitis. This
Vol. 2, No. 4 2005
Figure 4. Skeletal radiographs of bones of patients affected by ADOI and ADOII. Plain radiograph of the lumbar spine of a patient affected by ADOI. There is a generalized increased radio opacity of the vertebral bodies and the posterior elements of the spine. In ADOII, lateral X-ray of the lower spine reveals the classic ‘sandwich’ vertebrae of osteopetrosis and of the bone adjacent to the endplates (figure courtesy of Professor W. Van Hul).
form is usually diagnosed radiographically, although the patients have elevated serum levels of tartrate resistant acid phosphatase (TRAP) and the BB isoenzyme of creatine kinase (CK-BB) [33]. Although this disease was initially mapped to chromosome 1p21 [34], more recent studies have definitively shown that the gene responsible for the large majority of ADOII is the ClCN7 gene on chromosome 16p13.3 [30,35], coding for a putative chloride-channel protein highly expressed in osteoclasts, which had already been shown to cause a rare form of human recessive osteopetrosis [19]. In 2001, the group of Van Hul found heterozygous mutations in the ClCN7 gene in 12 out of 12 families affected by the ADOII form [35]. These results and other data from various groups demonstrated that ADOII is allelic with ClCN7-dependent ARO and that different mutations within the same gene, ClCN7, lead to different phenotypes of osteopetrosis [35–39]. When tested in vitro, osteoclasts from ADOII patients bearing the common G215R mutation maintain 10– 20% of normal resorption activity [40]. To explain the different picture, Cleiren and co-workers [35] hypothesized that the recessive form is the result of a complete loss of function of the chloride channel (null mutations), whereas the dominant form could be owing to
Drug Discovery Today: Disease Mechanisms | Dominant inheritance disorders
the presence of an abnormal molecule which might interfere, to various extents, with the normal channel. The intermediate form would therefore derive from the combination of two (recessive) hypomorphic mutations maintaining a partial chloride channel function. However, further studies on patients with ClCN7 mutations complicate the overall picture. First, Frattini and coworkers [36] have investigated a large number of osteopetrotic patients, both recessive and dominant, demonstrating that, although the presence of two recessive mutations is almost always linked to a very severe course of the disease, heterozygous ClCN7 mutations are also responsible for the intermediate forms as well as for severe clinical pictures. Although it is felt that the severity of the disease is somehow linked to the specific mutation, a precise correlation between molecular changes and phenotype is precluded by our current lack of data about the function and the structure of the channel. Second, even within the same family, patients bearing the same mutation can present with phenotypes ranging from asymptomatic to severely affected [36,41]. Although penetrance and genetic background have been invoked, this essentially masks our complete ignorance about the factor(s) modifying the phenotypes.
Conclusions The disruption of the delicate equilibrium between boneforming osteoblast cells and bone-resorbing osteoclast cells leads to a variety of pathological states (Fig. 5). Autosomal dominant osteopetrosis represents a heterogeneous group of inherited disorders whose molecular basis has recently been unraveled. These studies have shown that mutations mapping to the N-terminal portion of LRP5 lead to an increase of bone mineral density, known as ADOI, characterized by increased thickness of the skull and of the cortices of long bones. On the contrary, diffuse osteosclerosis and classical ‘bone within a bone’ appearance most commonly noted in the vertebrae, pelvis and the ends of long bones are the hallmarks of ADOII due to mutation in the ClCN7. Although the identification of disease-causing mutations in these two genes has enhanced our understanding of the dominant osteopetrotic forms, the therapeutic options are very limited (Table 1). In addition, many pathophysiological aspects still remain obscure. In particular, the exact function of the ClCN7 gene is unclear, as are the biochemical consequences of missense mutations. The reasons why the same mutation causes different phenotypes even in the same family and the extent to which the genetic and epigenetic background influences the manifestations of the disease are also unknown. Finally, a few patients still exist with a clinical picture compatible with an ADO diagnosis where no mutation has been found (A. Frattini, unpublished). These rare patients, who could be classified as ADOIII, are awaiting definition of their molecular defect. www.drugdiscoverytoday.com
507
Drug Discovery Today: Disease Mechanisms | Dominant inheritance disorders
Vol. 2, No. 4 2005
Figure 5. Pathophysiology of osteopetrosis. Panel a: Loss of function mutations of LRP5 lead to an impaired response to Wnt signaling with reduced osteoblastogenesis and consequent decreased bone mineralization. This leads a decreased bone formation and insurgence of osteoporosis–pseudoglioma syndrome (OPPG). Panel b: In contrast to the situation in OPPG, mutations in LRP5 impairing its interaction with Dkk1 result in activation of the Wnt signaling (Fig. 1) with a consequent increase in bone formation. This altered pathway is at the basis of ADOI and high bone mass. Panel c: Mutations in the ClCN7 gene are responsible for decreased osteoclast activity in bone resorption. Osteoclast cells are present but unable to resorb bone matrix. This condition leads to an increased bone density typical of ADOII.
Table 1. Various forms of osteopetrosis and related therapies Target
Strategic approach to target
Expected outcome
Who is working on target
Refs
ADOI and ADOII ADOI and ADOII ADOI and ADOII ADOI and ADOII ADOII and ARO ARO ARO and ADOII
Calcitrol (1,25-dihydroxy Vitamin D) Interferon gamma-1b Prednisone (Steroids) Parathyroid hormone Bone marrow transplantation Ventriculo-peritoneal shunt Decompression of the optic nerves
Stimulation of osteoclasts differentiation Increased bone resorption, hematopoiesis Increased hematopoiesis and bone resorption Stimulation of osteoclasts Increased hematopoiesis and bone resorption Treatment of hydrocephalus Treatment of visual impairment
Carolino et al. Key et al. Kocher et al.
[44] [45] [46]
Multiple investigators
[47]
Vanier et al.
[48]
Acknowledgements Partially funded by grants from FIRB-MIUR to A.V. and P.V. (RBNE019J9W) and from Progetto Nazionale Cellule Staminali (No. CS3 to P.V.). This is manuscript no. 87 of the Genoma 2000/ITB Project funded by Cariplo.
References 1 Ash, P. et al. (1980) Osteoclasts derived from haematopoietic stem cells. Nature 283, 669–670
508
www.drugdiscoverytoday.com
2 Karsenty, G. and Wagner, E.F. (2002) Reaching a genetic and molecular understanding of skeletal development. Dev. Cell 2, 389–406 3 Moon, R.T. et al. (2002) The promise and perils of Wnt signalling through beta-catenin. Science 296, 1644–1646 4 Glass, D.A., II et al. (2005) Canonical Wnt signalling in differentiated osteoblasts controls osteoclast differentiation. Dev. Cell 8, 751–764 5 Westendorf, J.J. et al. (2004) Wnt signaling in osteoblasts and bone diseases. Gene 341, 19–39 6 Logan, C.Y. and Nusse, R. (2004) The Wnt signalling pathway in development and disease. Annu. Rev. Cell Dev. Biol. 20, 781–810
Vol. 2, No. 4 2005
7 Gong, Y. et al. (2001) LDL receptor related protein 5(LRP5) affects bone accrual and eye development. Cell 107, 513–523 8 Kato, M. et al. (2002) Cbfa1-independent decrease in osteoblast proliferation, ostepenia and persistent embryonic eye vascularization in mice deficient in Lrp5 a Wnt coreceptor. J. Cell Biol. 157, 303–314 9 Little, R.D. et al. (2002) A mutation in the LDL receptor related protein 5 gene results in the autosomal dominant high bone mass trait. Am. J. Hum. Genet. 70, 11–19 10 Koay, M.A. and Brown, M.A. (2005) Genetic disorders of the LRP5-Wnt signalling pathway affecting the skeleton. Trends Mol. Med. 11, 129–137 11 Boley, W.J. et al. (2003) Osteoclast differentiation and activation. Nature 423, 337–342 12 Blair, H.C. and Athanasou, N.A. (2004) Recent advances in osteoclast biology and pathological bone resorption. Histol. Histopathol. 19, 189–199 13 Teitelbaum, S.L. and Ross, F.P. (2003) Genetic regulation of osteoclast development and function. Nat. Rev. Genet. 4, 638–649 14 Sly, W.S. et al. (1983) Carbonic anhydrase II deficiency identified as the primary defect in the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification. Proc. Natl. Acad. Sci. U S A 80, 2752–2756 15 Vaananen, K.H. et al. (2000) The cell biology of osteoclast function. J. Cell Sci. 113, 377–381 16 Ly, J.P. et al. (1999) Atp6i-deficient mice exhibit severe osteopetrosis due to loss of osteoclast-mediated extracellular acidification. Nat. Genet. 23, 447– 451 17 Scimeca, J.C. et al. (2000) The gene encoding the mouse homologue of the human osteoclast-specific 116-kDa V-ATPase subunit bears a deletion in osteosclerotic (oc/oc) mutants. Bone 26, 207–213 18 Frattini, A. et al. (2000) Defects in TCIRG1 subunit of the vacuolar proton pump are responsible for a subset of human autosomal recessive osteopetrosis. Nat. Genet. 25, 343–346 19 Kornak, U. et al. (2001) Loss of the ClC-7 chloride channel leads to osteopetrosis in mice and man. Cell 104, 205–215 20 Chalhoub, N. et al. (2003) Grey-lethal mutation induces severe malignant autosomal recessive osteopetrosis in mouse and human. Nat. Med. 9, 399– 406 21 Andersen, P.E. and Bollerslev, J. (1987) Heterogeneity of autosomal dominant osteopetrosis. Radiology 164, 223–225 22 Bollerslev, J. (1989) Autosomal dominant osteopetrosis: bone metabolism and epidemiological, clinical and hormonal aspects. Endocr. Rev. 10, 45–67 23 Van Hul, E. et al. (2002) Localization of the gene causing autosomal dominant osteopetrosis type I to chromosome 11q12–13. J. Bone Miner. Res. 17, 1111–1117 24 Hey, P.J. et al. (1998) Cloning of a novel member of the low density lipoprotein receptor family. Gene 216, 103–111 25 Johnson, M.L. et al. (1997) Linkage of a gene causing high bone mass to human chromosome 11 (11q12–13). Am. J. Hum. Genet. 60, 1326–1332 26 Boyden, L.M. et al. (2002) High bone density due to a mutation in LDLreceptor-related protein 5. N. Engl. J. Med. 346, 1513–1521 27 Ai, M. et al. (2005) Reduced affinity to and inhibition by DKK1 form a common mechanism by which high bone mass-associated missense mutations in LRP5 affect canonical Wnt signaling. Mol. Cell. Biol. 25, 4946–4955 28 Van Wesenbeeck, L. et al. (2003) Six novel missense mutations in the LDL receptor related protein 5 (LRP5) gene in different conditions with an increased bone density. Am. J. Hum. Genet. 72, 763–771
Drug Discovery Today: Disease Mechanisms | Dominant inheritance disorders
29
30
31 32
33
34
35
36
37
38
39
40
41
42
43
44 45 46 47
48
Kwee, M.L. et al. (2005) An autosomal dominant high bone mass phenotype in association with craniosynostosis in an extended family is caused by an LRP5 missense mutation. J. Bone Miner. Res. 20, 1254–1260 Benichou, O. et al. (2001) Mapping of autosomal dominant osteopetrosis type II (Albers–Schonberg disease) to chromosome 16p13.3. Am. J. Hum. Genet. 69, 647–654 El-Tawil, T. and Stoker, D.J. (1993) Benign osteopetrosis: a review of 42 cases showing two different patterns. Skeletal Radiol. 22, 587–593 Benichou, O.D. et al. (2000) Type II autosomal dominant osteopetrosis (Albers–Schonberg disease): clinical and radiological manifestations in 42 patients. Bone 26, 87–93 Waguespack, S. et al. (2003) Chloride channel (ClCN7) gene mutations and autosomal dominant osteopetrosis, type II. J. Bone Miner. Res. 18, 1513–1518 Van Hul, W. et al. (1997) Localization of a gene for autosomal dominant osteopetrosis (Albers–Schonberg disease) to chromosome 1p21. Am. J. Hum. Genet. 61, 363–369 Cleiren, E. et al. (2001) Albers–Schonberg disease (autosomal dominant osteopetrosis, type II) results from mutations in the ClCN7 chloride channel gene. Hum. Mol. Gen. 10, 2861–2867 Frattini, A. et al. (2003) Chloride channel ClCN7 mutations are responsible for severe recessive, dominant, and intermediate osteopetrosis. J. Bone Miner. Res. 18, 1740–1747 Campos-Xavier, A.B. et al. (2003) Chloride channel 7 (ClCN7) gene mutations in intermediate autosomal recessive osteopetrosis. Hum. Genet. 112, 186–189 Waguespack, S. et al. (2002) Measurement of tartrate resistant acid phosphatase and the brain isoenzyme of creatine kinase accurately diagnoses type II autosomal dominant osteopetrosis but does not identify gene carriers. J. Clin. Endocrinol. Metab. 87, 2212–2217 Letizia, C. et al. (2004) Type II benign osteopetrosis (Albers–Schonberg disease) caused by a novel mutation in ClCN7 presenting with unusual clinical manifestations. Calcif. Tissue Int. 74, 42–46 Henriksen, K. et al. (2004) Characterization of osteoclasts from patients harboring a G215R mutation in ClC-7 causing autosomal dominant osteopetrosis type II. Am. J. Pathol. 164, 1537–1545 Campos-Xavier, A.B. et al. (2005) Intrafamilial phenotypic variability of osteopetrosis due to chloride channel 7 (CLCN7) mutations. Am. J. Med. Genet. 133, 216–218 Toomes, C. et al. (2004) Mutations in LRP5 or FZD4 underlie the common familial exudative vitreoretinopathy locus on chromosome 11q. Am. J. Hum. Genet. 74, 721–730 Jiao, X. et al. (2004) Autosomal recessive familial exudative vitreoretinopathy is associated with mutations in LRP5. Am. J. Hum. Genet. 75, 878–884 Carolino, J. et al. (1998) Osteopetrosis. Am. Fam. Physician 57, 1293–1296 Key, L.L., Jr et al. (1995) Long-term treatment of osteopetrosis with recombinant human interferon gamma. N. Engl. J. Med. 15, 1594–1599 Kocher, M.S. and Kasser, J.R. (2003) Osteopetrosis. Am. J. Orthop. 32, 222– 228 Driessen, G.J. et al. (2003) Long-term outcome of haematopoietic stem cell transplantation in autosomal recessive osteopetrosis: an EBMT report. Bone Marrow Transplant. 32, 657–663 Vanier, V. et al. (2000) Bilateral visual improvement after unilateral optic canal decompression and cranial vault expansion in a patient with osteopetrosis, narrowed optic canals, and increased intracranial pressure. J. Neurol. Neurosurg. Psychiatry 69, 405–406
www.drugdiscoverytoday.com
509