Disease status in autosomal dominant osteopetrosis type 2 is determined by osteoclastic properties.
Asymptomatic gene carriers and clinically affected ADO2 subjects have the same ClCN7 mutation. We examined osteoclastic bone resorption in vitro as well as osteoclast formation, several markers, acid secretion, and cytoskeletal structure. We found that ADO2 expression results from osteoclast specific properties.


BACKGROUND
Autosomal dominant osteopetrosis type II (ADO2) is a heritable osteosclerotic disorder that results from heterozygous mutations in the ClCN7 gene. However, of those individuals with a ClCN7 mutation, one third are asymptomatic gene carriers who have no clinical, biochemical, or radiological manifestations. Disease severity in the remaining two thirds is highly variable.


METHODS
Human peripheral blood mononuclear cells were isolated and differentiated into osteoclasts by stimulation with hRANKL and human macrophage-colony stimulating factor (hM-CSF). Study subjects were clinically affected subjects, unaffected gene carriers, and normal controls (n = 6 in each group). Pit formation, TRACP staining, RANKL dose response, osteoclast markers, acid secretion, F-actin ring, and integrin alpha(v)beta3 expression and co-localization were studied.


RESULTS
Osteoclasts from clinically affected subjects had severely attenuated bone resorption compared with those from normal controls. However, osteoclasts from unaffected gene carriers displayed similar bone resorption to those from normal controls. In addition, the resorption lacunae from both unaffected gene carriers and normal controls appeared much earlier and spread much more rapidly than those from clinically affected subjects. As time progressed, the distinction between clinically affected subjects and the other two groups increased. No significant difference was found in acidic secretion or osteoclast formation between the three groups. Osteoclast cytoskeletal organization showed no difference between the three groups but there was low cellular motility in clinically affected subjects.


CONCLUSIONS
Osteoclasts from the unaffected gene carriers, in contrast to those from the clinically affected subjects, functioned normally in cell culture. This finding supports the hypothesis that intrinsic osteoclast factors determine disease expression in ADO2. Further understanding of this mechanism is likely to lead to the development of new approaches to the treatment of clinically affected patients.
Chu
Kang
K
Department of Medicine, Indiana University School of Medicine, Indianapolis, Indiana, USA.
Snyder
Richard
R
Econs
Michael J
MJ
D000135:Acid Phosphatase / Q000378:metabolism; D000293:Adolescent; D000328:Adult; D000368:Aged; D015415:Biomarkers / Q000378:metabolism; D001862:Bone Resorption / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D002352:Carrier Proteins / Q000494:pharmacology; D002454:Cell Differentiation / Q000187:drug effects / Q000235:genetics; D002478:Cells, Cultured; D002648:Child; D018118:Chloride Channels / Q000235:genetics* / Q000378:metabolism; D005260:Female; D005799:Genes, Dominant / Q000235:genetics; D006579:Heterozygote; D006801:Humans; D007527:Isoenzymes / Q000378:metabolism; D007963:Leukocytes, Mononuclear / Q000378:metabolism / Q000473:pathology; D008297:Male; D008562:Membrane Glycoproteins / Q000494:pharmacology; D008875:Middle Aged; D010010:Osteoclasts / Q000378:metabolism* / Q000473:pathology; D010022:Osteopetrosis / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D053245:RANK Ligand; D053246:Receptor Activator of Nuclear Factor-kappa B; D000071681:Tartrate-Resistant Acid Phosphatase
D015415:Biomarkers; C097540:CLCN7 protein, human; D002352:Carrier Proteins; D018118:Chloride Channels; D007527:Isoenzymes; D008562:Membrane Glycoproteins; D053245:RANK Ligand; D053246:Receptor Activator of Nuclear Factor-kappa B; C505686:TNFRSF11A protein, human; C505688:TNFSF11 protein, human; D000135:Acid Phosphatase; D000071681:Tartrate-Resistant Acid Phosphatase