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Upregulation of osteoclast α2β1 integrin compensates for lack of αvβ3 vitronectin receptor in Iraqi‐Jewish‐type Glanzmann thrombasthenia

Summary. Osteoclasts utilize αvβ3 integrin adhesion to bone matrix during bone resorption. We have generated osteoclasts from the peripheral blood of Iraqi‐Jewish patients with Glanzmann thrombasthenia (GT) who are completely deficient in β3 integrin and exhibit a haemorrhagic diathesis resulting from the absence of platelet αIIbβ3. We show that, in contrast to osteoclasts generated from normal subjects or patients with αIIb integrin deficiency, GT osteoclasts lack αvβ3. These osteoclasts exhibited a two‐ to fourfold increase in α2 and β1 integrin expression, whereas other αv integrins, including αvβ5, were not significantly affected. An accompanying decrease in bone resorption was observed, with 44% and 59% declines in pit number and depth, respectively, and resorption lacunae showed abnormal morphology on scanning electron microscopy. However, osteoclasts from GT developed in similar numbers to controls and exhibited an otherwise ‘normal’ phenotype. We conclude that the observed rise in α2β1 expression compensates for the chronic genetic deficiency of αvβ3 in osteoclasts from patients with GT and is sufficient to enable bone resorption to proceed, albeit to a submaximal extent. This explains why Iraqi‐Jewish patients with GT do not have osteopetrosis.