BoneKEy-Osteovision | Commentary
Double knock-outs and redundancy
DOI:10.1138/2002015
In the last few years our knowledge about gene functions has greatly improved due to the development of in vivo gene knock-out technology. Especially in the skeleton, where molecular studies are made difficult by the peculiar architecture of the mineralized tissue and by the highly specialized functions of cells, this advanced approach has provided a new impetus to the evaluation of genes otherwise non investigable by conventional methods. Nevertheless, for some genes, the information was scanty, due to the fact that, although in vitro studies had revealed important roles of a given gene in bone cell function, no obvious phenotype was apparent.
Recently, we learned that the null-of-function mutation of the Acp5 tartrate-resistant acid phosphatase, which is believed to play a relevant role in osteoclast function, resulted in an unexpected relatively mild bone phenotype in transgenic mice, consisting in developmental deformities likely due to disruption of endochondral ossification and mild osteopetrosis (). Some disordered macrophage inflammatory response was also observed. Similarly, knock-out of the tartrate-sensitive acid phosphatase LAP resulted in lysosomal storage in a number of organs, including the kidney and the central nervous system, but bone kyphoscoliotic malformations of the lower thoracic column were apparent only in mice older than 15 months (). In both mutants, vacuolization of osteoclasts was observed despite the fact that bone resorption was not severely altered.
Based on these observations, the question was raised of whether redundant activities could explain mildness of bone phenotype in loss-of-function trangenics. This indeed turned out to be the case in several regards. In a recent paper (), Suter and co-workers elegantly demonstrated that double knock-out of the LAP and the Acp5 acid phosphatases showed alterations of greater severity compared to the single deficient mice, and the lysosomal storage, as for example in Kupffer cells and bone marrow macrophages, was massive.
Bone malformations more severe than the single transgenics were evident. Foreshortening of long bones, narrower skull, expansion of cartilage growth plate with disruption, hyperplasia and hypertrophy of chondocytes were all observed in the double knock-outs. Not surprisingly, osteoblast activity was unaltered, whereas osteoclasts showed greater lysosomal storage but, unexpectedly, some amount of bone resorption occurred. In fact, metaphyseal subperiosteal as well as endosteal bone resorption was still appreciable. Interestingly, the two enzymes appeared to contribute to the dephosphorylation of osteopontin, although Acp5 is likely to represent the major enzyme implicated in this process.
Do redundant activities occur also in human pathologies? Of course the answer is yes. Just as an example, we recently characterized the phenotype of patients affected by autosomal recessive infantile ostepetrosis dependent on mutation of the a3 subunit of the osteoclast proton pump [manuscript in preparation]. Both iliac crest biopsies and in vitro studies of osteoclasts harvested from the peripheral blood monocytic fraction showed that bone resorption by a3-negative osteoclasts still occurs, thus suggesting that additional mechanisms may compensate for the a3 gene. This compensation is incomplete, as demonstrated by the severity of the disease. However, the observation may open an avenue to specific studies aimed at understanding the molecular basis of this complementation, with the perspective of pharmacological management to stimulate bone resorption acting on the alternative osteoclast acidifying mechanisms.
In conclusion, redundancy of function is an important mechanism underlying the capability of cells to compensate loss-of-function abnormalities, and the work of Suter et al. () is an elegant example demonstrating the relevance of this occurrence in bone.
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