The osteopetrotic mutation toothless ( tl ) is a loss-of-function frameshift mutation in the rat Csf1 gene: Evidence of a crucial role for CSF-1 in osteoclastogenesis and endochondral ossification

Abstract

The toothless (tl) mutation in the rat is a naturally occurring, autosomal recessive mutation resulting in a profound deficiency of bone-resorbing osteoclasts and peritoneal macrophages. The failure to resorb bone produces severe, unrelenting osteopetrosis, with a highly sclerotic skeleton, lack of marrow spaces, failure of tooth eruption, and other pathologies. Injections of CSF-1 improve some, but not all, of these. In this report we have used polymorphism mapping, sequencing, and expression studies to identify the genetic lesion in the tl rat. We found a 10-base insertion near the beginning of the open reading of the Csf1 gene that yields a truncated, nonfunctional protein and an early stop codon, thus rendering the tl rat CSF-1 null . All mutants were homozygous for the mutation and all carriers were heterozygous. No CSF-1 transcripts were identified in rat mRNA that would avoid the mutation via alternative splicing. The biology and actions of CSF-1 have been elucidated by many studies that use another naturally occurring mutation, the op mouse, in which a single base insertion also disrupts the reading frame. The op mouse has milder osteoclastopenia and osteopetrosis than the tl rat and recovers spontaneously over the first few months of life. Thus, the tl rat provides a second model in which the functions of CSF-1 can be studied. Understanding the similarities and differences in the phenotypes of these two models will be important to advancing our knowledge of the many actions of CSF-1.O steoclasts, multinucleated cells that resorb bone, differentiate via fusion of mononuclear precursors of the monocyte͞macrophage lineage, in large part under the local control of factors secreted by bone-forming osteoblasts (1). Insights into factors that regulate the formation and activation of osteoclasts have come from naturally occurring mutations and genetic manipulations that cause osteopetrosis, a condition in which defective bone resorption leads to a sclerotic skeleton (2). One such factor is the cytokine CSF-1 (M-CSF), originally identified as a monocyte-macrophage colony-stimulating factor (3). Identification of a frameshift mutation in the Csf1 gene of the osteopetrosis (op) mouse was a major step in understanding osteoclast ontogeny (4). (The op mutation in the mouse, which affects an osteoblast signal, is not to be confused with the op mutation in the rat, a severe, naturally occurring osteopetrotic mutation that affects osteoclast function.) Since then, many genes have been identified that do double-duty in the immune and skeletal systems. Tumor necrosis factor superfamily member 11 [TNFSF11, also called TRANCE (5), RANKL (6), ODF (7), and OPGL (8)], its receptor (RANK), and the receptorassociated intracellular signal initiator (TRAF6) are essential for lymph node organogenesis, maintaining antigen presenting dendritic cells, and formation of osteoclasts (refs. 9-12; reviewed in ref. 13). The transcription factor PU.1 functions in osteoclast differentiation and activation and in myeloid cells and B lymphocyt...

Expression of and Role for Ovarian Cancer G-protein-coupled Receptor 1 (OGR1) during Osteoclastogenesis

“…Briefly, 3-week-old csf1 tl /csf1 tl rats were obtained from the inbred colony maintained under specific pathogen-free conditions. Mutants were genotyped by neonatal x-ray and by PCR (6). Beginning at 21 days after birth, animals received daily injections of 10 6 units of recombinant human CSF-1 (generously provided by Chiron Corp., Emeryville, CA).…”

“…tl /csf1 tl ) osteopetrotic rats lack osteoclasts due to an inactivating frameshift mutation in the csf1 gene (5,6). Injections of CSF-1 restore osteoclast populations, with a few TRAP-positive cells first appearing in long bones 2 days after initiation of CSF-1 treatments and reaching a peak at 4 days (3).…”

“…One such model is the toothless (tl) rat, in which a frameshift mutation in the csf1 gene results in severe osteopetrosis in homozygous mutants due to a nearly complete lack of osteoclasts (5,6). Injections of recombinant CSF-1 restore osteoclast populations within several days in this animal model (3).…”

Genetics in Endocrinology: Autosomal dominant osteopetrosis revisited: lessons from recent studies

“…As the pathogenic defect is on the formative side of bone remodeling, bone marrow transplantation is not an option (151,158,159). Two considerations are of importance in relation to treatment of HBM: i) serious side effects are unacceptable given the relatively benign nature of the disease and ii) bone formation is increased and accompanied with some alteration in bone resorption (118,160).…”

Septoclast Deficiency Accompanies Postnatal Growth Plate Chondrodysplasia in the Toothless (tl) Osteopetrotic, Colony-Stimulating Factor-1 (CSF-1)-Deficient Rat and Is Partially Responsive to CSF-1 Injections

“…The toothless (Csf1 tl /Csf1 tl ) osteopetrotic rat has a recessive, loss-of-function frameshift mutation in the gene for colony-stimulating factor-1 (CSF-1 or macrophage-CSF). 11,12 The loss renders the Csf1 tl /Csf1 tl rat severely osteopetrotic because of profound osteoclastopenia. In this strain, as in a growing list of osteoclast-deficient mutant mice, 13 there is a progressive dysplasia in which chondrocytes fail to form their normal columns and differentiation zones, the central region of the growth plate thickens with time, and vascular invasion at the COJ is deficient.…”