Although apparently phenotypically normal at birth, mice heterozygous for inactivation of the gene encoding parathyroid hormone-related peptide (PTHrP) develop haplotype insufficiency by 3 months of age. In addition to histologic and morphologic abnormalities similar to those seen in homozygous mutants, heterozygous animals demonstrated alterations in trabecular bone and bone marrow. These included metaphyseal bone spicules which were diminished in volume, irregularly distributed, and less well developed than those seen in age-matched controls as well as bone marrow, which contained an inordinate number of adipocytes. A substantial reduction in PTHrP mRNA was detected in heterozygous tissue, while circulating parathyroid hormone (PTH) and calcium concentrations were normal. Thus, while a physiologic concentration of PTH was capable of maintaining calcium homeostasis, it was incapable of compensating for PTHrP haploinsufficiency in developing bone. In normal animals, both PTHrP and the PTH/PTHrP receptor were expressed predominantly in chondrocytes situated throughout the proliferative zone of the tibial growth plate. In the metaphysis, the PTH/PTHrP receptor was identified on osteoblasts and preosteoblastic cells situated in the bone marrow, while PTHrP was expressed only by osteoblasts. These observations indicate that postnatal bone development involves susceptible pathways that display exquisite sensitivity to critical levels of PTHrP and imply that the skeletal effects of PTH are influenced by locally produced PTHrP. Moreover, identification of both the ligand and its N-terminal receptor in metaphyseal osteoblasts and their progenitors suggests an autocrine/paracrine role for the protein in osteoblast differentiation and/or function. Impairment in this function as a consequence of PTHrP haploinsufficiency may critically influence the course of bone formation, resulting in altered trabecular architecture and perhaps low bone mass and increased bone fragility.

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Activation-induced cytidine deaminase (AID) and uracil DNA glycosylase (UNG) are required for class switch recombination (CSR). AID is involved in the DNA cleavage step of CSR, but the precise role of UNG is not yet understood. Mutations and deletions are footprints of abortive DNA cleavage in the immunoglobulin switch region in splenic B cells stimulated to undergo CSR. However, a UNG deficiency did not reduce the number of such footprints, indicating UNG is dispensable for the DNA cleavage step. Mutagenesis experiments revealed that the role of UNG in CSR depends on its WXXF motif. This motif is also essential for the interaction of UNG with the HIV viral peptide Vpr, which recruits UNG to the HIV particle. Furthermore, exogenous Vpr had a dominant-negative effect on CSR. These results suggest that UNG is recruited to the CSR machinery through its WXXF motif by a Vpr-like host factor and plays a novel non-canonical role in a CSR step that follows DNA cleavage. Activation-induced cytidine deaminase (AID)3 is necessary and sufficient for two major immunoglobulin (Ig) gene alterations, somatic hypermutation (SHM), and class switch recombination (CSR), which take place in the germinal center by the antigen stimulation of B cells (1-3). SHM introduces point mutations in the variable (V) region of Ig genes, resulting in an increase of the antibody affinity to antigens when coupled with selection of B cells by a limited amount of antigen. In the SHM reaction, DNA is first cleaved by single strand nicks, and mutations are subsequently introduced by erroneous DNA repair (4).CSR replaces the expressed heavy-chain constant (C H ) region exons from C with other downstream C H exons to generate antibodies with unaltered antigen specificity but with altered effector functions. In CSR, single strand nicks are introduced into both strands of the switch (S) regions to generate staggered cleavage ends with 3Ј or 5Ј overhangs, which are repaired to form blunt ends by gap-filling DNA synthesis and/or exonucleolytic digestion (4, 5). Finally, the blunt ends of the S and other S regions are joined by the non-homologous end joining (NHEJ) repair machinery (6 -9), giving rise to a large deletion in the chromosome and a looped-out circle of deleted DNA (10 -12). During this repair process, mutations and deletions are often introduced into the rearranged S regions. Some cleavages in the S regions are abortive and repaired without CSR, but mutations and deletions nonetheless arise during their repair (13-15).A deficiency of uracil DNA glycosylase (UNG), which removes U from DNA and triggers the base excision repair pathway, causes a severe reduction of CSR activity, to 10% of wild type (16). Interestingly, UNG deficiency does not significantly affect the SHM frequency, unlike deficiencies of the mismatch repair enzymes, which reduce the SHM frequency to 20 -50% (17-19). However, UNG deficiency does bias the target base specificity of SHM toward GC, as does a deficiency of MSH2 (16,20). The phenotypes of the UNG deficiency in CSR and SHM...