A lambda 1 transgene under the control of a heavy chain promoter and enhancer does not undergo somatic hypermutation

Targeting of somatic hypermutation

Somatic hypermutation (SHM) introduces mutations in the variable region of immunoglobulin genes at a rate of approximately 10(-3) mutations per base pair per cell division, which is 10(6)-fold higher than the spontaneous mutation rate in somatic cells. To ensure genomic integrity, SHM needs to be targeted specifically to immunoglobulin genes. The rare mistargeting of SHM can result in mutations and translocations in oncogenes, and is thought to contribute to the development of B-cell malignancies. Despite years of intensive investigation, the mechanism of SHM targeting is still unclear. We review and attempt to reconcile the numerous and sometimes conflicting studies on the targeting of SHM to immunoglobulin loci, and highlight areas that hold promise for further investigation.

DNA breaks in hypermutating immunoglobulin genes: evidence for a break-and-repair pathway of somatic hypermutation

To test the hypothesis that immunoglobulin gene hypermutation in vivo employs a pathway in which DNA breaks are introduced and subsequently repaired to produce mutations, we have used a PCR-based assay to detect and identify single-strand DNA breaks in lambda1 genes of actively hypermutating primary murine germinal center B cells. We find that there is a two- to threefold excess of breaks in lambda1 genes of hypermutating B cells, relative to nonhypermutating B cells, and that 1.3% of germinal center B cells contain breaks in the lambda1 gene that are associated with hypermutation. Breaks were found in both top and bottom DNA strands and were localized to the region of lambda1 that actively hypermutates, but duplex breaks accounted for only a subset of breaks identified. Almost half of the breaks in hypermutating B cells occurred at hotspots, sites at which two or more independent breaks were identified. Breaksite hotspots were associated with characteristic sequence motifs: a pyrimidine-rich motif, either RCTYT or CCYC; and RGYW, a sequence motif associated with hypermutation hotspots. The sequence motifs identified at breaksite hotspots should inform the design of substrates for characterization of activities that participate in the hypermutation pathway.

The reverse transcriptase model of somatic hypermutation

The evidence supporting the reverse transcriptase model of somatic hypermutation is critically reviewed. The model provides a coherent explanation for many apparently unrelated findings. We also show that the somatic hypermutation pattern in the human BCL-6 gene can be interpreted in terms of the reverse transcriptase model and the notion of feedback of somatically mutated sequences to the germline over evolutionary time.

PMS2-deficiency diminishes hypermutation of a lambda1 transgene in young but not older mice

The Pms2 gene is involved in DNA mismatch repair in mammalian cells, and has recently been shown to affect hypermutation of mammalian immunoglobulin genes. We have studied hypermutation of a lambda1 transgene in chronically stimulated Peyer's patch B cells of both young and old mice deficient in function of Pms2. In young (3-4 months) mice, somatic hypermutation is fourfold lower in PMS2-deficient mice than in control mice. This difference is statistically significant (P < 0.05). In contrast, in older mice (9 months of age), hypermutation levels are indistinguishable in the Pms2-/- and Pms2+/+ backgrounds. In the older mice, there was no clear difference in the fraction of clones carrying either any mutations or at least two mutations when PMS2-deficient mice were compared with their wild-type littermates. As genomic instability increases with age, this observation is difficult to reconcile with the hypothesis that highly mutated B cells cannot survive in Peyer's patches. Moreover, there were clear differences apparent in the mutation spectra of the Pms2-/- and Pms2+/+ mice. In the PMS2-deficient background, deletion and insertion mutations were found, and there was a significant decrease in the ratio of A mutations to T mutations in comparison with the Pms2+/+ controls. Our data support the hypothesis that PMS2 functions in somatic hypermutation, and are most consistent with the hypothesis that the role of PMS2 is direct rather than indirect.

A λ 3′ Enhancer Drives Active and Untemplated Somatic Hypermutation of a λ1 Transgene

Somatic hypermutation is a highly regulated process that targets mutations to the rearranged Ig genes. Little is known about the cis-elements required for somatic hypermutation of the λ light chain gene. We have studied somatic hypermutation of a rearranged λ1 transgene under the control of either a λ2-4 or κ 3′ enhancer. The mutations in the transgenes were analyzed by sequencing DNA amplified from hypermutating Peyer’s patch B cells. The results indicate that the λ 3′ enhancer can drive active hypermutation of a λ1 transgene in Peyer’s patch cells. The λ1 transgene under analysis carried two marked Vλ2 genes immediately upstream that could serve as sequence donors in possible gene conversion events. There was no evidence of sequence transfer to the hypermutated λ1 gene, suggesting that gene conversion is not a major mechanism for somatic hypermutation in mice.

Recombination-based mechanisms for somatic hypermutation

We review some experiments designed to test recombination-based mechanisms for somatic hypermutation in mice, particularly mechanisms involving templated mutation or gene conversion. As recombination and repair functions are highly conserved among prokaryotes and eukaryotes, pathways of mutation in microorganisms may prove relevant to the mechanism of somatic hypermutation. Escherichia coli initiates a recombination-based pathway of mutation in response to environmental stimuli, and this "adaptive" pathway of mutation has striking similarities with somatic hypermutation, as does a process of mutagenic repair that occurs at double-strand breaks in Saccharomyces cerevisiae. We present a model for recombination-based hypermutation of the immunoglobulin loci which could result in either templated or non-templated mutation.

Multiple sequences from downstream of the J kappa cluster can combine to recruit somatic hypermutation to a heterologous, upstream mutation domain

Recruitment of somatic hypermutation to the Ig kappa locus has previously been shown to depend on the enhancer elements, Ei/MAR and E3'. Here we show that these elements are not sufficient to confer mutability. However, hypermutation is effectively targeted to a chimeric beta-globin/Ig kappa transgene whose 5' end is composed of the human beta-globin gene (promoter and first two exons) and whose 3' end consists of selected sequences derived from downstream of the J kappa cluster (Ei/MAR, C kappa + flank and E3'). Thus, multiple downstream Ig kappa sequences (all derived from 3' of the J kappa cluster) can combine to recruit mutation to a heterologous mutation domain. The location of this hypermutation domain is defined by the position of the transcription start site and this applies even if the Ig kappa Ei/MAR is positioned upstream of the promoter. Hotspots within the mutation domain are, however, defined by local DNA sequence as evidenced by a new hotspot being created within the beta-globin domain by a mutation within the transgene. We propose that multiple, moveable Ig kappa sequences (that are normally located downstream of the transcription start site) cooperate to bring a hypermutation priming factor to the transcription initiation complex; a mutation domain is thereby created downstream of the promoter but the local sequence defines the detailed pattern of mutation within that domain.

Evaluation of the role of the 3'alpha heavy chain enhancer [3'alpha E(hs1,2)] in Vh gene somatic hypermutation

Previous work on the cis-acting elements that control heavy chain variable region (VH) gene somatic hypermutation has indicated the presence of an as yet unidentified element(s) 3' of the intron enhancer that is necessary for high rate mutation. Examination of cis-acting elements involved in kappa light chain V gene hypermutation has demonstrated a requirement for both the intronic and 3' kappa enhancers in this process. To examine whether the 3'alpha heavy chain enhancer [3'alpha E(hs1,2)] is required for somatic hypermutation of VH genes, we generated two types of transgenic mice. One type was generated using a construct containing a VH promoter, a rearranged VDJ, the heavy chain intronic enhancer, and the murine heavy chain 3'alpha E(hs1,2). The transgenes in the second lines were similar to the transgenes in the first with the addition of a second complete matrix attachment region (MAR) 3' of the heavy chain intronic enhancer, and splice acceptor and polyadenylation sites between the two enhancers. Analysis of both transgenes revealed levels of mutation at least 10-fold lower than endogenous VH genes. These data suggest that the 3'alpha E(hs1,2) does not play a role analogous to the 3' kappa enhancer in the regulation of the hypermutation process. Moreover, in one of the transgenes, the presence of the 3'alpha E(hs1,2) resulted in a lack of transcription in vivo, suggesting a negative regulatory role for this enhancer in certain contexts.

Developmental specificity of immunoglobulin heavy chain switch region recombination activities

To understand the regulation of enzymes that carry out immunoglobulin heavy chain class switch recombination, we have assayed recombination of extrachromosomal substrates carrying switch region sequences in cell lines representing different stages of lymphoid cell development. Both pre-B and mature B cell lines supported switch substrate recombination, but B cell lines derived from later stages of cell development did not. Recombination did not occur in an erythroid or a macrophage line. Most recombination junctions in the substrates recovered from transfection of pre-B and B cells mapped to heterogeneous sites within the S mu and Sgamma regions, as do chromosomal switch junctions. Some recombination did occur in T cell lines, but most recombination junctions involved an upstream promoter and did not map preferentially to S regions. Culture of the pre-B cell lines PD31 and 70Z/3 with LPS increased recombination two-fold, to levels approaching those observed in LPS-cultured primary B cells. These results show that the full complement of factors necessary for switch recombination is present only in cells representing a limited spectrum of B cell development and that LPS, which can activate resting splenic B cells to carry out chromosomal recombination, can also stimulate recombination activity in immortalized pre-B cell lines.

Mechanism of antigen-driven somatic hypermutation of rearranged immunoglobulin V(D)J genes in the mouse

Available data relevant to the mechanism of somatic hypermutation have been critically evaluated in the context of alternative models: (i) error-generating reverse transcription (RT) followed by homologous recombination; and (ii) error-prone DNA replication/repair. A set of basic principles concerning somatic hypermutation has also been formulated and a revised and expanded "RT-Mutatorsome" concept (analogous to telomerase) is presented which is consistent with these principles and all data on the distribution of somatic mutations in normal and Ig transgenic mice carrying particular V(D)J and flanking region constructs. It is predicted that in the mouse VH and Vk loci. the J-C intronic Enhancer-Nuclear Matrix Attachment Region (Ei/MAR) contains a unique sequence motif or secondary structure which ensures that only V(D)J sequences mutate whilst other regions of the genome are not mutated.

Somatic hypermutation of immunoglobulin genes

The relationship between somatic hypermutation and affinity maturation in the mouse is delineated. Recent work on the anatomical and cellular site of this process is surveyed. The molecular characteristics of somatic hypermutation are described in terms of the region mutated and the distinctive patterns of nucleotide changes that are observed. The results of experiments utilizing transgenic mice to find out the minimum cis-acting sequences required to recruit hypermutation are summarized. The hypothesis that V gene sequences have evolved in order to target mutation to certain sites but not others is discussed. The use that different species make of somatic hypermutation to generate either the primary or secondary B cell repertoire is considered. Possible molecular mechanisms for the hypermutation process and future goals of research are outlined.

Loss of the p16/MTS1 tumor suppressor gene causes E2F-mediated deregulation of essential enzymes of the DNA precursor metabolism

Homozygous deletions of the tumor suppressor gene p16/MTS1 were reported in a wide variety of tumors and tumor cell lines. Its product inhibits the phosphorylation of the retinoblastoma protein (pRb) by CDK4 and CDK6. Because phosphorylation of pRb is a major regulatory event in the activation of the transcription factor E2F, a role for p16 in the regulation of E2F-dependent transcription was presumed. We investigated the effect of the loss of p16 on E2F-mediated transcription in a tumor progression model consisting of three cell lines originating from a common precursor cell--one p16-positive cell line established from the primary biopsy and two lines derived from more advanced stages of the tumor representing the same cell clone after loss of p16. We observed up- and deregulation of E2F-dependent transcription during the cell cycle of the p16-negative cell clones, which returned to normal after transient expression of p16. This p16-dependent regulation affects a set of enzymes necessary for the activation of all four DNA precursors; it is paralleled by the interconversion of transcriptionally active free E2F and transcriptionally inactive higher molecular complexes of E2F and is dependent on the existence of endogenous pRb. Furthermore, we show that p16-negative cell clones exhibit a growth advantage compared to their p16-positive counterparts. One might speculate that one feature of tumor progression could be deregulation of E2F-dependent transcription caused by loss of p16.

Somatic hypermutation

For the generation of secondary response antibodies, immunoglobulin genes are subjected to hypermutation. Cells expressing antibodies with higher affinity are then selected by antigen. Recent clues to the mechanism of hypermutation come from experiments using transgenic mice enabling analysis of the controlling cis-acting elements and the intrinsic features of the hypermutation, dissociated from the effects of antigenic selection.

Isotype exclusion in lambda 1 transgenic mice depends on transgene copy number and diminishes with down-regulation of transgene transcripts

We have compared expression of the endogenous kappa and transgenic lambda 1 light chains in three lines of mice carrying one, four and eight copies of a lambda 1 transgene. We have found that in very young mice, even a single rearranged transgenic lambda allele excludes expression of the endogenous kappa loci. As the lambda 1 transgenic mice age, the proportion of lambda-positive cells decreases, as has been reported by others (Neuberger et al., Nature 1989, 338:350; Pettersson et al., Nature 1990. 344:165; Hagman et al., J. Exp. Med. 1989. 169:1911; Bogen and Weiss, Eur. J. Immunol. 1991. 21:2391). The decrease in lambda-positive B cells is accompanied by an increase in kappa-positive cells. We show that the decrease in B cells bearing surface lambda immunoglobulin depends on transgene copy number and occurs most rapidly in lower copy number lines. The decrease in surface lambda expression correlates with a dramatic decrease in the level of lambda mRNA in splenic B cells. Transgene down-regulation cannot be alleviated by stimulation of splenocytes with the mitogen lipopolysaccharide. These are the first data to establish that a single copy transgene can effect isotype exclusion. In addition, these results provide strong evidence that down-regulation of transgene expression is controlled at the level of transcription, and that it is the level of expressed light chain that regulates isotype exclusion.