The 3′ end of the heavy chain constant region locus enhances germline transcription and switch recombination of the four γ genes

The switch in immunoglobulin (Ig) heavy chain class is preceded by germline transcription and then mediated by a DNA recombination event. To study germline transcription and class switch recombination we used transgenic mice with a 230-kilobase bacterial artificial chromosome that included a rearranged VDJ gene and the entire heavy chain constant region locus. In addition to several lines with intact transgenes, we identified two lines in which the heavy chain locus transgene lacked Cα and everything 3′ of it, including the regulatory elements HS3a, HS1-2, HS3b, and HS4. B cells from both lines with the truncated transgenes make abundant transgenic (Tg) VDJCμ transcripts and IgM protein. Deletion of the 3′ end of the locus results in dramatically reduced expression of both germline transcripts and switched VDJCH transcripts of the γ3, γ2b, γ2a, and ɛ genes. In addition, the transgenes lacking the 3′ end of the locus express reduced amounts of γ1 germline transcripts and 2–3% of the amount of Tg IgG1 in tissue culture compared with intact transgenes. Finally, switch recombination to γ1 is undetectable in the transgenes lacking the 3′ elements, as measured by digestion circularization–polymerase chain reaction or by the expression of VDJCγ1 transcripts

Methylation of plasmacytoma c-myc genes

The chromosomal translocation associated with many tumors of immunoglobulin-producing cells frequently results in the joining of the immunoglobulin heavy-chain locus and the c-myc oncogene. This translocation of c-myc has profound structural and functional consequences for the oncogene, including loss of the 5 ' end of the gene and transcriptional deregulation. We report in this communication that translocation results in a new methylation pattern of c-myc. In normal kidney and liver tissue, the c-myc gene is methylated at its 3' end. The translocated gene in plasmacytoma DNA is extensively demethylated. On the other hand, the nonrearranged c-myc gene in plasmacytoma DNA (which is transcriptionally silent) is extensively methylated. In addition, we confirm the nucleotide sequence (with 19 discrepancies out of 1400 bp) 5' to the murine c-myc gene, as reported by Corcoran et al. [Cell 40 (1985) 71-79].Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25882/1/0000445.pd

Two subfamilies of murine retrotransposon ETn sequences

Early transposon (ETn) elements are 5.7-kb retrotransposons found in the murine genome. We have sequenced large portions of two ETn elements that have apparently transposed within the DNA of a murine myeloma cell line, P3.26Bu4. One of the transposed ETn elements has 5' and 3' long terminal repeats (LTRs) that are exact duplicates of each other and has a 6-bp target site duplication. These results suggest that this element, which inserted into an immunoglobulin [gamma]1 switch region, moved by a retrotransposition process. Our nucleotide sequences confirm that individual ETn elements are very similar to one another and lack open reading frames. However, the ETn sequences reported here and those previously described differ significantly near their 5' LTRs, including 200 bp of weak similarity and 240 bp of complete disparity. Southern hybridization analysis suggests that both subfamilies of ETn sequences are represented many times in the mouse genome. The possibility that the disparate sequences have a role in transposition by ETn elements is discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28722/1/0000543.pd

Transgenes of the Mouse Immunoglobulin Heavy Chain Locus, Lacking Distal Elements in the 3′ Regulatory Region, Are Impaired for Class Switch Recombination

<div><p>The immunoglobulin heavy (H) chain class switch is mediated by a deletional recombination event between µ and γ, α, or ε constant region genes. This recombination event is upregulated during immune responses by a regulatory region that lies 3′ of the constant region genes. We study switch recombination using a transgene of the entire murine H chain constant region locus. We isolated two lines of mice in which the H chain transgenes were truncated at their 3′ ends. The truncation in both transgenic lines results in deletion of the 3′-most enhancer (HS4) and a region with insulator-like structure and activities. Even though both truncated transgenes express the µ H chain gene well, they undergo very low or undetectable switch recombination to transgenic γ and α constant region genes. For both transgenic lines, germline transcription of some H chain constant regions genes is severely impaired. However, the germline transcription of the γ1 and γ2a genes is at wild type levels for the transgenic line with the larger truncation, but at reduced levels for the transgenic line with the smaller truncation. The dramatic reduction in class switch recombination for all H chain genes and the varied reduction in germline transcription for some H chain genes could be caused by (i) insertion site effects or (ii) deletion of enhancer elements for class switch recombination and transcription, or (iii) a combination of both effects.</p> </div