In 1976, S. Tonegawa and N. Hozumi found the first direct evidence that separate genes encode the V and C regions of immunoglobulins and that the genes are rearranged in the course of B-cell differentiation. This work changed the field of immunology. In 1987, Tonegawa was awarded the Nobel Prize for this work.
Selecting DNA from embryonic cells and adult myeloma cells—cells at widely different stages of development— Tonegawa and Hozumi used various restriction endonucle-ases to generate DNA fragments. The fragments were then separated by size and analyzed for their ability to hybridize with a radiolabeled mRNA probe. Two separate restriction fragments from the embryonic DNA hybridized with the mRNA, whereas only a single restriction fragment of the adult myeloma DNA hybridized with the same probe. Tone-gawa and Hozumi suggested that, during differentiation of lymphocytes from the embryonic state to the fully differentiated plasma-cell stage (represented in their system by the myeloma cells), the V and C genes undergo rearrangement. In the embryo, the V and C genes are separated by a large DNA segment that contains a restriction-endonuclease site; during differentiation, the V and C genes are brought closer together and the intervening DNA sequence is eliminated.
The pioneering experiments of Tonegawa and Hozumi employed a tedious and time-consuming procedure that has since been replaced by the much more powerful approach of Southern-blot analysis. This method, now universally used to investigate the rearrangement of immunoglobulin genes, eliminates the need to elute the separated DNA restriction fragments from gel slices prior to analysis by hybridization with an immunoglobulin gene segment probe. Figure 5-2 shows the detection of rearrangement at the k light-chain locus by comparing the fragments produced by digestion of DNA from a clone of B-lineage cells with the pattern obtained by digestion of non-B cells (e.g., sperm or liver cells). The rearrangement of a V gene deletes an extensive section of germ-line DNA, thereby creating differences between rearranged and unrearranged Ig loci in the distribution and number of restriction sites. This results in the generation of
RE RE RE RE
Experimental basis for diagnosis of rearrangement at an immunoglobulin locus. The number and size of restriction fragments generated by the treatment of DNA with a restriction enzyme is determined by the sequence of the DNA.The digestion of rearranged DNA with a restriction enzyme (RE) yields a pattern of restriction fragments that differ from those obtained by digestion of an unrearranged locus with the same RE. Typically, the fragments are analyzed by the technique of Southern blotting. In this example, a probe that includes a J gene segment is used to identify RE digestion fragments that include all or portions of this segment. As shown, rearrangement results in the deletion of a segment of germ-line DNA and the loss of the restriction sites that it includes. It also results in the joining of gene segments, in this case a V and a J segment, that are separated in the germ line. Consequently, fragments dependent on the presence of this segment for their generation are absent from the restriction-enzyme digest ofDNA from the rearranged locus. Furthermore, rearranged DNA gives rise to novel fragments that are absent from digests of DNA in the germ-line configuration. This can be useful because both B cells and non-B cells have two immunoglobulin loci. One of these is rearranged and the other is not. Consequently, unless a genetic accident has resulted in the loss of the germ-line locus, digestion of DNA from a myeloma or normal B-cell clone will produce a pattern of restriction that includes all of those in a germline digest plus any novel fragments that are generated from the change in DNA sequence that accompanies rearrangement. Note that only one of the several J gene segements present is shown.
different restriction patterns by rearranged and unre-arranged loci. Extensive application of this approach has demonstrated that the Dreyer and Bennett two-gene model—one gene encoding the variable region and another encoding the constant region—applied to both heavy and light-chain genes.
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