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The DNA coding for the human immunoglobulin D(IgD) heavy chain (delta, delta) has been sequenced including the membrane and secreted termini. Human delta, like that of the mouse, has a separate exon for the carboxyl terminus of the secreted form. This feature of human and mouse IgD distinguishes it from all other immunoglobulins regardless of species or class. The human gene is different from that of the mouse; it has three, rather than two, constant region domains; and its lengthy hinge is encoded by two exons rather than one. Except for the third constant region, the human and mouse genes are only distantly related.

Cystic fibrosis (CF) is a common recessive lethal genetic disorder, affecting 1 in 1,600 Caucasians. The disease causes defective regulation of chloride-ion transport in exocrine cells. Although in all CF families the disease is linked to a locus on chromosome 7q31, there is clinical heterogeneity in the severity of the disease and the age at which it is diagnosed. CF is caused by mutations in the CF transmembrane conductance regulator (CFTR) gene. A three-nucleotide deletion (delta F508) causing the loss of a phenylalanine residue in the tenth exon of the CFTR gene has been found on 70% of CF chromosomes. We have now characterized a CF family in which neither parent of the affected individual carries the common mutation, and identified a two-nucleotide insertion in the CF allele of the mother. The mutation introduces a termination codon in exon 13 of the CFTR gene at residue 821, and is predicted to result in the production of a severely truncated nonfunctional protein.

Prototypical class switching in mouse and human immunoglobulin heavy chains occurs through recombination of tandem blocks of short repeats located 5' to each heavy chain constant region (CH) except C8. Deletion of C, in immunoglobulin D (IgD)-secreting murine plasmacytomas occurs illegitimately. We demonstrate here that in human IgD-secreting myeloma cells freshly isolated from patient bone marrow and in normal peripheral blood B lymphocytes, an IgD switch can occur through homologous recombination of a direct repeat consisting of a 442-bp sequence 1.5 kbp 3' of the JH complex and a 443-bp sequence that is duplicated almost perfectly (96% similarity) 1.7 kbp 5' of the Cc, gene (442/443-base-pair [bp] repeat). This homologous recombination mechanism is not exclusive for IgD switching, since C^deletion endpoints in two established IgD-secreting myeloma cell lines fall outside the 442/443-bp repeat. The 442/443-bp mediated recombination shows cell type specificity, and we propose that it represents a unique mode for increased levels of IgD secretion in humans.The immunoglobulin heavy chain locus is located on chromosome 14 in humans. A heavy chain variable region (VH) gene is composed of one member from each of three gene families designated variable (V), diversity (D), and joining (J). The heavy chain constant region (CH) genes are located downstream of these VH genes in the order C t-CS-Cy3-cyl-C+£-Cal-CyC-y2-cy4-Ce,C.2 (3,12,25). In the primary immune response, the VH gene is transcribed along with the nearest downstream constant region gene, C, , to form a ,u heavy chain mRNA (50). During further B-cell differentiation, however, a given B cell and its progeny may shift from the production of immunoglobulin M (IgM) to one of the other heavy chain isotypes by the expression of downstream CH genes (reviewed in references 28 and 53). Studies primarily with myelomas have indicated that the switch phenomenon is most often the result of an intrachromosomal rearrangement between switch site (S) sequences located upstream of the heavy chain genes. Upon switching, the assembled VDJ gene is brought adjacent to the heavy chain gene to be expressed, resulting in the deletion of the intervening CH genes. Studies, at the DNA sequence level, of mice and humans have demonstrated that these S regions consist of 2 to 10 kilobases (kb) of tandem arrays of repeated sequences (8, 12, 20, 34-36, 41, 42, 48, 56-58). This description of the heavy chain class switch applies to all antibody classes except IgD. To date, the only biological function attributed to IgD with any certainty is serving as an antigen receptor along with IgM on the surface of virgin B cells (22,26,27,33 within the JH-C,, intron some 1.5 kbp 5' of S,, (30).To determine whether either of these sequences is implicated in the secretion of IgD, we have examined primary IgD-secreting myelomas from bone marrow, two established IgD-secreting myeloma cell lines, and normal B lymphocytes from peripheral blood. We found evidence for homologous recombination of the repeat co...

The Li-Fraumeni Syndrome (LFS) is a rare, dominantly inherited syndrome that features high risk of cancers in childhood and early adulthood. Aected families tend to develop bone and soft tissue sarcomas, breast cancers, brain tumors, leukemias, and adrenocortical carcinomas. In some kindreds, the genetic abnormality associated with this cancer phenotype is a heterozygous germline mutation in the p53 tumor suppressor gene. Recently, we identi®ed one patient who presented in early childhood with multiple primary cancers and who harbored three germline p53 alterations (R156H and R267Q on the maternal allele and R290H on the paternal allele). To classify the biologic eects of these alterations, functional properties of each of the p53 mutants were examined using in vitro assays of cellular growth suppression and transcriptional activation. Each amino acid substitution conferred partial or complete loss of wild-type p53 function, but the child completed normal embryonic development. This observation has not been previously reported in a human, but is consistent with observations of normal embryogenesis in p53-de®cient mice.

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The entire nucleotide sequence (approximately 20 kbp) spanning the human immunoglobulin IgM (mu) and IgD (delta) heavy chain constant region genes has been determined from DNA of mu-delta producing chronic lymphocytic leukemic B cells. As in the murine IgM + IgD double-producing B cells, no rearrangement has occurred in the C mu-C delta region in the leukemic cells. The C mu locus is highly conserved between mouse and human with the exception of the nucleotide sequence between the C mu 4 and mu M1 exons, which has diverged dramatically. The intergenic sequence between human C mu and C delta is three times larger than the analogous region in the mouse and contains notable features absent from the mouse, including a 443 bp segment that is 96% identical to a 442 bp sequence that occurs just 3' to the heavy chain enhancer, a 366 bp sequence that is directly repeated with 76% homology, and 12 tandem copies of a 35 bp sequence. The human C delta gene contains two additional exons relative to mouse C delta, but shares with the mouse the unique distal location of both secreted and membrane coding segments. Several polymorphisms in the human population have been identified in the intergenic region and in C delta but not in C mu.