A growing family of genes that share homology with the bcl-2 proto-oncogene is involved in the regulation of cell death. Many of these proteins show widespread expression and are expressed in the nervous system in developing and adult organisms. A physiologic role for Bcl-2 and Bcl-x in neuron survival has been shown. In addition, these proteins have been shown to protect neurons from a wide array of toxic insults. In this review, we discuss the Bcl-2 family of proteins with regard to their structure and interactions. We then discuss the role of apoptotic cell death in the development of the nervous system and as a response to neuronal injury. Lastly, we discuss the evidence for a role for these cell death regulators in neuronal death decisions.

Acute lymphoblastic leukemia (ALL) in infants generally shows distinctive biologic features and has a poor prognosis. Cytogenetic studies indicate that many infant leukemias have chromosome 11q23 translocations. Because of these findings and the distinct clinical features of infant leukemia, we investigated 30 cases of infant ALL for molecular defects of 11q23. Fourteen cases had cytogenetic abnormalities of 11q23, and all of them showed 11q23 rearrangements at the molecular level. An additional seven cases also had 11q23 molecular rearrangements, including one with normal cytogenetic analysis. Molecular abnormalities of 11q23 were significantly correlated with adverse prognostic factors, including age under 6 months, hyperleukocytosis, CD10- phenotype, and early treatment failure. Molecular analysis identified a group of infants with germline 11q23 that had a very good treatment outcome with a projected event-free survival of 80% at median follow-up of 46 months compared to 15% in infants with rearranged 11q23 (P < .001). These findings suggest that a high proportion (70%) of infants with ALL have 11q23 rearrangements and that these rearrangements are not always detectable by cytogenetic analysis. The presence of germline 11q23 DNA may identify a subgroup of infant ALL patients with a good outcome using current therapy and a different etiology for their ALL.

Past studies have shown that serum-free cultures of PC12 cells are a useful model system for studying the mechanisms of neuronal death after neurotrophic factor deprivation. These cultures, as well as NGF-deprived cultures of sympathetic neurons, manifest and endonuclease activity that leads to "apoptotic" internucleosomal DNA cleavage. Overexpression of the proto-oncogene bcl-2 blocks apoptotic death in various cell types. To study the actions of this protein in neuronal cells, we derived PC12 cell lines stably transfected with a cDNA encoding human bcl-2. It is reported here that lines expressing high levels of the exogenous bcl-2 protein are protected from both death and apoptotic DNA fragmentation caused by removal of trophic support. However, expression of high levels of exogenous bcl-2 neither mimics nor interferes with promotion of neurite outgrowth by NGF.

As part of an ongoing investigation of the regulation of gene expression in B cell development, we have obtained a genomic DNA clone encoding the human J chain protein. The nucleotide sequence of exons encoding the mature protein defines a 137 amino acid primary sequence similar to that previously determined at the protein level. Probes from the gene have been used to analyze J chain expression in human cell lines corresponding to pre-B and B lymphocytes. J chain RNA was detected in two of six human pre-B cell lines and in 8 of 10 B cell lines expressing various Ig isotypes. The expression of the J chain gene is, thus, not tightly linked to IgM or IgA secretion. Our data do not, however, support the recent suggestion (7) that synthesis of J chain precedes that of mu chain in B lymphocyte differentiation. Because of the presence of nine candidate polyadenylation signals (AATAAA or AATTAAA) downstream of the C-terminal coding block of the J chain gene, the 3' end of the gene could not be determined from sequence data alone. To define the 3' end, J chain RNA from a human B lymphocyte line was used to protect an end-labelled DNA fragment from S1 nuclease digestion. The sequence 40 basepairs 5' of the functional polyadenylation site identified by these S1 experiments is homologous the same region of a previously reported mouse J chain complementary DNA clone.

SummaryThe bcl-2 gene encodes an intracellular, membrane-associated protein that protects immature cortical thymocytes from a wide variety of apoptotic stimuli, including glucocorticoids, radiation, and anti-CD3 treatment. Since cortical thymocytes are the primary target cells for thymic positive and negative selection processes, and since these processes are associated with cell death, we evaluated the role of bcl-2 in T cell development in two ways. In the first approach, transgenic mice expressing high levels of Bcl-2 in cortical thymocytes were mated with H-Y T cell receptor (TCR) transgenic mice, the latter being a well-defined system for the study of positive and negative selection of T cells. We found that the bcl-2 transgene had a dramatic effect on positive selection. This was manifested by a greatly increased production of mature thymocytes that were highly skewed towards the CD4-8 + lineage. The change involving CD4-8 § thymocytes occurred not only in bcl-2 transgenic mice, but was also observed in H-Y TCR/bcl-2 doubly transgenic mice, regardless of whether the H-Y TCR was expressed in the selecting (H-2 b) or nonselecting (1-1-2 d) environments. Furthermore, a large proportion of CD4-8 § thymocytes produced in H-2 b H-Y TCR/bcl-2 doubly transgenic female mice expressed endogenous TCR ot chains rather than the transgenic TCR ot chain. These observations are consistent with the model that high expression of Bcl-2 in cortical thymocytes overrides the normal apoptotic pathway. This then allows the selection of CD4-8 § thymocytes expressing TCRs that are otherwise nonselectable. However, the bcl-2 transgene did not protect CD4 + 8 + thymocytes expressing the male-specific TCR from deletion in male doubly transgenic mice. In the second approach, we determined the level of bcl-2 mRNA expression in populations of thymocytes defined by their CD4/CD8 phenotypes using quantitative reversed transcriptase PCIL techniques. Our results indicate that bcl-2 mRNA was expressed at a high level in immature CD4-8-thymocytes and in mature CD4 § -thymocytes. There is a dramatic downregulation of bcl-2 mRNA in CD4+8 + thymocytes, particularly those expressing a low level of TCR. CD4+8 + thymocytes that upregulated their TCR, likely as a result of receiving positive selection signals, also upregulated bcl-2 mP, NA. This observation suggests that rescue of immature thymocytes from the programmed cell death pathway by positive selection signals is accompanied by the upregulation of bcl-2 mRNA.

A cell line, designated RS4;11, was established from the bone marrow of a patient in relapse with an acute leukemia that was characterized by the t(4;11) chromosomal abnormality. The cell line and the patient's fresh leukemic cells both had the t(4;11)(q21;q23) and an isochromosome for the long arm of No. 7. Morphologically, all cells were lymphoid in appearance. Ultrastructurally and cytochemically, approximately 30% of the cells possessed myeloid features. The cells were strongly positive for terminal deoxynucleotidyl transferase. They were HLA-DR positive and expressed surface antigens characteristic for B lineage cells, including those detected by anti-B4, BA-1, BA-2, and PI153/3. Immunoglobulin gene analysis revealed rearrangements of the heavy chain and kappa chain genes. The cells lacked the common acute lymphoblastic leukemia antigen and antigenic markers characteristic of T lineage cells. The cells reacted with the myeloid antibody 1G10 but not with other myeloid monoclonal antibodies. Treatment with 12-O-tetradecanoyl- phorbol-13-acetate induced a monocyte-like phenotype demonstrated by cytochemical, functional, immunologic, and electron microscopic studies. The expression of markers of both early lymphoid and early myeloid cells represents an unusual phenotype and suggests that RS4;11 represents a cell with dual lineage capabilities. To our knowledge, RS4;11 is the first cell line established from t(4;11)-associated acute leukemia.

S rently found in distinct types of ma1ignan~ies.l.~ Indeed, many of these interchromosomal translocations are essentially pathognomonic for a given tumor. The molecular cloning of chromosomal breakpoints has proven a rich source of novel proto-oncogenes. Experimental approaches indicate that deregulation of these genes represents a primary pathogenic event in the generation of tumors. Determining the normal developmental role of each gene promises to deliver insights into their oncogenic mechanism. The lessons provided from the study of one such oncogene, Bcl-2, argue that understanding the functional roles of genes found at breakpoints will have an enormous impact upon mammalian biology. Bcl-2 was discovered at the t(14; 18)(q32;q21) breakpoint, the cytogenetic hallmark of human follicular l y m p h~m a .~-~ Bcl-2 is novel among proto-oncogenes in that it localizes to mitochondria.8 Moreover, Bcl-2 shows the unique functional role of blocking programmed cell death independent of affecting proliferation.8-10 MULTIPLE ROADS TO NEOPLASIA PROVIDES THE BcI-2 GENE REGULATORS OF PROGRAMMED CELL DEATH: A NEW CATEGORY OF ONCOGENES Malignancies usually possess aberrations in more than a single pathway.13 Either increased proliferation or decreased death might result in an expansion of cell numbers (Fig 1). To date, most of our knowledge concerning oncogenic events has concentrated on mechanisms of

We examined a-, ,(-, and 'y-T cell receptor (TCR) gene activation within acute lymphoblastic leukemias (ALLs) that represent early stages of B and T cell development. We wished to determine if TCR rearrangement and expression was lineage restricted, showed any developmental hierarchy, or could identify new subsets of T cells. Rearrangement of 'y and , ( TCR genes occurred early in development but in no set order, and most T-ALLs (22/ 26) were of sufficient maturity to have rearranged both genes.T-ALLs preferentially rearranged C.y2 versus the Cj1 complex; no preference within thef( locus was apparent. Once rearranged, the TCR continued to be expressed (11/13), whereas the y TCR was rarely expressed (3/14). The a TCR was expressed only in more mature T-ALLs (8/14) that usually displayed T3. The 3A-1 T cell associated antigen appeared earliest in development followed by T111 and T3. Within pre-B cell ALL a higher incidence of lineage spillover was noted for y TCR rearrangements (8/17) than for rearrangements (3/17). This also contrasts with the only occasional rearrangement of immunoglobulin (Ig) heavy chains (3/25) in T-ALL. However, in pre-B ALL the pattern of y TCR usage was distinct from that of T cells, with the C,1 complex utilized more frequently. Almost all ALLs could be classified as pre-B or T cell in type by combining Ig and TCR genes with monoclonal antibodies recognizing surface antigens, although examples of lineage duality were noted. Unique subpopulations of cells were discovered including two genetically uncommitted ALLs that failed to rearrange either Ig or TCR loci. Moreover, two T lymphoblasts were identified that possessed the T3 molecule but failed to express a plus , ( TCR genes. These T-ALLs may represent a fortuitous transformation of T cell subsets with alternative T3-Ti complexes.