Characterization of the immunoglobulin light chain variable region gene expressed in multiple myeloma

We studied the organization, diversification and clinical significance of the immunoglobulin light chain (IgL) variable region genes expressed in 17 kappa-chain and 16 lambda-chain producing multiple myeloma (MM) samples. The V genes from 31 MM samples had over 84.9% homology to the known germline Vkappa/lambda genes, whereas one Vkappa and one Vlambda gene had only 75.5% and 65.9% homology, respectively. While all five Jkappa segments were equally used, only Jlambda-1 or Jlambda-2/3 was used among seven Jlambda segments. N nucleotide addition was found at two Vkappa-Jkappa and five Vlambda-Jlambda junctions. The lambda-chain complementarity determining region (CDR)-3 was longer and more variable than the kappa-chain CDR-3 mainly due to junctional flexibility of Vlambda and Jlambda segments. Somatic mutations were more frequent in the Jlambda than the Jkappa segments, and were distributed in the CDR-3 as well as the frame work region (FWR)-4. Those of the Jkappa segments, however, were limited to FWR-4. In FWR-4, replacement mutations were clustered at codon 106 of kappa-chain and 103 of lambda-chain. Thus nucleotide mutation or conservation was dependent on position, indicating a structural necessity of IgL for the development of myeloma cells in addition to a non-random distribution of mutations. There was no characteristic IgL sequence according to the isotype of M-protein, clinical stage or renal complication.

Mutant AF-2 domain of PML-RARalpha in retinoic acid-resistant NB4 cells: differentiation induced by RA is triggered directly through PML-RARalpha and its down-regulation in acute promyelocytic leukemia

To study the molecular mechanism of the differentiation induced by retinoic acid (RA) in acute promyelocytic leukemia (APL), we established a new RA-resistant NB4 subline, NB4/RA. The NB4/RA cells were neither differentiated by a single or a combination of RA isoforms, nor by the addition of clotrimazole (P450-inhibitor) or interferon gamma. However, the combination of RA and 8-(4-chlorophenylthio) adenosine cyclic 3',5'-monophosphate (a cAMP analog, 8-CPT-cAMP) induced differentiation. Immunostaining of NB4/RA cells using anti-PML antibody showed a microgranular pattern which was not restored even by the combination of RA and 8-CPT-cAMP, whereas the microgranular pattern in NB4 cells was rapidly restored to the normal speckled pattern by RA. Western blot analysis revealed that RA alone or the combination with 8-CPT-cAMP did not down-regulate PML-RARalpha in NB4/RA cells, which was in contrast to NB4 cells. The PML-RARalpha fusion gene and transcript in NB4/RA cells were conserved as well as the RARalpha gene and transcripts. Sequence analysis of the PML-RARalpha transcript in NB4/RA cells indicated a Pro (CCG) to Leu (CTG) mutation at codon 900 (type L) in AF-2 domain, while the RARalpha transcript had a normal sequence. These data suggest that differentiation of APL by RA is triggered directly through PML-RARalpha, and is associated with its degradation. Furthermore, there might be another mechanism of differentiation which does not require the down-regulation of PML-RARalpha and the restoration of the PML-staining pattern.

Internal tandem duplication of the FLT3 gene is preferentially seen in acute myeloid leukemia and myelodysplastic syndrome among various hematological malignancies. A study on a large series of patients and cell lines

In this study, we examined a large number of patients to clarify the distribution and frequency of a recently described FLT3 tandem duplication among hematopoietic malignancies, including 112 acute myelocytic leukemia (AML), 55 acute lymphoblastic leukemia (ALL), 37 myelodysplastic syndrome (MDS), 20 chronic myelogenous leukemia (CML), 30 non-Hodgkin's lymphoma (NHL), 14 adult T cell leukemia, 15 chronic lymphocytic leukemia (CLL) and 38 multiple myeloma (MM). We also evaluated 71 cell lines derived from 11 AML, 31 ALL, two hairy cell leukemia, three acute unclassified leukemia, 10 CML, 12 NHL including six Burkitt's lymphoma, and two MM. Using genomic PCR of exon 11 coding for the juxtamembrane (JM) domain and first amino acids of the 5'-tyrosine kinase (TK) domain, this length mutation was found only in AML (22/112, 20%) and MDS (1/37). According to the FAB subclassification, they were 5/18 (28%) of M1, 4/29 (14%) of M2, 3/17 (18%) of M3, 6/24 (25%) of M4, 4/20 (20%) of M5 and 1/9 of refractory anemia with excess of blast in transformation. In the various cell lines examined, this abnormality was determined in only one derived from AML and never found in other hematological malignancies. The sequence analysis of the abnormal PCR products revealed that 23 of 24 showed internal tandem duplication with or without insertion of nucleotides. In one AML, insertion and deletion without duplication was determined. All 24 lengthened sequences were in-frame. Duplication takes place in the sequence coding for the JM domain and leaves the TK domain intact. In conclusion, we emphasize that the length mutation of FLT3 at JM/TK-I domains were restricted to AML and MDS. Since all these mutations resulted in in-frame, this abnormality might function for the proliferation of leukemic cells.

Internal tandem duplication of FLT3 associated with leukocytosis in acute promyelocytic leukemia. Leukemia Study Group of the Ministry of Health and Welfare (Kohseisho)

FLT3 is a member of receptor tyrosine kinases expressed in leukemia cells, as well as in hematopoietic stem cells. Recently, a somatic alteration of the FLT3 gene was found in acute myeloid leukemia, as an internal tandem duplication (FLT3/ITD) which caused elongation of the juxtamembrane (JM) domain of FLT3. Here we characterized the FLT3/ITD and investigated its clinical significance in acute promyelocytic leukemia (APL). Seventy-four newly diagnosed patients with APL, who were treated with the same protocol in a multi-institutional study, were studied for the FLT3/ITD. Genomic and message sequences of the FLT3 gene were amplified by means of polymerase chain reaction (PCR), and elongated PCR products were sequenced. Fifteen patients (20.3%) had FLT3/ITD, all of which were transcribed in frame. Location of the duplicated fragments (six to 30 amino acids) varied from patient to patient. However, they always contained either Y591 or Y599, but the tyrosine kinase domain was not significantly affected. This finding implied that signal transduction of FLT3 is amplified by the duplication. Clinically, the presence of FLT3/ITD was related to high peripheral white blood cell counts as well as peripheral leukemia cell counts (P < 0.0001), high LDH level (P = 0.04), and low fibrinogen concentration (P = 0.04). These data suggest that FLT3/ITD plays a significant role in progression of APL.

Comparison of the immunoglobulin gene transcripts between immature B lineage acute lymphoblastic leukemia and the normal phenotypic counterparts in the bone marrow

Immature B lineage acute lymphoblastic leukemia (ALL) is divided into two subtypes, 'pre-B' and 'early pre-B' ALL, by the presence or absence of cytoplasmic immunoglobulin (cIg). To study their clonal origin, we compared mu-chain transcripts in six cIg+ and eight cIg- ALL samples (CD10+/- CD19+ surface Ig-) with those in the normal phenotypic counterparts (CD10+ CD19+ surface Ig-) sorted from the bone marrow (BM). Northern blot analysis showed that the cIg+ ALL samples expressed greater amounts of mu-chain transcripts than the cIg- ALL samples. In the ALL samples and their counterparts, sequence analysis of mu-chain transcripts revealed infrequent somatic mutations of the V(H) genes and the similar usage of D and J(H) gene segments, but the length of complementarity determining region (CDR)-3 in the ALL samples was longer than that in the counterparts (50.0 +/- 15.5 vs 40.8 +/- 12.7 bp, P = 0.01). The mu-chain transcripts in the six cIg+ ALL samples and the counterparts (119/120 clones) had productive sequences, whereas those in the eight cIg- ALL samples had nonsense codons and/or frame shifts in their CDR-3. Our data suggest that a phenotype of ALL, 'pre-B' or 'early pre-B', is associated with V(H)-D-J(H) gene recombinatorial events, and that the CD10+ CD19+ surface Ig- population in the BM is not simply the cellular origin of ALL.

Unusual feature of the T-cell receptor genes in T-lineage acute lymphoblastic leukemia

We characterized the T-cell receptor (TCR) gene rearrangements and sequences in 15 T-lineage acute lymphoblastic leukemia (T-ALL) and seven adult T-cell leukemia (ATL) samples. Southern blot analysis showed that neither of the two TCR delta alleles was deleted in two T-ALL samples, suggesting that the TCR alpha loci have a germ line configuration. The TCR alpha and beta sequences were cloned and sequenced by reverse transcriptase-inverse polymerase chain reaction. Two T-ALL samples had a long complementarity determining region (CDR), three of the alpha chain and the other two T-ALL samples had long CDR3 of the beta chain, compared with normal peripheral blood lymphocytes (PBL). Thus, a total of six T-ALL samples had unusual TCR gene structure, which was unrelated to the immunophenotype. On the other hand, CDR3 length in ATL samples was similar to normal PBL. These data suggest that T-ALL is derived from an immature T-cell repertoire which undergoes TCR gene rearrangement or has not been negatively selected.

New retinoids and arsenic compounds for the treatment of refractory acute promyelocytic leukemia: clinical and basic studies for the next generation

All-trans retinoic acid (ATRA) is a potent differentiation drug for acute promyelocytic leukemia (APL) and is now incorporated into first-line therapy. However, ATRA resistance has become a major clinical problem. This limitation has prompted the development of alternative agents with desirable pharmacologic properties. We describe (1) our recent clinical trial using the new synthetic retinoid Am80 to overcome acquired resistance to ATRA and (2) basic in vitro effects of arsenic trioxide, a possible alternative to ATRA, on APL cells. A total of 19 APL patients who had relapsed after ATRA-induced complete remissions (CRs) received 6 mg/m2 Am80 p.o. daily until CR; 11 (58%) patients achieved a CR between days 20 and 58 (median day 37). The in vitro sensitivity to Am80, based on PML immunostaining, correlated well with the clinical effect in all patients tested. All three patients whose blasts were sensitive to Am80 in vitro despite a poor response to ATRA achieved CRs. Thus, Am80 might be an effective compound for the treatment of refractory APL and is a promising alternative retinoid. Since arsenic compounds have reportedly induced CRs in APL patients in China, we studied the in vitro effect of arsenic and other metal ions on myeloid leukemia cell lines. The effects of arsenic were limited mainly to APL cells, and the arsenic concentration was critical for the APL cell line NB4: 1 microM As3+ induced time-dependent apoptosis, whereas 0.1 microM As3+ allowed partial NB4 cell differentiation. Arsenic trioxide was equally effective when used on ATRA-resistant NB4 cells. Among the clinical leukemia samples tested, the in vitro cytotoxic effects of As3+ were observed selectively in APL cells, regardless of their ATRA sensitivity. These data suggest that APL cells are sensitive to As3+ and that As3+ acts on APL cells via a different pathway to ATRA.

Comparison of the immunoglobulin heavy-chain complementarity determining region-3 structure among the DNA sequences and the mu- and gamma-transcripts in human B-lineage cells

To study the recombinational significance of the (immunoglobulin heavy) IgH chain gene in human B-cell development, we compared the complementarity determining region (CDR)-3 sequences of the DNA and the mu-transcripts from human normal pre-B cells and mature B cells, and the gamma-transcripts from bone marrow cells. The CDR-3 sequences were longer in the DNA than in the mu- and gamma-transcripts, and this was independent of whether or not the rearrangement was productive. The DLR family genes were less frequently used in the mu- and gamma-transcripts. When translated into amino acids, all CDR-3 sequences from the mu- and gamma-transcripts were productive, although 26.2% of the DNA sequences had stop codons in the D element and/or frameshifts of the JH gene segments. The CDR-3 of the productive DNA sequences in pre-B cells frequently (26.6%) contained at least three continuous hydrophobic amino acids, which were mainly coded by the DLR and DXP family genes at the third reading frame. However, such motifs were rate in the mu-transcripts of pre-B (7.7%) and mature B cells (3.9%), and in the gamma-transcripts of bone marrow cells (1.1%) as well as in the DNA of mature B cells (10.4%). These findings suggested that the length and/or hydrophobicity of the IgH CDR-3 might play a role in the selection mechanisms of B-cell development.

Comparable gene structure of the immunoglobulin heavy chain variable region between multiple myeloma and normal bone marrow lymphocytes

To characterize multiple myeloma (MM) from the viewpoint of the immunoglobulin (Ig) gene structure, we compared the transcripts of the Ig heavy chain variable region from 23 MM samples with 221 clones of the gamma, alpha and mu chain transcripts amplified by the reverse transcriptase-polymerase chain reaction (RT-PCR) from normal bone marrow (BM) cells. The usage of D and JH gene segments and the length of the N regions were the same between MM and the normal gamma, alpha and mu transcripts. Compared with the known germline VH genes, the frame work regions (FWRs) and complementarity determining regions (CDRs) of the VH segments mutated at rates of 8.3 +/- 4.7% and 15.9 +/- 7.7%, respectively, which were the same as the normal gamma and alpha (gamma/alpha) transcripts and higher than the normal mu transcripts. The replacement/silent (R/S) ratios of the mutations in FWRs and CDRs were 1.9 +/- 1.3 and 2.7 +/- 1.8, respectively, which were the same as the gamma/alpha and mu transcripts. On the other hand, we detected the clone-specific mu transcripts by RT-PCR using the primers corresponding to the each respective CDR-III and the constant region of the mu chain in three of the studied six MM samples, suggesting the involvement of a pre-switched B cell in some cases of MM. These findings suggested that the cellular origin of MM is heterogeneous, but that the Ig structure in MM reflects normal B cell maturation to plasma cell through mutation and selection.

Different T-cell receptor repertoires between lesions and peripheral blood in acute graft-versus-host disease after allogeneic bone marrow transplantation

From the viewpoint of T-cell receptor (TCR) repertoire, we studied the role of T cells in acute graft-versus-host disease (GVHD) after allogeneic bone marrow transplantation (allo-BMT) from an HLA-identical sibling. By means of inverse polymerase chain reaction method and DNA sequencing, we analyzed TCR-alpha and -beta transcripts from GVHD lesions and peripheral blood (PB) in a patient with typical GVHD together with PB from donor. At the initial onset of GVHD, V alpha-7 and -19 subfamilies were oligoclonally expanded in the PB compared with those in the oral mucosal lesions. At the second onset, V alpha-2, and V beta-6 subfamilies were more frequently detected in the cutaneous lesion than in the PB. Some TCR transcripts were recurrently found either in the mucosal or cutaneous lesions (or in both) and not in the PB. Furthermore, some of recurrent TCR transcripts in the lesions shared V gene segments and common motifs of complementarity determining region-3. These findings suggested that T cells infiltrating the GVHD lesions recognized a limited kind of antigens presented by patient's tissues with GVHD, and that T-cell repertoire in the GVHD lesions was different from that in the PB.

Detection of minimal residual disease in patients with childhood common acute lymphoblastic leukemia after autologous bone marrow transplantation with ex vivo purging and systemic IL-2 infusion: unsuccessful prediction of subsequent relapse

We sequentially analyzed minimal residual disease (MRD) in 7 children with common acute lymphoblastic leukemia (cALL) after autologous bone marrow transplantation (ABMT) with ex vivo purging followed by systemic interleukin-2 infusion. After ABMT, 3 of the 7 patients remained in complete remission (CR) for more than 1 year, and 4 subsequently relapsed. MRD was estimated by polymerase chain reaction amplification to detect the leukemia clone-specific immunoglobulin heavy chain third complementarity determining region (IgH CDR-III). The IgH CDR-III sequences from the relapsed patients were identical with those determined at each respective initial diagnosis. In 2 patients, the levels of MRD were 10(-2) and 10(-5) in the harvested bone marrow (BM) cells, and even after purging the levels were 10(-4) and 10(-5) cells, respectively. One of the 2 patients relapsed 3 months after ABMT, while the other remained in CR for 33 months after ABMT. Among the 4 patients who subsequently relapsed after ABMT, MRD was not detected in the BM samples even 1 month before relapse. Our results suggest that PCR-negativity does not necessarily indicate a lower risk of subsequent relapse. Detection of MRD tends to favor the assessment of the therapeutic effects rather than prediction of relapse.

Comparable profiles of the immunoglobulin heavy chain complementarity determining region (CDR)-3 in CD5+ and CD5- human cord blood B lymphocytes

It has been suggested that CD5+ B cells are closely related to the pathogenesis of autoimmune disease or chronic lymphocytic leukaemia, whereas the origin and physiological role of CD5+ B cells remain controversial. To study the molecular differences between CD5+ and CD5- B cells in terms of immunoglobulin gene structure, we sorted both subsets from new-born cord blood and analysed the complementarity determining region (CDR)-3 profiles of the immunoglobulin heavy chain (IgH) gene. The CDR-3 sequences from both CD5+ and CD5- B cells represented the same incidence and length of N-region, and the same usage of D and JH segments. When translated into amino acids, 24 of 32 clones (75%) and 25 of 37 clones (65.8%) from their respective subset were productive, and the composition of the deduced amino acids were similar between both subsets. These data suggest that CD5+ B cells are not a distinct lineage presenting a biased immunoglobulin repertoire in B cells.

Prognostic significance of the RT-PCR assay of PML-RARA transcripts in acute promyelocytic leukemia. The Leukemia Study Group of the Ministry of Health and Welfare (Kouseisho)

Minimal residual disease (MRD) was prospectively monitored at the 10(-5) level by the reverse transcriptase-polymerase chain reaction (RT-PCR) of PML-retinoic acid receptor alpha (RARA) transcripts from 27 acute promyelocytic leukemia (APL) patients who achieved complete remission (CR) with all-trans retinoic acid and chemotherapy (previously untreated patients, 15; refractory to chemotherapy or relapsed, 12). The RNA quality from bone marrow cells was firstly assessed by gel electrophoresis to avoid false negativity because of the fragility of the APL cells and the PML-RARA transcripts. In 12 of 15 untreated patients, RT-PCR became negative during consolidation and intensification therapy 4-16 months after the initiation of therapy, whereas it remained positive in nine of 12 refractory patients. At the end of therapy, RT-PCR was negative in 14 patients and positive in 13 patients. The former patients remained in CR at median follow-up of 9 months after the end of therapy. In the latter, however, 10 patients relapsed at a median of 5 months after the end of therapy. These results suggest that the RT-PCR assay can evaluate the quality of CR in APL and predict subsequent relapse.

[Characterization of the immunoglobulin heavy and light chain variable region sequences from human multiple myeloma cells]

Although multiple myeloma (MM) morphologically represents the terminal B cell differentiated stage as a plasma cell, it remains controversy that MM cells have arisen from a B cell at which stage of B cell development. The immunoglobulin heavy chain (IgH) variable region genes can serve as markers in clonal analysis because unique combinations of VH, D and JH gene elements and as a genealogical record of clonal selection and expansion, and molecular diversification during maturation of the immune response. We analyzed the IgH variable region sequences from human MM cells. Furthermore, to compare with normal bone marrow (BM) plasma cell repertoire, we determined the IgH variable region sequences of PCR amplified cDNA libraries consisting of C mu, C gamma, and C alpha transcripts from human normal BM cells. The VH segments in MM cells, except for Bence-Jones protein (BJP) type, were extensively mutated, and the characteristic structure of the IgH variable region essentially reflect those from normal BM C gamma and C alpha transcripts. Although the replacement/silent (R/S) mutation ration in MM cells was under the value which reflect antigen selected substitutions, such a lower value was also observed in normal BM C gamma and C alpha transcripts, suggesting that the transformation of MM dose not necessarily take place before the antigen-dependent selection, rather can after clonal expansion. However, BJP type MM might arise from an earlier stage of B cell development than the other types because of lower incidence of somatic mutation in their VH segments which was the same as normal BM C mu transcripts.

Continuing immunoglobulin heavy chain gene rearrangements in chronic myeloid leukemia with recurrent B-lymphoid blast crises after bone marrow transplantation

We sequentially analyzed the immunoglobulin heavy chain variable (IgH V) region gene of leukemia cells obtained from a chronic myeloid leukemia (CML) patient who had three episodes of B-lymphoid crisis after bone marrow transplantation. Southern blot analysis using the JH probe showed different rearranged bands at each crisis, although the same rearranged bands of the BCR gene were observed. We amplified and sequenced the IgH V region gene of the leukemia cells by reverse transcriptase polymerase chain reaction (RT-PCR) using the primers corresponding to the consensus 5'VH and mu constant regions. The dominant leukemia clone at each crisis had a unique VH-D-JH rearrangement; VH4A (V79)-DLR2-J5 (clone-1), VH4B (DP70)-DK4-J6 (clone-2) and VH4A (V79)-DN4-J6 (clone-3) at the first, second and third crises, respectively. Further analysis by PCR amplification using the consensus 5'VH and clone-specific primers revealed that clone-1 underwent VH4-->VH3 replacement at the second crisis, and that clone-3 was already in existence at the first crisis. Moreover, the DN4-J6 joining clone, in which the sequence was the same as that of clone-3, was identified at the first and third crises by PCR amplification using primers corresponding to the region upstream of the DN4 segment and DN4-J6 boundary of clone-3. These observations suggest that multiple clones were generated from the progenitor cells of blast crisis, which were transformed at a very early stage of B-lymphocyte ontogeny, by continuing rearrangement mechanisms of the IgH genes, and that the dominant clone at each crisis was undergoing change.

Analysis of the joining sequences of the t(15;17) translocation in human acute promyelocytic leukemia: sequence non-specific recombination between the PML and RARA genes within identical short stretches

Molecular analysis of the t(15;17) translocation in 70 patients with acute promyelocytic leukemia (APL) confirmed that the breakpoints of chromosome 15 were located in two regions of the promyelocytic leukemia (PML) gene, mainly introns 3 and 6, whereas the breakpoints of chromosome 17 were consistently in intron 2 of the retinoic acid receptor alpha (RARA) gene. To study the reason for the clustering of the breakpoints and the underlying mechanism of the chromosomal translocation, we characterized the joining sequences of der(15) and der (17) by polymerase chain reaction in samples from eight patients with APL. There was no cluster of the breakpoints within the introns, and no consensus sequence-motif was found around them. One or nine extra nucleotides were inserted into two joining sites. There were identical stretches of one to seven nucleotides between the PML and RARA genes in the majority of the joining sequences. These data provide a potential model of the t(15;17) translocation: random DNA double strand cleavage, modification of DNA ends by enzymes including terminal deoxynucleotidyl transferase, and single strand base-pairing within identical short stretches. Furthermore, APL develops only when the PML and RARA genes are rearranged, within restricted genomic regions and a functional PML-RARA chimeric product is produced, and this might lead to a clustering of the breakpoints.

Characterization of immunoglobulin heavy chain gene rearrangements in chronic myeloid leukemia with recurrent B-lymphoid blast crisis following bone marrow transplantation

We sequentially analyzed the immunoglobulin heavy chain (IgH) variable region gene of leukemia cells obtained from a chronic myeloid leukemia (CML) patient who had three episodes of B-lymphoid crisis after bone marrow transplantation. Southern blots using the JH probe showed a single rearranged band which differed at each crisis, although the rearranged bands of the BCR gene were the same at each crisis. The IgH variable region sequences of the leukemia cells at each crisis were different. These observations suggested that multiple clones were generated from the progenitor cells of the blast crisis, which were transformed at a very early stage of B-lymphocyte ontogeny.

Minimal residual disease status in pre-B acute lymphoblastic leukemia patients after chemotherapy and bone marrow transplantation: assessment of the anti-leukemic effects of chemotherapy and BMT

We prospectively analyzed minimal residual disease (MRD) in four patients with B-cell precursor acute lymphoblastic leukemia who had been in complete remission for more than one year after chemotherapy and allogenic or autologous bone marrow transplantation (BMT). MRD was quantitatively estimated using polymerase chain reaction amplification to detect the complementarity-determining region III of the immunoglobulin heavy chain gene at limiting dilution DNA samples. Our study showed that remission induction chemotherapy reduced at most 2-logs of leukemia cells, and that subsequent consolidation chemotherapy induced further reduction of leukemia cells. In two cases, 10(-5) levels of MRD were detected two months after BMT. However, no MRD was detected four months after BMT. We also showed the effectiveness of ex vivo purging with anti-CALLA monoclonal antibodies which eliminated at least 2-logs of leukemia cells in autologous BMT. Our results suggest that this detection system is useful for assessing the reduction of the original leukemia clone, and that the presence of MRD within three months after BMT is not related to clinical outcome.

Clonal analysis of multiple point mutations in the N-ras gene in patients with acute myeloid leukemia

We have screened mutations of the N-ras gene at codons 12, 13, and 61 in leukemia cells obtained from 100 patients with acute myeloid leukemia (AML), and found mutated N-ras alleles in 9 patients. We further analyzed the polyclonality of multiple N-ras gene mutations in 4 AML patients. One patient, who had the monoclonal karyotype, t(11;17), had two types of double missense mutations at codons 13 and 61 in the same allele. Each of the remaining three patients, one of whom had t(15;17) with a monoclonal rearrangement of the retinoic acid receptor alpha and PML genes, carried two missense mutations in a relatively small population of leukemia cells. We have demonstrated that multiple clonality of the N-ras gene is occasionally observed in leukemia with a monoclonal karyotype. These findings indicate that the N-ras mutations may not always be characterized simply by an accumulative process and that the activated N-ras gene alone is not sufficient to cause leukemia.

Characterization of the immunoglobulin heavy chain complementarity determining region (CDR)-III sequences from human B cell precursor acute lymphoblastic leukemia cells

Sequence analysis of the immunoglobulin heavy chain complementarity determining region (CDR)-III of B-lineage cells at various stages has provided important insights concerning B cell maturation and selection. Knowledge of human CDR-III sequences has been relatively limited compared with that of the murine system. We analyzed the CDR-III sequences of B cell precursor acute lymphoblastic leukemia (pre-B ALL) cells in 23 newly diagnosed and 10 relapsed patients, in order to elucidate the organization of CDR-III in B cell precursors. We found a very low frequency of somatic mutations in D and JH regions, preferential use of DLR, DXP, DHQ52, and DN elements, and of 3' side JH segments, and no predominant usage of D coding frames. Unusual joinings such as VH-D-D-JH and VH-JH were observed in three, and one sequences, respectively. We compared the CDR-III sequences derived from 10 patients between diagnosis and relapse. Two of them had three spots of mutated nucleotides at relapse, all of which were found in the N region near the D segments. Our data showed the possibility of somatic mutation at relapse, in addition to developmentally regulated rearrangement of the immunoglobulin gene at the stage of B cell precursors.