Christine M. Morris

Telomeric association of chromosomes in B-cell lymphoid leukemia

SummaryAbout 20% of leukemic bone marrow cells from each of two patients with B-cell lymphoid leukemias showed apparent translocations which appeared to be the result of telomeric association. In one patient, whole chromosomes were associated telomere to telomere in pairs; in the other patient, telomeres of whole chromosomes were associated with breakpoints located close to the proximal or distal ends of the heterochromatic band 1q12. Repeated base sequences, particularly (CA)n sequences, are believed to be the basis of telomere pairing, and likewise repeated base sequences of heterochromatin may explain the association of 1qh and telomeres. Telomeric association may be considered as a potential origin of new stable cytogenetic combinations that have a role in oncogene transposition and tumor etiology.

Nonrandom cytogenetic changes in New Zealand patients with acute myeloid leukemia

Bone marrow clones with abnormal chromosomes were observed in 56% of 66 patients with forms of acute myeloid leukemia [French-American-British (FAB) M1-M6]. Acute myeloblastic leukemia (AML, M1 and M2) was the most common form, and 65% of these patients showed chromosomal abnormalities compared with 41% of patients with acute myelomonocytic leukemia (AMMoL, M4). The recognized nonrandom chromosomal abnormalities found were trisomy 8, monosomy 5 or 7, trisomy 1q, t(6;9), t(8;21), t(15;17), and abnormalities in 17q. There was also a strong involvement of chromosome No. 11: Abnormalities were found in eight patients when their leukemia was diagnosed and in a further three patients during the course of karyotypic evolution. Six of these patients had AMMoL or AMoL. Complex or multiple clones were found in 37% of AML patients at diagnosis. Our AML patients had a reduced frequency of abnormalities in chromosome No. 5 or 7 and an increased frequency of abnormalities in chromosome No. 8 compared with studies reported in other countries (p = 0.01). This difference suggests that in New Zealand AML might be caused by factors different from those operating in more industrialized centers.

Complex chromosomal translocations in the Philadelphia chromosome leukemias: Serial translocations or a concerted genomic rearrangement?☆

Joining of the BCR and ABL genes is an essential feature of the group of human leukemias characterized by the Philadelphia chromosome and there is recent evidence that the human BCR-ABL fusion gene induces leukemia in experimental animals. Joining of these two genes is the result of cytogenetic translocation, usually the t(9;22)(q34;q11), but sometimes of more complex translocations involving one or more chromosomes in addition to chromosomes 9 and 22. The leukemic cells of some patients carry the BCR-ABL fusion gene but have an apparently normal karyotype. Recent studies show that these cells conceal complex chromosome rearrangements. Because the BCR-ABL fusion gene appears to be the result of cytogenetic rearrangement in all cases of these leukemias, the causes and mechanism of chromosome rearrangement will be relevant to the development of leukemia in man. We examine mechanisms of chromosome rearrangement and propose that both simple and complex chromosome translocations result from a single, though sometimes complex, interchange event.

Evidence for the repeated primary non-disjunction of chromosome 21 as a result of premature centromere division (PCD)

SummaryA clinically normal 28-year-old woman had three conceptuses with trisomy 21 and one normal child. She showed minimal cytogenetic evidence of mosaicism: 4% of her blood cells and 6% of skin fibroblasts had trisomy 21. Also, 7% of her blood cells showed aneuploidy of the X chromosome which was associated with premature centromere division (PCD, X); 6% of fibroblasts showed trisomy 18, 10% of fibroblasts showed PCD,21, and 1% PCD, 18. It is unlikely that this woman is a constitutional mosaic for trisomies X, 18, and 21, all at low levels. We suggest that she has a predisposition to irregular centromere separation and that chromosomes X, 18, and 21 are most susceptible to its action.

Familial mutations of the transcription factor RUNX1 (AML1, CBFA2) predispose to acute myeloid leukemia.

RUNX1 (AML1, CBFA2) is mutated in affected members of families with autosomal dominant thrombocytopenia and platelet dense granule storage pool deficiency. Many of those affected, usually by point mutations in one allele, are predisposed to the development of acute myeloid leukemia (AML) in adult life. The RUNX1 protein complexes with core binding factor beta (CBFB) to form a heterodimeric core binding transcription factor (CBF) that regulates many genes important in hematopoiesis. RUNX1 was first identified as the gene on chromosome 21 that is rearranged by the translocation t(8;21)(q22;q22.12) recurrently found in the leukemic cells of patients with AML. In addition to the t(8;21), RUNX1 is rearranged with one of several partner genes on other chromosomes by somatically acquired translocations associated with hematological malignancies. Point mutations of RUNX1 are also found in sporadic leukemias to reinforce the important position of this gene on the multi-step path to leukemia. In animal models, at least one functional copy of RUNX1 is required to effect definitive embryonic hematopoiesis. Cells expressing dominant-negative mutants of RUNX1 are readily immortalized and transformed, and those RUNX1 mutants which retain CBFB binding ability may possess dominant-negative function. However, in some families there is transmitted one mutated allele of RUNX1 with no dominant-negative function, demonstrating that simple haploinsufficiency of RUNX1 predisposes to AML and also causes a generalized hematopoietic stem cell disorder most recognizable as thrombocytopenia.

A novel inherited mutation of the transcription factor RUNX1 causes thrombocytopenia and may predispose to acute myeloid leukaemia

Summary.u2002 The RUNX1 (AML1, CBFA2) gene is a member of the runt transcription factor family, responsible for DNA binding and heterodimerization of other non‐DNA binding transcription factors. RUNX1 plays an important part in regulating haematopoiesis and it is frequently disrupted by illegitimate somatic recombination in both acute myeloid and lymphoblastic leukaemia. Germline mutations of RUNX1 have also recently been described and are dominantly associated with inherited leukaemic conditions. We have identified a unique point mutation of the RUNX1 gene (A107P) in members of a family with autosomal dominant inheritance of thrombocytopenia. One member has developed acute myeloid leukaemia (AML).

Does multisomy of chromosome 1q confer a proliferative advantage in B-cell acute lymphoblastic leukemia?

Two patients fulfilled the clinical and hematologic criteria for B‐cell acute lymphoblastic leukemia: the malignant cells had L3 morphology, bore B‐cell markers, and carried the specific t(8;14) translocation. The leukemic cells of one patient were tetrasomic for lq, and those of the other patient showed several separate cell lines with complete or partial trisomy of lq. In the latter patient it appeared that a break close to the heterochromatin of lq produced an unstable chromosome end which formed associations with the telomeres of at least seven other chromosomes. It is suggested that multisomy of lq gives tumor cells a proliferative advantage and is secondary to the basic neoplastic event.

Essential thrombocythaemia and the Philadelphia chromosome

Six adult patients presented with clinical features of essential thrombocythaemia. Five of the patients, although Ph‐positive, have maintained these features without evidence of leukaemia; in one case for 9 years. A sixth patient developed leukaemic blast crisis following a persistently high platelet count over 4 years. Her cells were Ph‐negative, but hybridization of gene probes to chromosomes in situ and to leukaemic DNA showed that the abl oncogene had moved to the breakpoint cluster region (bcr) on the normal chromosome 22. This patient has the same molecular gene change as occurs in some cases of Ph‐negative chronic myeloid leukaemia (CML) whose leukaemic cells likewise show no evidence of chromosomal translocation. Molecular studies are essential for the correct diagnosis of these patients. The Ph genomic lesion appears to have a range of leukaemic expression which includes thrombocythaemia as well as chronic myeloid leukaemia and acute lymphatic leukaemia.

Fine mapping of an apparently targeted latent human herpesvirus type 6 integration site in chromosome band 17p13.3

An unusually high level of latent HHV‐6 infection has been documented in the peripheral blood and/or bone marrow cells of a small group of patients with predominantly malignant lymphoid disorders, and in at least one healthy individual. We have shown previously in peripheral blood mononuclear cells (PBMCs) of three patients, two with a history of lymphoma and one with multiple sclerosis, a specific target site for latent integration of the full‐length HHV‐6 viral genome on the distal short arm of chromosome 17, in band p13.3. Fluorescence in situ hybridization (FISH) procedures were used to map more precisely the location of the viral integration site in one of those patients, relative to two known oncogenes mapped previously, namely CRK, and the more telomeric ABR oncogene. It is shown that the HHV‐6 integration site is located at least 1,000 kb telomeric of ABR, and is very likely to map close to or within the telomeric sequences of 17p. This finding is significant given that human telomeric‐like repeats flank the terminal ends of the HHV‐6 genome. Cytogenetic studies showed evidence of karyotype instability in the peripheral blood cells infected latently. J. Med. Virol. 58:69–75, 1999.

Localization of a gamma-glutamyl-transferase-related gene family on chromosome 22

A gene family encompassing a minimum of four genes or pseudogenes for gamma-glutamyl transferase (GGT; EC 2.3.2.2) is present on chromosome 22q11. We have previously isolated a cDNA related to GGT but clearly not belonging to its gene family. The chromosomal location of this related gene, GGTLA1, has been determined by both isotopic and fluorescence in situ hybridization to metaphase cells and by Southern blot analysis of somatic cell hybrid DNAs. We show that GGTLA1 is part of a distinct gene family, which has at least four members (GGTLA1, GGTLA2, GGTLA3, GGTLA4). At least two loci are located on chromosome 22 within band q11 and proximal to the chronic myelogenous leukemia (CML) breakpoint in BCR (breakpoint cluster region gene). At least one other member is located more distally between the breakpoints found in Ewings sarcoma and CML. Some of the GGT and GGTLA family members are located on NotI restriction enzyme fragments of a similar size. Combined results indicate that a segment of human chromosome 22q11 has undergone largescale amplification events relatively recently in evolution.