Chronic myeloid leukemia (CML) is genetically characterized by the presence of the reciprocal translocation t(9;22)(q34;q11), resulting in a BCR/ABL gene fusion on the derivative chromosome 22 called the Philadelphia (Ph) chromosome. In 2-10% of the cases, this chimeric gene is generated by variant rearrangements, involving 9q34, 22q11, and one or several other genomic regions. All chromosomes have been described as participating in these variants, but there is a marked breakpoint clustering to chromosome bands 1p36, 3p21, 5q13, 6p21, 9q22, 11q13, 12p13, 17p13, 17q21, 17q25, 19q13, 21q22, 22q12, and 22q13. Despite their genetically complex nature, available data indicate that variant rearrangements do not confer any specific phenotypic or prognostic impact as compared to CML with a standard Ph chromosome. In most instances, the t(9;22), or a variant thereof, is the sole chromosomal anomaly during the chronic phase (CP) of the disease, whereas additional genetic changes are demonstrable in 60-80% of cases in blast crisis (BC). The secondary chromosomal aberrations are clearly nonrandom, with the most common chromosomal abnormalities being +8 (34% of cases with additional changes), +Ph (30%), i(17q) (20%), +19 (13%), -Y (8% of males), +21 (7%), +17 (5%), and monosomy 7 (5%). We suggest that all these aberrations, occurring in >5% of CML with secondary changes, should be denoted major route abnormalities. Chromosome segments often involved in structural rearrangements include 1q, 3q21, 3q26, 7p, 9p, 11q23, 12p13, 13q11-14, 17p11, 17q10, 21q22, and 22q10. No clear-cut differences as regards type and prevalence of additional aberrations seem to exist between CML with standard t(9;22) and CML with variants, except for slightly lower frequencies of the most common changes in the latter group. The temporal order of the secondary changes varies, but the preferred pathway appears to start with i(17q), followed by +8 and +Ph, and then +19. Molecular genetic abnormalities preceding, or occurring during, BC include overexpression of the BCR/ABL transcript, upregulation of the EVI1 gene, increased telomerase activity, and mutations of the tumor suppressor genes RB1, TP53, and CDKN2A. The cytogenetic evolution patterns vary significantly in relation to treatment given during CP. For example, +8 is more common after busulfan than hydroxyurea therapy, and the secondary changes seen after interferon-alpha treatment or bone marrow transplantation are often unusual, seemingly random, and occasionally transient. Apart from the strong phenotypic impact of addition of acute myeloid leukemia/myelodysplasia-associated translocations and inversions, such as inv(3)(q21q26), t(3;21)(q26;q22), and t(15;17)(q22;q12-21), in CML BC, only a few significant differences between myeloid and lymphoid BC are discerned, with i(17q) and TP53 mutations being more common in myeloid BC and monosomy 7, hypodiploidy, and CDKN2A deletions being more frequent in lymphoid BC. The prognostic significance of the secondary genetic changes is not uniform, although abnormalities involving chromosome 17, e.g., i(17q), have repeatedly been shown to be ominous. However, the clinical impact of additional cytogenetic and molecular genetic aberrations is most likely modified by the treatment modalities used.

Cytogenetic and Molecular Genetic Evolution of Chronic Myeloid Leukemia

Clustering of aberrations to specific chromosomes in human neoplasms. IV. A survey of 1,871 cases.

Deletions of the derivative chromosome 9 occur at the time of the Philadelphia translocation and provide a powerful and independent prognostic indicator in chronic myeloid leukemia

Loss of p53 sensitizes to antimicrotubule agents in human tumor cells, but little is known about its role during mitosis. We have identified the Polo-like kinase family member serum inducible kinase (Snk/Plk2) as a novel p53 target gene. Snk/Plk2 mutagenesis demonstrated that its kinase activity is negatively regulated by its C terminus. Small interfering RNA (siRNA)-mediated Snk/Plk2 silencing in the presence of the mitotic poisons paclitaxel (Taxol) or nocodazole significantly increased apoptosis, similar to p53 mutations, which confer paclitaxel sensitivity. Furthermore, we have demonstrated that the apoptosis due to silencing of Snk/Plk2 in the face of spindle damage occurs in mitotic cells and not in cells that have progressed to a G1-like state without dividing. Since siRNA directed against Snk/Plk2 promoted death of paclitaxel-treated cells in mitosis, we envision a mitotic checkpoint wherein p53-dependent activation of Snk/Plk2 prevents mitotic catastrophe following spindle damage. Finally, these studies suggest that disruption of Snk/Plk2 may be of therapeutic value in sensitizing paclitaxel-resistant tumors.

Silencing of the novel p53 target gene Snk/Plk2 leads to mitotic catastrophe in paclitaxel (taxol)-exposed cells.

This survey of chromosomal aberrations in human neoplasms comprises 1,871 cases distributed on 15 groups of malignant disease, viz. 4 groups of myeloproliferative disorders with 1,328 cases, 5 groups of lymphoproliferative disorders with 369 cases and 6 groups of solid tumors with 174 cases. The material is presented in 17 tables, in which the variation of each chromosome is listed separately for each geographic region. In the discussion part a number of special problems are looked into. The difference in variation between the individual chromosomes was highly signifcant statistically. Within each tumor, all chromosomes which exceeded the average variation with more than 1 standard deviation were considered selectively involved. In all neoplasms taken together, the chromosomes selectively involved clustered to 15 of the 24 human chromosome types. Other topics taken up for discussion were the following: possible differences in chromosome variation between geographic regions; the specificity of certain marker chromosomes; the hypothesis of 2 essentially different kinds of chromosome aberration in cancer. The phenomena of genic amplification and recurrent translocation often found in cancer require the assumption of unorthodox cytogenetic mechanisms. One main conclusion of the paper is that the aberrations of the selectively involved chromosomes influence cancer development because of the genes they carry.

Clustering of aberrations to specific chromosomes in human neoplasms

One hundred and nineteen unselected and similarly treated patients with Ph1-positive chronic granulocytic leukaemia (CGL) had the precise nature of their chromosome rearrangements producing the Ph1 studied to determine whether this had any clinical relevance. Eighteen (15%) did not have the usual 9/22 translocation and these, by life-table analysis, had a significantly shorter benign phase of their disease than the others (P less than 0.01). It further appeared that possession of a non-standard Ph1 was related to age, in that whereas only 24 patients were over 60 at diagnosis, 9 (33%) had a non-9/22 translocation (P less than 0.01). As the duration of the benign phase seemed to be shorter in those over 60 irrespective of Ph1 type (P less than 0.01), the questions arose whether non-standard PhI chromosomes were simply occurring in older patients or whether they were affecting prognosis independently. Their independent effect was suggested by the 11 patients under 60 with a non-9/22 Ph1 who still had a significantly shorter benign phase than the 84 of similar age with a standard Ph1 (P less than 0.01). It is concluded that the myeloid karyotype can provide prognostic as well as diagnostic information in patients with CGL.

Significance of non-standard Philadelphia chromosomes in chronic granulocytic leukaemia.

During the course of a 30 month study period, 26 patients with typical chronic granulocytic leukaemia (CGL) developed karyotype abnormalities in addition to the Philadelphia (Ph1) chromosome. All cases had received at least 14 months continuous low dose busulphan, and the chromosome changes were found before clinical transformation in six patients and at the time this occurred in 20.



Survival following the discovery of these additional abnormalities was short, with a median of 11 weeks for the whole group. Trisomy 8 was the commonest additional chromosome abnormality, but no one karyotype change was clearly associated with shorter survival than another.



23 of the 26 have died as a direct result of their disease, forming part of a total of 40 deaths from typical CGL encountered during the study period where karyotype analyses were performed during the terminal stages of the disease. Of these 40, only two (5%) patients showed no chromosome abnormalities extra to the Ph1 prior to death. (The 17 non-study patients who died were similar in all respects to the 23 from the study group, but had no prior chromosome studies performed during the chronic phase of the disease.)



It is suggested that an expanding aneuploid or pseudodiploid clone arising from the leukaemic cells during the benign phase of CGL can be used to mark the sometimes ill-defined onset of the malignant phase in all but a small proportion of cases.

Myeloid karyotype and the malignant phase of chronic granulocytic leukaemia.

To define further the clinical importance of cytogenetic analysis in acute lymphoblastic leukaemia (ALL) a prospective study was performed on 139 unselected children. Analyses were considered adequate in 104, of whom 35 were normal and 69 had clonal abnormalities. Abnormalities were categorised according to banded chromosome analysis as well as chromosome count. Karyotypes were correlated with clinical and laboratory features at diagnosis and with survival. Of the successful analyses, thirty five (34%) children had no abnormalities; this group contained an excess of T cell disease. Twenty five (24%) had a "characteristic" hyperdiploid karyotype and as a group had lower presenting white counts, a tendency to CD10, and periodic acid schiff positivity of the blast cells and smaller spleens. None was an infant and only one was over 10 years old. Seven (7%) children with t(9; 22), t(8; 14), or t(4; 11) translocations were grouped together as "specific" translocations. Collectively they had a significantly worse prognosis than the remainder. Nine children developed central nervous system relapse, six of whom had either t(4; 11) or abnormalities of 9p or 19p. A descriptive classification taking into account chromosome bonding pattern is cytogenetically more appropriate and may be more clinically useful than grouping children simply by chromosome number. As knowledge and techniques improve, the classification of cytogenetic abnormalities in ALL will need to be kept under frequent review.

https://jcp.bmj.com/content/jclinpath/44/1/48.full.pdf

Chromosomes in childhood acute lymphoblastic leukaemia: karyotypic patterns in disease subtypes.

Value of cytogenetic studies in prediction of acute phase CML.

Summary Chromosomal abnormalities in three cases of chronic granulocytic leukaemia are presented. The relative importance of ‘specific’ chromosomal abnormalities, additional to the Ph1 in the karyotypic evolution of chronic granulocytic leukaemia is discussed. A new abnormal metacentric chromosome is described.

A.M. Potter

Papers

Significance of non-standard Philadelphia chromosomes in chronic granulocytic leukaemia.

Myeloid karyotype and the malignant phase of chronic granulocytic leukaemia.

Chromosomes in childhood acute lymphoblastic leukaemia: karyotypic patterns in disease subtypes.

Value of cytogenetic studies in prediction of acute phase CML.

Karyotypic Abnormalities in Transformed Chronic Granulocytic Leukaemia