Pathology > Basic Hematology > White Cell Disorders > Leukemia: Laboratory Evaluation - Karyotyping

Leukemia: Laboratory Evaluation - Karyotyping

Cytogenetic chromosomal analysis and DNA ploidy studies are important diagnostic and prognostic factors in evaluating leukemia.

More than half of all leukemias have detectable chromosomal abnormalities by karyotypic analysis. Many chromosomal abnormalities are associated with particular subtypes of leukemia, and appear to have prognostic significance.

The Philadelphia Chromosome Commonly Used Abbreviations

Oncogenes, genes for growth factors and genes for growth factor receptors are often located in the region of the breakpoints. In the example above, the c-abl oncogene at 9q34, coding for the tyrosine protein kinase p145, is translocated to the bcr (breakpoint cluster region) on 22q. This results in a new and abnormal fusion gene, producing bcr/c-abl mRNA. This mRNA synthesizes a potent tyrosine protein kinase p210 unique to chronic myelogenous leukemia. The p210 protein may be instrumental in the genesis of CML.

The following is a partial list of chromosomal abnormalities associated with particular subtypes of leukemia. Abnormalities are detected in 50-60% of de novo acute myeloblastic leukemias and more than 66% of acute lymphoblastic leukemias.

Chromosomal Abnormality

Leukemia

t(8;21)

M2

t(15;17)

M3

inv,del,t(16q)

M4

t(9;11)

M5 especially M5a; M4

t(9;22)

CML

t(1;19)

ALL-preB

t(11;14)

ALL-T

t(8;14), t(2;8), t(8;22)

ALL-Burkitt's (L3)

For a more exhaustive list for the inquisitive student, click here.

****2YMS should know t(8;21), t(15;17), t(8;14), t(9;22), and the 5q- chromosome abnormalities and the diseases associated with each. See course objectives.

 

The frequency of chromosomal abnormalities in patients with a previous myelodysplastic syndrome, history of radiotherapy or history of chemotherapy approaches 100%

The prognosis of acute leukemia arising in these settings is poor.

The following chromosomal abnormalities are poor prognostic indicators:

  • del(7q)
  • del(5q)
  • t(1;3)(p36;q21)*
  • t(1;7)(p11;p11)*
  • t(2;11)(p21;q23)*
  • del(11)(q14)
  • del(13)(q12-q32)

*associated with myelodysplastic syndromes

Other poor prognostic factor are advanced age at the time of diagnosis, and severe leukocytosis.

 

Different chromosomal abnormalities are associated with different characteristic features. For example, Auer rods are often associated with t(8;21) and t(15;17). Eosinophils have been associated with abnormalities of 16q22, and this is a good prognostic indicator.

inv(16), t(8,21) - respond better to high dose cytarabine.
del(5q), del(7q) trisomy 8, and abnormal IIq - poor response to chemotherapy.

Certain translocations are closely associated with ALL:

For example:

B-cell lymphoid malignancies:

The following translocations place the c-myc oncogene next to different immunoglobulin genes
- t(8;14) (q24;q32) - IgH on 14
- t(8;22) (q24;q11) - IgL on 22
- t(2;8) (p12; q24) - IgK on 2


FYI: T-cell lymphoid malignancy - t(10;14) (q24; q11): a protooncogene on chromosome 10 is translocated to a region of TCR alpha-chain (14q11).

 

Practice Question #1

A 31-year-old white male coal miner, comes to you because of a persistent cold that began 3 wks ago. There is no lymphadenopathy or hepatomegaly.

His WBC was 12.0 x10 /L with 45% blasts. A marrow aspirate & biopsy showed 80% blasts. The morphologic and cytochemical studies are equivocal, (ie. nondescript blasts with negative MPO, PAS, and NSE. Immunologic studies) were unavailable.

What is the most likely diagnosis based on the karyotype?

A. Acute myeloid leukemia, M2
B. Acute myeloid leukemia, M3
C. Acute lymphoblastic leukemia, L1
D. Acute lymphoblastic leukemia, L3
E. Chronic myelocytic leukemia

Leukemia: Laboratory Evaluation - DNA Index

The number of chromosomes or ploidy of leukemic cells is especially important in childhood ALL, where the hyperdiploid ALLs have a better prognosis than diploid or hypodiploid ALL.

ALL Ploidy
Median Survival

Hyperdiploid

58 months

Diploid

28 months

Hypodiploid

12 months

Approximately 25-30% of childhood ALLs are hyperdiploid (>50 chromosomes); 15% are hyperdiploid (47-50 chromosomes); 5-10% are hypodiploid (<46 chromosomes); <1% are tetraploid or near tetraploid; and the remainder diploid.

Ploidy is determined by karyotype or by measurement of cellular DNA content. The following discussion will teach you how to determine ploidy by measuring cellular DNA content.


Cells are stained with a fluorescent dye (propidium iodide) that binds to DNA. The fluorescence is measured by flow cytometry. The degree of fluorescence reflects the amount of cellular DNA. A DNA index of >1.15 corresponds to >52 chromosomes or hyperdiploidy. A DNA index of 1.0-1.15 is diploid or hyperdiploid 47-50. A DNA index of <1.0 corresponds with hypodiploidy.

The Cell CycleNormal DNA Histogram

Examples:

In A, the DNA index of a leukemic bone marrow (60% blasts; 40% normal myeloid & erythroid) is abnormally high (>1.15) indicating hyperdiploidy. Note the normal G0/G1 peak from the normal marrow cells present.

In B, marrow cells (>90% blasts) with features of ALL-L3 Burkitt's are hypodiploid (DNA index <1.0).There is a high proliferative fraction, as measured by the cells in S phase. This is characteristic of high grade leukemia/lymphoma.

****2YMS should know ALL-L3 are hypodiploid with a high proliferative fraction characteristic of high grade leukemia/lymphoma. See course objectives.

Practice Question #2

A 5-year-old boy is noted by his mother to have tiny "pin-point" bruises on his legs and arms. You ask his mother to bring him to the office today. He appears pale and frightened. No hepatosplenomegaly, nor lymph-nodes are noted. His WBC is 74.0 x10 /L with 86% blasts. A marrow aspirate shows >90% blasts. Morphologic and cytochemical (negative MPO & NSE; granular PAS positivity) studies are consistent with lymphoblasts. Immunologic studies are c/w a pro-B cell ALL.

What is the prognosis based on the bone marrow DNA index?

A. average <8 week survival
B. average <12 month survival
C. average 24-36 month survival
D. average 60 month survival
E. average 10 year survival

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