Philadelphia chromosome

The Philadelphia chromosome, also called the Philadelphia translocation, is a genetic abnormality associated with chronic myeloid leukemia (CML) and acute lymphoid leukemia in children. The Philadelphia chromosome is the result of an abnormality resulting from the reciprocal translocation between chromosomes 9 and 22. At the cytogenetic level, a larger than normal chromosome 9 and a smaller chromosome 22 or Philadelphia chromosome are observed.

History

The phenomenon was discovered and described in 1960 by Philadelphia scientists Peter Nowell of the University of Pennsylvania School of Medicine and David Hungerford of the Fox Chase Cancer Center, giving it the name from the city where both research centers are located.

In 1973, Janet Rowley at the University of Chicago identified genetic translocation as the source of the abnormality.

Pathophysiology

Translocation to the Philadelphia chromosome

This abnormality affects chromosomes 9 and 22. 90 percent of patients with chronic myeloid leukemia (the first malignant disease in which it was possible to demonstrate an acquired genetic abnormality) present this abnormality, while the rest of the patients suffer cryptic translocations invisible to G-band preparations or other translocations that affect one or more other chromosomes in the same way as chromosomes 9 and 22. Cases of the Philadelphia chromosome are also found in patients with acute lymphoblastic leukemia (25 to 30 percent in adults and 2 to 10 percent in children), and occasionally, in cases of acute myelocytic leukemia (AML).

The genetic defect of the Philadelphia chromosome consists of a phenomenon known as translocation, that is, a chromosomal break occurs in two specific regions of chromosome 9 and 22 (translocation 9-22), exchanging their positions. Specifically, the breakpoint occurs in the ABL gene (Abelson) of chromosome 9 (region q34) and in the BCR gene (Breakpoint Cluster Region, in English) of chromosome 22 (region q11), giving rise to to an altered chromosome 9 and an also altered chromosome 22 (Philadelphia chromosome), but characterized by the fusion of these two genes (BCR-ABL), which encode a chimeric protein. The ABL gene takes its name from "Abelson", the name of a leukemia-causing virus that precursors of a protein similar to the one produced by this gene.

The result of this translocation is the production of a protein of p230, p210, or p190 weight (p is a measure of weight for cellular proteins in atomic mass units). The ABL gene in its normal situation expresses a tyrosine kinase protein, when the fusion with the BCR gene occurs, said tyrosine kinase activity continues to be maintained. Although the BCR gene codes for a serine/threonine protein kinase enzyme, the really important activity for the development of the disease is the altered tyrosine kinase function, since it has been shown to play an important role in the genesis of the leukemic process.

The protein resulting from the BCR-ABL fusion interacts with the Interleukin 3beta(c) receptor subunit. BCR-ABL transcription remains continuously active, without needing to be activated by other messenger proteins. This continuous transcription leads to an uncontrolled alteration of proteins and enzymes that govern the regularity of the cell division cycle and consequently DNA repair is inhibited, causing genome instability and being a potential factor triggering the "chain crisis" of leukemia. chronic myeloid, with a high mortality rate.

Nomenclature

The Philadelphia chromosome is designated the Ph (or Ph') chromosome. The exact breakpoints were established by Prakash and Yunis in 1984, and are located at q34.1 on the chromosome 9 and q11.2 on chromosome 22. Using the correct ISCN nomenclature (International System for Human Cytogenetic Nomenclature) it would be 45,XY,t(9;22)(q34.1;q11.2).

Diagnostic techniques

The oncogenes (malignant genes responsible for the transformation of a normal cell to a malignant cell) are mainly located at the chromosomal break point, which are involved in the generation of neoplasms, in the case of this disease.

Fish or Fluorescent in Situ Hybridization

FISH is a technology that uses DNA probes labeled with a fluorophore to detect or confirm gene or chromosomal abnormalities. First, the DNA sample (metaphase chromosomes or interphase nuclei) is denatured, a process that separates the complementary strands of the double-stranded structure. The probe of interest is added to the already denatured sample, which will associate with the DNA of the sample at the target site, during the hybridization process. The probe is covalently linked (labelled) with a fluorophore, which emits a signal observable under a fluorescence microscope.

The genes that we consider to be targets are the BCR and ABL genes, so the BCR-ABL oncogene will be present if under a microscope we can observe two luminescent points together, which corresponds to the genetic exchange that occurs in the formation of the Ph chromosome. It is It is important to understand that performing FISH with metaphase nuclei offers us more information and more specific results than with interphase nuclei.

Southern Blot

This technique is very similar to the previous one, but in this case the labeled probe targets the fused ABL-BCR gene. Southern blot will use in situ hybridization of said probe with metaphase chromosomes from patients with Chronic Myeloid Leukemia. In short, if hybridization occurs, it is proof that the fused gene is present, so we can say that it has the Philadelphia chromosome and by analyzing the symptoms it will be possible to diagnose the disease or not.

PCR

In this technique you can start from RNA (you transform it to cDNA thanks to reverse transcriptase) or DNA, the characteristic is the primers or oligonucleotides used to produce the amplification of your target genes, in this case the ABL genes and BCR.

Treatment

In the late 1990s, STI-571 (Imatinib, Gleevec) was identified by the pharmaceutical company Novartis as a broad-spectrum, high-performance tyrosine kinase inhibitor. Imatinib is a highly effective therapy for chronic myeloid leukemia, however there is a high relapse rate among patients in advanced stages due to the development of mutations in the quinada domain of ABL that cause resistance to current drugs. Imatinib is a 2-phenyl amino pyrimidine derivative and functions as a specific inhibitor of several tyrosine kinase enzymes. This occupies the active site of the kinase, which leads to a decrease in activity, that is, Imatinib binds near the ATP binding site from where it picks up phosphates to pass it on to tyrosine residues of different substrates. This fact explains why mutations in the BCR-ABL gene can cause resistance to treatment.

Subsequent medical tests conducted by Dr. Brian J. Druker, in collaboration with Dr. Charles Sawyers and Dr. Moshe Talpaz, demonstrated that STI-571 inhibited the proliferation of BCR-ABL-positive hematopoietic cells. Although this did not eradicate the CML cells, it significantly limited the growth of the tumor clones and reduced the risk of the dreaded "chain crisis." This product was marketed in 2001 by the pharmaceutical company Novartis as “Imatinib Mesylate” (Gleevec® in the United States and Glivec® in Europe). Other pharmacological inhibitors have also been developed.