Discussion

Analysis of chimerism levels can be used to monitor the proportion of donor and recipient cells after allogeneic hematopoietic stem cell transplant (HSCT). It can identify graft rejection and can predict early relapse and GVHD.[2] There are various methods by which chimerism can be estimated. These include XY-FISH, conventional karyotyping, restriction fragment length polymorphism (RFLP), red blood cell phenotyping, analysis of STR by PCR, single nucleotide polymorphism analysis, and Y chromosome PCR.[3]

The sensitivity of XY-FISH for sex chromosomes is between 0.1% and 0.5% and conventional karyotyping is around 5%–10% for the detection of mixed chimerism.[3] XY-FISH is done on interphase preparations with two differently labeled probes for X and Y chromosomes. It allows for rapid screening of many cells, and therefore, it is considered that high sensitivity can be achieved. Conventional cytogenetics depends on the culture of cells and analysis of mitoses and successful analysis depends on the presence of dividing cells and hence was considered to have low sensitivity. The relapse of leukemia initially starts in the bone marrow, and then, the leukemic cells spill over to the peripheral blood. Hence, in the context of detecting relapse, the sensitivity estimate of these methods is questionable. In the early stage of relapse, in the peripheral blood, the proportion of malignant cells is very low compared to the normal donor cells. Consequently, the analysis by XY-FISH would be done on majorly on donor cells which are in the G0 phase, and hence, there is a chance to miss early relapse. Moreover, the leukemic cells tend to have a high proliferative index, and hence, an interphase FISH would miss such cells during analysis. However, in conventional cytogenetics, the cells will be cultured, which leads to the replication of the rapidly dividing malignant recipient cells as compared to the quiescent donor cells and hence analyze recipient malignant cells majorly. This is reflected in our patient where chimerism by XY-FISH in the peripheral blood was 98% donor cells; however, the conventional karyotyping detected malignant recipient cells in all the mitoses.

Studies are done comparing XY-FISH to PCR analysis of Y chromosome-specific sequences and found that the XY-FISH missed a significant number of relapses compared to Y chromosome PCR.[4] In conditions such as myelodysplastic syndrome and benign conditions such as pregnancy, XY-FISH is as good as conventional cytogenetics to detect chromosomal abnormalities.[5] A study has been done for monitoring of chimerism posttransplant and was found that FISH is more sensitive.[6] However, studies have not been done comparing the utility of these in the context of detecting relapse after ASCT.

We conclude that, in the context of detecting relapse after allogeneic HSCT, XY-FISH might not be an ideal modality for analyzing chimerism. Conventional karyotyping, given its ability to select rapidly dividing cells, might be better when compared to XY-FISH, but karyotyping lacks sensitivity. Hence, more sensitive methods such as RFLP, STR PCR, variable number tandem repeat, and Y chromosome PCR which are the current standard of care should be considered in the context of ASCT for malignant disorders over XY-FISH.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Conflict of Interest

There are no conflicts of interest.

References

1. Stikvoort A, Sundin M, Uzunel M, Gertow J, Sundberg B, Schaffer M. et al. Long-term stable mixed chimerism after hematopoietic stem cell transplantation in patients with non-malignant disease, shall we be tolerant?. PLOS One 2016; 11: e0154737
2. Balon J, Hałaburda K, Bieniaszewska M, Reichert M, Bieniaszewski L, Piekarska A. et al. Early complete donor hematopoietic chimerism in peripheral blood indicates the risk of extensive graft-versus-host disease. Bone Marrow Transplant 2005; 35: 1083-8
3. Van Deerlin VM, Reshef R. Chimerism testing in allogeneic hematopoietic stem cell transplantation. In: Leonard DG. editor Molecular Pathology in Clinical Practice. Cham: Springer International Publishing; 2016: 823-48
4. Koldehoff M, Steckel NK, Hlinka M, Beelen DW, Elmaagacli AH. Quantitative analysis of chimerism after allogeneic stem cell transplantation by real-time polymerase chain reaction with single nucleotide polymorphisms, standard tandem repeats, and Y-chromosome-specific sequences. Am J Hematol 2006; 81: 735-46
5. He R, Wiktor AE, Durnick DK, Kurtin PJ, Van Dyke DL, Tefferi A. et al. Bone Marrow conventional karyotyping and fluorescence in situ hybridization: Defining an effective utilization strategy for evaluation of myelodysplastic syndromes. Am J Clin Pathol 2016; 146: 86-94
6. Díez-Martín JL, Llamas P, Gosálvez J, López-Fernández C, Polo N, de la Fuente MS. et al. Conventional cytogenetics and FISH evaluation of chimerism after sex-mismatched bone marrow transplantation (BMT) and donor leukocyte infusion (DLI). Haematologica 1998; 83: 408-15

Address for correspondence

Publication History

Received: 13 February 2020

Accepted: 07 April 2020

Article published online:
17 May 2021

© 2020. Indian Society of Medical and Paediatric Oncology. This is an open access article published by Thieme under the terms of the Creative Commons Attribution-NonDerivative-NonCommercial-License, permitting copying and reproduction so long as the original work is given appropriate credit. Contents may not be used for commercial purposes, or adapted, remixed, transformed or built upon. (https://creativecommons.org/licenses/by-nc-nd/4.0/.)

Thieme Medical and Scientific Publishers Pvt. Ltd.
A-12, 2nd Floor, Sector 2, Noida-201301 UP, India