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From a medical point of view, the word chimera is used to define an individual in which there is a population of cells from genetically different individuals. This phenomenon can occur spontaneously, such as embryo-to-embryo transfer or embryo-to-mother transfer, or artificially in the case of allogeneic hematopoietic stem cell transplantation (HSCT).
The success of allogeneic HSCT is assessed by chimerism analysis, which monitors the relative amount of viable donor cells and residual recipient cells in peripheral blood or bone marrow samples. What is clinically useful is not just determining it, but monitoring the dynamics or changes in the percentage of donor and recipient cells between different time points over time. Analysis of chimerism after stem cell transplantation following HSCT is necessary to evaluate engraftment status of transplanted cells and eventual disease relapse. In addition, minimal residual disease (MRD) analysis can also provide significant prognostic information about disease recurrence, thus influencing clinical decisions. Peripheral blood (PB) and bone marrow (BM) are the preferred biological sources used to determine chimerism after transplantation. In chimerism, markers that are specific to the recipient are quantitatively detected in a DNA sample from the recipient’s peripheral blood and/or bone marrow. The presence of persistent or emerging host cells may indicate impending disease recurrence.
Classification of chimerism
Chimerism status is classified as follows:
“Complete chimerism” (CC) when only the genotyping donor is detected.
“Mixed chimerism” (MC) when donor and recipient genotypes are identified together and the recipient genotype is at least 1%.
“microchimerism” (Mc) When the recipient genotype is less than 1%
“split chimerism” in which one or more recipient and donor lineages are present simultaneously.
MC is divided into transient MC, stable MC and progressive MC. Different studies show that there is a different relationship between CC, MC, GVHD and relapse. The presence of progressive MC (typically a 5% increase in recipient DNA) appears to indicate disease recurrence, which is different from transient or persistent MC. Interestingly, an early CC has been reported to be associated with the occurrence of acute GVHD. Furthermore, it seems clear that MC can be considered a high risk factor for recurrence. As molecular technology advances, studies investigating MC have defined highly sensitive techniques for analyzing chimerism at levels below the 1% detection limit. Therefore, early detection of MC may lead to a beneficial effect thanks to a rapid and appropriate medical intervention that reduces the probability of progression to the high-risk category of MC.
Chimerism analysis
Chimerism analysis is based on the detection and quantification of specific genetic differences (i.e., polymorphic markers) that distinguish donor cells from recipient cells. It essentially quantifies the ratio of hematopoietic cells of donor origin to those of recipient origin in peripheral blood, bone marrow, and other potential tissues. Observing trends in these ratios over time is used to monitor and track linker status. Currently, chimerism analysis is usually performed by evaluating short tandem repeat (STR) profiles. However, other DNA polymorphisms such as variable number of tandem repeats (VNTR), single nucleotide polymorphisms (SNPs), or insertion/deletion (Indel) polymorphisms can be used. The main reason STRs are more suitable for chimerism analysis compared to other polymorphisms is their higher information rate. Nevertheless, careful selection of STR alleles is required to select the most suitable STR for chimerism monitoring, taking into account all variables that can affect the analysis. STR analysis is considered the gold standard technology for monitoring chimerism after allogeneic HSCT. However, the sensitivity of about 1% of STR-PCR analysis may be an obstacle for timely intervention. A 2002 study proved the higher sensitivity of quantitative real-time PCR (qPCR) applied to SNPs compared to conventional STR-PCR analysis. Further improvements in sensitivity were observed when qPCR was applied to indel polymorphisms. Two recently developed technologies, namely digital PCR (dPCR) and next-generation sequencing (NGS), can be very promising in chimerism analysis in the future due to their high sensitivity, accuracy, and wide applicability.
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