**2.8 Specific features of chimerism assessment in non-malignant diseases monitored by 'FA'**

#### *2.8.1 Introduction*

Allo-hSCT is a well-established treatment not only for high risk leukaemias and myelodysplastic syndrome (MDS) but also for several non-malignant diseases, including acquired and inherited bone marrow failure (BMF) syndromes like aplastic anaemia (AA) and Fanconi anaemia (FA), immunodeficiencies like severe combined immunodeficiency (SCID), Wiskott–Aldrich syndrome (WAS), chronic granulomatous disease (CGD), haemoglobinopathies and metabolic disorders, mostly transplanted in childhood. The aim of transplantation in these diseases is to achieve sustained engraftment of donor stem cells to improve haematopoietic function, provide immune competence and increase or normalise enzyme deficiency. It can completely transform the lives of children for whom life expectancy or quality of life would otherwise be very poor. Although complete donor haematopoiesis is a desirable outcome of SCT in malignant disorders, complete replacement of recipient's haematopoietic system is not considered necessary to improve the underlying disease state in patients with non-malignant disorders [33, 34].

#### *2.8.2 Cohort*

The cohort consisted of 74 different adult and paediatric non-malignant patients from three Slovak bone marrow transplantation centres who underwent allo-hSCT.

#### *2.8.3 Methods*

The method included PCR-based analysis (with fluorescent allele specific primers) of short tandem repeats (PCR-STR) markers by fragment analysis ('FA') following capillary electrophoresis in Genetic analyser. Surveillance of chimerism was done within total peripheral blood leukocytes or as lineage-specific chimerism in selected T cells, B cells and myeloid cells after an allogeneic SCT according to Park et al. and Thiede [35, 36].

*Monitoring of Chimerism in Rare Haematological Malignant Diseases after Allogeneic… DOI: http://dx.doi.org/10.5772/intechopen.89845*

#### *2.8.4 Results*

Definition of chimerism, as it is described above considers a patient to have CC, when he does not show any evidence or less than 1% of autologous—recipient DNA at any time after allo-hSCT. Patients with both donor and recipient DNA (that increased 5% or more) in any of the samples analysed were defined as having MC. Split chimerism is present if one or more leukocyte lineages are of host and one or more leukocyte lineages are of donor origin [4, 36, 37].

As other studies mentioned above, also our results show there is a relationship between chimerism status and clinical course as well as outcome of allogeneic SCT in non-malignant diseases. From our cohort, 75% (55 patients) reached complete chimerism (CC) monitored by 'FA' and 25% (19 patients) mixed chimerism on different levels. Most of the patients with CC and decreasing or stable MC are alive doing well and are in remission, with a median follow-up time of 4.3 years. All of the patients with high MC (>40% or increasing MC) experienced transplant rejections and almost half of them died. Patients with CC had a higher risk of acute graft-versus host disease GVHD compared with MC patients.

#### *2.8.5 Conclusion*

Therefore, mixed chimerism (if there are still low levels of recipient cells) may be welcomed in these patients, as it reflects a decreased allo-response with less acute GVHD. Reduced intensity conditioning (RIC) was more often associated with decreasing or low stable MC compared to myeloablative conditioning, but importantly high MC was not different when using reduced intensity conditioning than with myeloablative conditioning [33, 34].
