Unravelling the molecular pathogenesis of Myelodysplastic Syndrome Patients with Ringed Sideroblasts

Abstract

Myelodysplastic syndromes (MDS) are a collection of clonal hematopoietic stem cell disorders with diverse phenotypes, characterized mainly by ineffective haematopoiesis. Refractory anaemia with ringed sideroblasts (RARS) is an acquired form of MDS with accumulation of mitochondrial iron that gives rise to ringed sideroblasts. In this study, whole-exome sequencing performed in 12 patients all of whom had >25% ringed sideroblasts identified mutations in one particular gene, splicing factor 3b subunit 1 (SF3B1), a component of the major and minor spliceosomes, in 11/12 cases. No SF3B1 mutations were observed in a single case of congenital sideroblastic anaemia, however, a constitutional mutation in ALAS1 gene was confirmed in this patient. Subsequently, amplicon sequencing using a 22 myeloid-gene panel in 154 MDS patients revealed 76% (n=117) of the patients had mutations in at least one of the genes, with 38% (n=59) having splicing gene mutations and 49% (n=75) patients harbouring more than one gene mutation. Interestingly, single and specific epigenetic modifier mutations tended to coexist with SF3B1 and SRSF2 mutations (p<0.03). Moreover, patients with splicing factor mutations alone had a better overall survival than those with coexisting additional mutations. Next, combination of techniques including xenotransplantation and single cell clonogenic assays were used to study the characteristics of SF3B1 mutant cells. Mutational analysis of hematopoietic stem/progenitor cells showed that SF3B1 mutations arise in phenotypically defined CD34+CD38-CD45RA-CD90+CD49f+ stem cells, therefore revealing the existence of MDS-initiating stem cells that propagate mutations to myeloid progenitors only. A persistent engraftment restricted to myeloid lineage was observed which was initiated only via the xenotransplantation of HSCs. Genetically diverse evolving subclones of mutant SF3B1 were detected in mice, indicating a branching multi-clonal as well as ancestral evolutionary paradigm. Subclonal evolution in mice was also mirrored in the clinical evolution in patients. Sequential sample analysis uncovered a clonal evolution in hematopoietic progeny and selection of the malignant driving clone leading to AML transformation.

Date of Award	2016
Original language	English
Awarding Institution	King's College London
Supervisor	Ghulam Mufti (Supervisor)