IMPLICATIONS OF REVERTANT SOMATIC MOSAICISM IN BONE MARROW FAILURE SYNDROMES.

Revertant somatic mosaicism (RM) has been reported in genetic diseases including skin disorders and bone marrow failure (BMF) syndromes. We present examples of RM of haematopoietic cells in the context of inherited BMF syndromes with dramatically different clinical outcomes. Diamond Blackfan Anaemia (DBA) is an inherited BMF disorder caused by mutations/deletions in ribosomal proteins. We have a 9 year old male who required regular transfusions until he became transfusion independent at age 4. We identified a 184kb deletion on chromosome 12 encompassing 11 genes including RPS26 (known DBA gene). Significantly, we observed multiple independent copy neutral loss of heterozygosity (CNLOH) events on chromosome 12, correcting the 184kb deletion in a subset of blood cells, but not in hair. We propose RM as a mechanism for correction of the germline deletion resulting in spontaneous remission in the patient. We also recently identified germline mutations in SAMD9L, in a family with thrombocytopaenia, macrocytosis and ALL. In this family, we see examples for somatic mosaicism leading to a severe blood phenotype as well as CNLOH RM causing milder phenotypes in different family members. RM needs to be considered when selecting tissue source for diagnosis in carriers with mild phenotype/remission, and use of patient-derived cell lines for drug screens with multiple rounds of cell replication. We are currently trying to understand and manipulate revertant clone selection and hope this will open windows for rational correction and selection protocols for effective therapeutic intervention. Experimental manipulations to select clones that improve clinical phenotype have substantive implications on efficiencies of gene manipulations, manufacture and quality control requirements of autologous cellular therapies. [ABSTRACT FROM AUTHOR]