Clonal Events in Normal Aging Hematopoiesis

Chronological aging of the hematopoietic compartment is associated with decreased bone marrow cellularity, reduced lymphopoiesis, increased anemia, a myeloid proliferation bias and an increased incidence of myeloid cancers. Beerman et al. proposed that this age-related myeloid lineage favoritism may be explained by clonal expansion of intrinsically myeloid-biased hematopoietic stem cells with robust self-renewal potential(1). This age-associated clonal expansion was initially suspected by X-chromosome inactivation (XCI) studies performed in the normal aging population, which documented a skewed XCI pattern in a significant proportion of women over 60 year-old(2).  More recently, genome wide approaches led several groups to document au augmented prevalence of acquired clonal copy number changes(3,4,5) or clonal somatic mutations with increasing age(6,7,8,9). The most frequently mutated genes are the same as those documented in myeloid cancers, such as TET2, DNMT3A, ASXL1, PPM1D, GNAS, TP53, JAK2 and SF3B1 among others. The prevalence of these age-associated mutations may reach > 10% of older individuals, and is associated with an 11-12 fold increased relative risk of developing hematological malignancies. However, the actual problematic is to define the prognostic significance of these clonal mutations in the aging population.  Steensma et al.  proposed to consider these mutations as «Clonal Hematopoiesis of Indeterminate Potential (CHIP)»(10).   The goal of our research group is to define the oncogenic penetrance of CHIP by applying a precision medicine approach in a large prospective cohort (n=4000) of aging individuals comprised of related and unrelated subjects. The variables under investigation include, clonality by XCI in women, deep sequencing (NGS) of myeloid cancer associated genes, epigenetic markers (5hmC, 5mC), telomere length, blood counts, heritability and outcome. PRELIMINARY RESULTS. XCI analyses  Acquired skewing of XCI predominantly affects the myeloid lineage with a prevalence of 41.4% for PMN and is age dependent (r=0.15, P<10^-4), in contrast to T cells 22.5%. These results support the idea of an age-associated clonal myeloid expansion. NGS of myeloid gene panel. We documented a prevalence of 17.9% of mutated individuals. Mutations were mainly documented in TET2 and DNMT3A which accounted for 90% of all identified mutations. Other significantly mutated genes included JAK2, ASXL1, CBL, TP53 and KRAS. Double mutations were identified in 2.5% of individuals (14% of the mutated individuals) and half of them had concomitant mutation in TET2 and DNMT3A. Age and XCI skewing was similar between subjects with mutation in TET2 or DNMT3A, but slightly higher in double mutants. Epigenetic markers. Subjects with mutation in TET2 had a significant reduction in 5hmC level that correlated with Variable Allele Frequency (VAF) of the mutation. No specific global epigenetic phenotype was documented in the DNMT3A mutation subgroup. We also documented an age-associated reduction in 5hmC that was independent of acquired mutation in the TET2 gene. Taken together these results indicate that age-associated clonal mutations involves predominantly two genes (TET2 and DNMT3A), suggesting that alteration of epigenetic maintenance is a central to the initiation of clonal dominance. Completion of investigation of the aging cohort and prospective follow-up will help characterize the link between aging hematopoiesis and the development of myeloid cancers.   

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9.Steensma DP, Bejar R, Jaiswal S, et al. Clonal hematopoiesis of indeterminate potential and its distinction from myelodysplastic syndromes. Blood. 2015;126(1):9-16