In about 1/3 of older anemic adults, no clinical etiology can be found and this group has been labeled unexplained anemia in the elderly (UAE). UAE has been associated with poor outcomes, including decreased functional status, increased rate of hospitalization, and increased mortality. We hypothesize that intrinsic functional impairment of hematopoietic stem and progenitor cells is a significant contributor to the pathogenesis of UAE. Hematopoietic stem cells (HSCs) reside at the top of the hematopoietic hierarchy and can differentiate, via committed downstream progenitors, into all the mature cells of the hematopoietic system. Human erythroid development proceeds through a set of oligopotent progenitors that can be isolated by immunophenotypic markers: HSCs (Lin-CD34+CD38-CD90+CD45RA-) give rise to multipotent progenitors (MPPs), which give rise to common myeloid progenitors (CMPs), which give rise to granulocyte-macrophage progenitors (GMPs) and megakaryocyte-erythrocyte progenitors (MEPs; Lin-CD34+CD38+CD123-CD45RA-). In addition, MEPs can be further fractionated into erythroid-biased MEPs (E-MEPs; Lin-CD34+CD38+CD123-CD45RA-CD71+CD105-), which give rise to erythroid-committed progenitors (EPs; Lin-CD34+CD38+CD123-CD45RA-CD71+CD105+), which subsequently differentiate into early erythroid precursors (pre-Es; CD34-CD71+GPA-). We compare immunophenotypic hematopoietic stem, progenitor, and erythroid precursor populations obtained from hematologically normal young (25 samples; age 20-35), normal elderly (11 samples; age 65+), and UAE (9 samples; age 65+) bone marrow samples. We find that UAE patients contain increased frequencies of MEP compared to normal (1.4-fold; p<0.006). However, there are fewer E-MEPs in UAE patients, suggesting that MEPs from UAE patients do not readily differentiate towards the erythroid lineage (p<.04). Interestingly, we find that one cohort (n = 3) of UAE patients exhibits an expanded EP population and a reduced pre-E population while another cohort (n =4) exhibits the converse. We hypothesize that UAE patients with an expanded EP population have a defect in the ability of EPs to differentiate into downstream pre-Es, whereas UAE patients with a reduced EP population have a defect in the ability of E-MEPs to differentiate into downstream EPs. Functionally, HSCs from UAE patients produce relatively more granulocyte-monocyte (CFU-GM) colonies and fewer erythroid (CFU-E and BFU-E) colonies compared to HSCs from normal young and elderly samples (p<0.008). Similarly, CMPs and MEPs from UAE patients yield skewed distributions of myeloid-erythroid colonies when plated in methylcellulose, significantly favoring production of CFU-GM colonies over CFU-E and BFU-E colonies (CMP p<0.003; MEP p<0.02). Under conditions for erythroid differentiation in liquid culture, E-MEPs from UAE patients with a reduced EP population, but not from patients with an expanded EP population, produce fewer mature erythroid (GPA+) cells compared to normal (p<0.03). EPs and pre-Es from both UAE patient cohorts display defective mature erythroid differentiation potential (EP p<0.05; pre-E p<0.05). We find a functional impairment in the ability of UAE HSCs to differentiate into more mature erythroid forms in both methylcellulose and liquid culture. Thus, we hypothesize that the underlying pathophysiology in UAE is at least in part due to an intrinsic HSC defect. Additionally, although the pathophysiology of UAE remains obscure, we find that UAE can be divided into two surprisingly uniform hematopoietic progenitor phenotypes, suggesting two possible underlying impairments in hematopoiesis. Better understanding of underlying pathophysiology is the first step leading towards rational targeted therapeutic interventions.


No relevant conflicts of interest to declare.

Author notes


Asterisk with author names denotes non-ASH members.