Things Are Not Always What They Seem: Marrow Failure From a Hematopoietic Cell-Extrinsic Mechanism Due to Germline Mutations in Thrombopoietin

Clinical and histopathologic distinction between inherited versus acquired bone marrow failure is challenging. Both are characterized by peripheral cytopenias and marrow hypocellularity; the distinction is important, as risks of and responses to various therapies differ between the two, as do their natural histories.^1,2  This is particularly relevant in the context of hematopoietic stem cell transplantation (HSCT) because the recognition of an underling inherited disorder informs the timing and indication for HSCT, transplant approaches (i.e., many inherited disorders are associated with excessive transplant regimen-related toxicities and may require specialized reduced-intensity conditioning regimens), and appropriate donor selection.¹  Both the importance of accurately recognizing inherited from acquired marrow failure and its therapeutic relevance are highlighted in recent work by Dr. Aaron Seo and colleagues.³  These investigators identified and characterized germline biallelic-loss-of-function mutations in thrombopoietin (THPO) as a cause of bone marrow failure unresponsive to HSCT, and responsive to thrombopoietin-mimetic therapy in an informative family.

Thrombopoietin is a glycoprotein class 1 hematopoietic cytokine produced primarily in the liver. TPHO signaling, via its receptor MPL, is essential for both megakaryopoiesis and hematopoietic stem cell maintenance.⁴  In mice, knockout studies of Thpo or Mpl result in reduced numbers of hematopoietic cell progenitors and thrombocytopenia.^5-7  In humans, autosomal recessive loss-of-function mutations in MPL result in congenital amegakaryocytic thrombocytopenia, with affected children presenting initially with isolated severe thrombocytopenia and subsequently developing progressive pancytopenia due to a reduction in bone marrow hematopoietic stem cells.⁸ 

Seo and colleagues report on five children from three consanguineous families presenting with early-onset thrombocytopenia and subsequent bone marrow failure. Four of the children underwent HSCTs from various related and unrelated donor sources for progressive marrow failure using both aplastic anemia and myelodysplastic preparatory regimens. This resulted in engraftment failure or persistent bone marrow failure despite donor cell engraftment, consistent with the cause of the marrow failure being extrinsic to the hematopoietic cell. Three of the four children treated with HSCT died posttransplant. Notably, serum thrombopoietin levels were measured in two of the five patients studied and were found to be undetectable. Thrombopoietin levels are typically high in bone marrow failure syndromes such as aplastic anemia, where levels are inversely proportional to megakaryocytic mass as opposed to peripheral platelet counts.⁹  These two individuals (one of whom failed HSCT) were treated with the thrombopoietin receptor agonist romiplostim (final dose 5 μg/kg subcutaneously weekly) and responded with improved trilineage hematopoiesis, including normal count recovery in one individual.

Whole exome or targeted sequencing of known inherited marrow failure genes in the patients identified homozygous germline mutations in THPO in affected family members, which segregated with the phenotype in an autosomal recessive fashion and are predicted to be damaging by in silico analyses (THPO: c.295C>T; R99W and c.469C>T; R157X: NM_000460.3). To determine whether the R99W affects THPO function, R99W and wild-type control THPO were generated and used to assay MPL-dependent cell survival and proliferation in vitro. Both wild-type and mutant THPO supported MPL-dependent cell survival and proliferation. The authors noted that the R157W allele introduces a stop codon in the middle of the protein that results in the loss of a critical protein domain known to be required for normal THPO secretion. They concluded that both mutations likely affect THPO serum levels without affecting THPO function, consistent with their functional analysis of the R99W mutant and the undetectable thrombopoietin levels measured in the patients.