Bone marrow failure unresponsive to bone marrow transplant is caused by mutations in thrombopoietin

Abstract

We report 5 individuals in 3 unrelated families with severe thrombocytopenia progressing to trilineage bone marrow failure (BMF). Four of the children received hematopoietic stem cell transplants and all showed poor graft function with persistent severe cytopenias even after repeated transplants with different donors. Exome and targeted sequencing identified mutations in the gene encoding thrombopoietin (THPO): THPO R99W, homozygous in affected children in 2 families, and THPO R157X, homozygous in the affected child in the third family. Both mutations result in a lack of THPO in the patients' serum. For the 2 surviving patients, improvement in trilineage hematopoiesis was achieved following treatment with a THPO receptor agonist. These studies demonstrate that biallelic loss-of-function mutations in THPO cause BMF, which is unresponsive to transplant due to a hematopoietic cell-extrinsic mechanism. These studies provide further support for the critical role of the MPL-THPO pathway in hematopoiesis and highlight the importance of accurate genetic diagnosis to inform treatment decisions for BMF.

Figure 1.

Family pedigrees. (A) The arrow indicates the proband. Family A and Family B: Asterisks indicate individuals evaluated with targeted gene capture panel/high-throughput sequencing. The variant allele (295C>T; R99W) is indicated with V₁, and reference alleles are indicated with an N. Family C: Asterisk indicates individual evaluated with whole-exome sequencing. The variant allele (469C>T; R157X) is indicated with V₂, and reference alleles are indicated with an N. (B) Serum THPO levels in affected individuals. Serum THPO concentrations (picograms per milliliter) are given for the affected child along with the testing laboratory’s control ranges. (C) THPO protein domains. The signal peptide (amino acid residues 1-21) is shaded in light red. The positions of the THPO mutations are indicated on the structure. (D) THPO amino acid conservation across vertebrate species. Bold font indicates the arginine residue at position 99 of the human THPO sequence. Twenty–amino acid residues flanking position 99 are shown (NCBI RefSeq). EPO, erythropoietin; N/A, not applicable.

Figure 2.

Clinical response to romiplostim. Trilineage response to romiplostim therapy. (A) PLT, (B) ANC, (C) Hb, and (D) WBC counts are shown for affected individuals: Family A, III-6 and Family C, II-5. Arrow indicates date of BMT. Gray shading indicates timeframe of platelet and erythrocyte transfusion dependence. Dotted line indicates initiation of romiplostim therapy. Note difference in time scale (x-axis) between left and right panels.