RUNX1 associated Familial Platelet Disorder with Myeloid Malignancy (FPD-MM) in Children: A Novel New Phenotype with Juvenile and Chronic Myelomonocytic Leukemia (JMML/CMML) Characteristics

Introduction:RUNX1 (aka AML1; 21q22.12) is indispensable in the establishment of definitive hematopoiesis in humans. Activating RUNX1 mutations are associated with both Acute Myeloid and Lymphoblastic Leukemias (AML, ALL). On the other hand, hypofunctioning RUNX1 mutations cause dominantly inherited Familial Platelet Disorder (FPD). RUNX1 FPD has a high risk for progression to pancytopenia, myeloproliferative disorders (MPD) or AML, hence the new WHO category FPD with myeloid malignancy (FPD-MM). Those with MM carry mutations in other genes seen in AML, MDS. It is a relatively rare disorder with ~75 affected kindreds reported worldwide (Sood, et al. Blood 2017). Detailed reviews of pediatric cases are few.

Case Histories: We encountered two children with RUNX1 associated thrombocytopenia; the mutations are novel. The first family is that of 14 yr old AAF, presenting with fainting- blood counts are shown in Table 1; fetal hemoglobin (HbF) was elevated; bone marrow was hypercellular with 6% type 1 blasts, extreme paucity of megakaryocytes, erythroid hyperplasia and large numbers of sea blue histiocytes. The high HbF suggested JMML while the monocyte CD16;14 profile (95.6% CD14+ cells) was similar to that seen in the adult type Chronic Myelomonocytic Leukemia (CMML). Her mother has pancytopenia without excess blasts in the marrow. The second case presented with neonatal thrombocytopenia; father has history of excessive bruising.

Results: Blood counts and values for HbF are listed in Table 1.

Molecular testing:

Case1: A Myeloid gene panel showed RUNX1 - NM_001754.4:c.501delT, p.Ser167Argfs*9; PHF6 - NM_032458.2:c.902dupA, p.Tyr301*; CUX1- NM_001202543.1:c.2378delC, p.Pro793Argfs*26. No mutations were noted in PTPN11, CBL or RAS genes, the latter confirmed by JMML panel done at University of California, San Francisco. UCSF panel identified a mutation in SH2B3, a gene linked to erythrocytosis not caused by JAK2 mutations. Her mother has the same RUNX1 mutation, thus identifying a germline mutation of RUNX1 in her and her child but not the PHF6, CUX1 or the SH2B3 mutations seen in her daughter. A half sibling is unaffected and is a potential transplant donor for the mother.

Case2: No coding sequence mutations were detected in genes associated with familial thrombocytopenia including ETV6, GATA1 and RUNX1. Array Comparative Genomic Hybridization studies (Prevention Genetics) identified a heterozygous deletion of the entire exon 5 of RUNX1.

To understand the complex findings in family 1 additional studies were done- DRAQ5, CD71, Fetal Hb staining showed that NRBC in Case 1 contained predominantly high HBF cells. LIN28B was markedly elevated in the proband but not the mother (HbF- normal); LIN28B expression was normal in Case 2.

Treatment/Outcome: In Case 1, low dose decitabine therapy resulted in the control of MPD features with good Hb recovery and normalization of the monocyte CD16;14 profiles. There was no platelet response to decitabine nor to a course of valproic acid. The child died of fulminant acute graft vs host disease affecting the liver following a 4/6 cord mismatch transplantation. Mother continues to show moderately severe pancytopenia requiring frequent transfusion support. The second child is symptom free with mild thrombocytopenia.

Discussion: The hybrid JMML/CMML features in the index child are likely caused by the concurrent CUX1/PHF6/SH2B3 mutations. We are unable to establish if these are true de novo mutations as the father was not available for study; she had no full siblings. Neither high HbF nor high LIN28B are known feature of FPD by itself nor CMML or Polycythemia Vera (p Vera). Recently, the high HbF in JMML has been linked to high expression of LIN28B. SH2B3 mutation may have contributed to the high erythroid proliferation observed in our case. Induced CUX1 haploinsufficiency in mice causes MPD akin to CMML and megakaryocytic (Meg) proliferation (An N, et al. Blood 2018). The virtual absence of Megs in our case indicates that the CUX1 mutation was unable to overcome the Meg ploidization defect caused by the RUNX1 mutation. PHF6 mutations occur in T-ALL and AML but have not been linked to high HbF.

Conclusions: Normal HbF and normal LIN28B expression in the mother of Case1 and in Case2 indicate that increased LIN28B is linked to the high HbF in Case 1 and that high LIN28B itself is a consequence of the malignant transformation caused by the concurrent CUX1/PHF6/SH2B3 mutations.

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