Clinical risk features and genetic findings in essential thrombocythemia and polycythemia vera of childhood and adolescence: prospective insights from the German MPN Childhood Registry

Our understanding of essential thrombocythemia (ET) and polycythemia vera (PV) in children and adolescents is limited to retrospective analyses partly including young adults. Here, we present the largest set of prospectively acquired clinical and genetic data of pediatric patients up to the age of 18 years at diagnosis of ET and PV from the German myeloproliferative neoplasms (MPN) Childhood Registry. Forty-five patients were diagnosed with ET (n=39; 87%) or PV (n=6; 13%). Thirty-one cases (69%) had at least one established clinical adverse prognostic marker at diagnosis represented by symptoms of hyperviscosity/microcirculatory disturbance and/or splenomegaly in 22 patients (49%) each. Twenty patients (44%) carried JAK2V617F (ET n=14; PV n=6), one patient (2%) showed a non-V617F JAK2 variant of uncertain significance, eight (18%) had a pathogenic CALR variant (Type I/II), and 14 (31%) did not show a known somatic (likely) pathogenic driver variant in the classical gene regions of JAK2, CALR, or MPL. No additional cooperating non-driver variants commonly observed in adult patients with MPN were detected using targeted sequencing. However, we identified likely pathogenic germline variants in five patients without a known somatic driver variant: JAK2:c.2624C>A p.(Thr875Asn), JAK2:c.2047A>G p.(Arg683Gly), THPO:c.13+1G>A r.spl p.?, FLT3:c.2525A>G p.(Tyr842Cys), and SAMHD1:c.500A>C p.(His167Pro). Neither the high prevalence of clinical risk features nor the observed enrichment of germline variants identified through targeted sequencing have been previously described in the context of pediatric MPN. This study confirms that pediatric MPN carries a substantial early disease burden and identified potential predisposing variants with possible implications for cancer surveillance and therapy.