Mutation Analysis of JAK-1, 2 and 3 in Children with Down Syndrome and Acute Leukemia.
Authors: Blink Marjolein , Buitenkamp Trudy , Danen-van Oorschot Astrid A , de Haas Valerie , Reinhardt Dirk , Hasle Henrik , van den Heuvel Marry M. , Zwaan C. Michel 
Published: October 2019

Abstract
Abstract 5035
Children with Down Syndrome (DS) have an increased risk of developing leukemia, including both acute myeloid (ML-DS), as well as acute lymphoblastic leukemia (DS-ALL). ML-DS can be preceded by a pre-leukemic clone in newborns (transient leukemia-TL), which in most cases resolves spontaneously. Janus Kinase (JAK) 1-3 belongs to a family of intracellular non-receptor protein tyrosine kinases that transduce cytokine-mediated signals via the JAK-STAT pathway. JAK plays an important role in regulating the processes of cell proliferation, differentiation and apoptosis in response to cytokines and growth factors. Mullighan et al. described JAK 1-3 mutations in non-DS high-risk childhood B-cell precursor acute lymphoblastic leukemia (BCP-ALL; PNAS, 2009). In T-ALL, JAK-1 mutations are a frequent event (∼25%) as reported among others by Jeong et al (Clinical Cancer Research, 2008). Mutations in JAK-2 and JAK-3 have been described in TL and ML-DS. Bercovich et al. recently reported mutations within the pseudokinase domain of JAK-2 in DS-ALL patients (Lancet 2008). This activating JAK-2 mutation differs from the V617F exon 14 mutation found in myeloproliferative diseases. However, JAK-1 has never been investigated in Down syndrome leukemias. Therefore we performed mutational analysis of the pseudokinase and kinase domains of JAK-1, 2 and 3 by direct sequencing in 8 TL, 16 ML-DS and 35 DS-ALL samples taken at initial diagnosis. The TL and ML-DS samples were unpaired. In the ML-DS group, 12 patients were classified as FAB M7, 3 as FAB M0 and 1 as FAB M6; all 35 DS-ALL patients were classified as BCP-ALL. Mutations in JAK-1 were found in 1 ML-DS patient (D625R) and in 1 DS-ALL patient (V651M). These mutations were localised in the same region of the pseudokinase domain, but not identical to the activating mutations in JAK1 described in high-risk ALL (Mullighan et al., PNAS 2009). The JAK-1 mutated ML-DS patient had a complex karyogram, and the DS ALL patient a normal karyotype. No events occurred in either of the patients with a follow-up of 2.4 and 3.1 years, respectively. JAK-2 activating mutations at position R683 were found in 5/35 (14.3%) of the DS-ALL patients. These patients had diverse cytogenetic aberrations, and had no events at a median follow up of 4.4 years. In the TL and ML-DS patients no mutations were identified in JAK-2. For JAK-3, 1 TL-patient (13%) and 1 ML-DS patient (6.3%) harboured the A573V-mutation. This activating mutation is previously described in ML-DS patients and the megakaroyblastic cell line CMY ((Kiyoi et al, Leukemia 2007). Because the mutations occur in both TL and ML-DS, this suggests that they do not play a role in the clonal progression model from TL to ML-DS. A mutation at JAK3 R1092C, which to our knowledge has never been reported before, was found in 1 DS-ALL patient. This patient had a deletion on chromosome 12 (p11p13), and was in CCR with a follow up of 5 years. In conclusion, JAK-mutations are rare in DS-leukemias, except for JAK-2 mutations in DS-ALL, which occur in approximately 15% of cases. The rarity of JAK-1 mutations in DS is in accordance with the rarity of T-ALL in DS. Of interest, none of the DS ALL cases with a JAK-2 mutation relapsed so far, which differs from the patients with JAK-2 mutations that were recently in high-risk BCP-ALL. Hence, JAK-2 may be an interesting novel therapeutic target.

Disclosures
No relevant conflicts of interest to declare.