Noonan syndrome (NS) is a common autosomal dominant condition characterized by short stature, congenital heart defects, and dysmorphic facial features caused in approximately 50% of cases by missense mutations in the PTPN11 gene. NS patients are predisposed to malignancies including myeloproliferative disorders or leukemias. We report a female NS patient carrying a PTPN11 germline mutation c.417 G > C (p.E139D), who developed in her second year of life an acute lymphoblastic leukemia (ALL) and after remission, she developed at 4 years of age a juvenile myelomonocytic leukemia (JMML). Molecular genetic analysis of lymphoblastic blasts at the time of the ALL diagnosis revealed the germline mutation in a heterozygous state, while in the myelomonocytic blasts occurring with JMML diagnosis, the mutation p.E139D was found in a homozygous state due to a uniparental disomy (UPD). These findings lead to the suggestion that the pathogenesis of ALL and JMML in our patient is due to different mechanisms including somatically acquired secondary chromosomal abnormalities.
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