Clinical and Cytogenomic Features of Lymphoblastic Leukemia with Intrachromosomal Amplification of Chromosome 21 (iAMP21) in the Context of Constitutional Ring Chromosome 21

Constitutional ring chromosome 21, r(21)c, is a rare chromosome abnormality associated with variable clinical features that range from mild dysmorphism to severe congenital anomalies and intellectual disability. Recently, r(21)c has been reported to predispose to B-cell acute lymphoblastic leukemia (B-ALL) with intrachromosomal amplification of chromosome 21 (iAMP21), a distinct subgroup of high-risk pediatric B-ALL. Only a few iAMP21-B-ALL cases with r(21)c have been reported to date. The mechanism of leukemogenesis of r(21)c has not been entirely elucidated. Here we report an 11-year-old boy with iAMP21-B-ALL carrying an atypical r(21)c. The patient has a history of attention-deficit/hyperactivity disorder, sensorineural hearing loss s/p cochlear implant, intellectual disability and scoliosis. Three months before admission he developed a soft tissue nodule on the occipital scalp deemed to possibly represent an enlarged lymph node. Subsequently, he presented with spontaneous bruising followed by severe epistaxis. The initial CBC showed a white blood cell count of 4.3K/uL with circulating blasts, absolute neutropenia, profound normocytic normochromic anemia (hemoglobin of 5.2 g/dL), and marked thrombocytopenia (platelets, 12K/uL ). Peripheral blood flow cytometry showed 17.9% phenotypically abnormal B lymphoblasts which were negative for CD45, and positive for CD34, nuclear TdT, CD19, CD22, CD79a, CD10, HLA-DR , as well as CD20 (21% positive). The bone marrow aspirate contained 98% blasts. CNS status was 2a (RBC 0, WBC 0, 8% blasts) and clear after the second lumbar puncture. Fine-needle aspiration of the scalp mass demonstrated B-lymphoblastic leukemia/lymphoma. The patient was treated per COG protocol AALL1131 and was assigned to the very high-risk arm when bone marrow interphase FISH showed iAMP21. The chromosome analysis failed to yield metaphase cells on the diagnostic bone marrow sample. A concurrent genomic microarray showed chromothripsis with multiple non-contiguous losses and high copy gains on 21q involving the RUNX1 gene, as well as mosaic deletions within 7q22.3q36.3, 9p24.3p24.1 (including JAK2), 12q12 (several exons of ARID2), 13q14.2q21.2 (RB1, DLEU1/2/7, miR-15a/miR-16-1 cluster), 14q32.33 (IGH locus), 19p13.2 (several exons of the SMARCA4), and mosaic gains within 3q22.3q29 and Xp22.33p11.3. Day 29 end of induction bone marrow examination was positive for minimal residual disease (MRD) at 0.13% of nucleated mononuclear cells, but FISH was