Genomic Characterization Of Histiocytic Lesions Following Pediatric T-Cell Acute Lymphoblastic Leukemia
Authors: McMasters Richard , Turpin Brian K. , Absalon Michael J. , Phillips Christine L. , Burns Karen , Dandoy Christopher E. , Bakeer Nihal , Mo Jun , Perentesis John , O'Brien Maureen M. 
Published: October 2019

Abstract

Introduction
The development of histiocytic lesions after acute lymphoblastic leukemia (ALL) is rare, occurring in 6 of 971 patients enrolled on BFM treatment regimens for T-ALL (Trebo 2005). Few cases of histiocytosis arising after a history of T-ALL have been characterized at the molecular level for genomic alterations. One case of fatal Langerhans cell histiocytosis (LCH) following treatment of T-ALL revealed activating mutations in NOTCH1; in contrast, no NOTCH1 mutations were identified in 24 other cases of LCH or Rosai-Dorfman disease without a previous history of T-ALL (Rodig 2008). In patients without prior leukemia, BRAF V600 mutations have been identified in a significant proportion of patients with LCH or Erdheim-Chester Disease but not in other histiocytoses (Haroche 2012).

Methodology and Principal Findings
Next generation focused exomic sequencing of 236 genes and 47 Introns was conducted on samples of histiocytic lesions from two patients with a previous history of T-ALL.

Case 1
A 2 year-old male presented with marked adenopathy, mediastinal mass and white blood cell count 67,000 cells/uL with 30% blasts. The blasts expressed CD45, CD2, CD3 (surface/cyto), CD5, CD7, CD38, CD45 (bright), TCR gamma-delta, but were negative for CD4, CD8, and TdT. Karyotype was 46,XY,t(8;14)(q24;q11.2),der(12)t(12;20)(q11;q13.3),der(20)t(12;20)(q21;q13.3) with rearrangement of MYC (8q24) confirmed by FISH. He was treated for T-ALL per Children’s Oncology Group (COG) protocol AALL0434 and had 5% residual bone marrow blasts at day 29 of induction. MRD-negative remission was ultimately achieved with high-dose cytarabine and methotrexate followed by consolidation with nelarabine. He underwent matched unrelated cord blood transplant following conditioning with cyclophosphamide and total body irradiation with cranial boost, and engrafted at day +14. Surveillance bone marrow at day +110 revealed systemic juvenile xanthogranuloma (JXG) without T-ALL. PET/CT revealed FDG-uptake in the diffusely enlarged spleen and throughout the skeleton. Due to progressive cytopenias, therapy was initiated with vinblastine and prednisone as per LCHIII. However, refractory cytopenias exacerbated by splenic sequestration developed following induction, and he was then treated with thalidomide and splenectomy. The spleen weighed 404 grams (expected 40 grams) and was diffusely infiltrated with JXG. The cytopenias dramatically improved and he continued thalidomide for 2 months until PET scan demonstrated progression. Genomic analysis of the JXG lesion revealed NRAS G13D mutation, and FISH demonstrated MYC rearrangement identical to the initial T-ALL sample.

Case 2
A 12-year-old male presented with WBC 142,700 cells/uL and CNS leukemia. Flow cytometry showed T-ALL with CD2, surface CD3, CD4, CD5, CD7, CD8, CD24 (subset), CD71, HLA-DR (subset) and TdT (partial). There was a clonal TCR gamma gene rearrangement and a biallelic CDKN2A (p16) deletion by FISH. He was enrolled on COG AALL0434 and had a rapid response with remission in both CNS and marrow at induction day 29. Following completion of high-dose methotrexate interim maintenance he developed hepatosplenomegaly, pancytopenia and elevated serum bilirubin, ferritin, and triglycerides. Bone marrow aspirate showed rare hemophagocytosis but no evidence of T-ALL. He was treated with dexamethasone and etoposide with no response. Follow-up bone marrow revealed brisk hemophagocytosis and a diffuse histiocytic neoplasm. Karyotype was 48,XY,+7,+11[2] /49,idem,+18[3] /46,XY[14]. PET/CT showed hepatosplenomegaly with FDG uptake in anterior mediastinum, hepatic nodules, spleen, and bone marrow. He was treated with Campath and then with intensive chemotherapy with fludarabine, cytarabine, and liposomal daunorubicin with no response and ultimately succumbed to disease. Genomic analysis of the clonal histiocytic infiltrate revealed KRAS G12C, BRAF G469V, NOTCH1 Q2440, and CCND2 G268R mutations, and FISH positive for biallelic CDKN2A (p16) deletion similar to original T-ALL.

Conclusions
Our extensive genomic characterization suggests a unique molecular pathogenesis for histiocytic disorders arising after T-cell ALL and identified RAS signaling pathway and NOTCH1 mutations. Furthermore, these findings strongly indicate a potential derivation or trans differentiation from the malignant leukemic stem cell clone.

Disclosures:
No relevant conflicts of interest to declare.