Acute Lymphoblastic Leukemia Natural History in Neurofibromatosis Type 1 Monozygotic Twins
Authors: Galbiati Marta , Lettieri Antonella , Songia Simona , Morerio Cristina , Micalizzi Concetta , Dufour Carlo , Biondi Andrea , Cazzaniga Giovanni 
Published: October 2019

Abstract
Abstract 2414

Introduction:
Neurofibromatosis 1 (NF1) is an autosomal dominant disease, caused by mutation of neurofibromin (NF1) gene, located in the 17q11.2 chromosomal region. Children affected by NF1 have an increased risk to develop tumors, including leukemia. It has been suggested that NF1 aberrancy is a tumor predisposing factor, and that secondary events in somatic cells gives rise to the formation of neoplasm. However, the clonal evolution from NF1 mutation to tumor development has never been elucidated. Monozygotic twins constitute an ideal tool to understand the clonal evolution of leukemia in the context of a common genetic background.

Patients:
We performed the genomic characterization of a pair of monozygotic twins with diagnosis of NF1, who developed concordant B-cell precursor ALL at the age of 6 (Twin 1, T1) and 6.5 years (T2). The diagnostic and relapse samples showed common and unique lesions, allowing us to hypothesize a model for leukemia clonal evolution. At diagnosis, T1 had common ALL with 45,XX,-7,del(9)(p12),del(10)(q23)[8]/46,XX[12] whereas T2 had common ALL with 45,XX,-7,del(10)(q22)[14]. Both were treated according to AIEOP-BFM ALL 2000 protocol; T1 was classified as MRD high risk (HR), and T2 as MRD Intermediate risk (IR). T1 underwent BMT 8 months after diagnosis, and achieved clinical remission, still persisting, whilst T2 relapsed 13 months after diagnosis with 47,XX,del(9)(p21),+21[11]/46,XX[5]; she entered AIEOP REC 2003 protocol and, after achieving second complete remission and negativity for MRD, underwent allogeneic HLA-compatible BMT; she is in clinical remission 3.5 years after BMT.

Results:
Genome-wide copy number alteration (CNA) analysis by Cytogenetics Whole Genome 2.7M Arrays (Affymetrix) indicated that T1 and T2 shared a copy number neutral Loss Of Heterozygosity (LOH) of 17q arm, were the NF1 gene is located. At diagnosis, both had chromosome 7 monosomy but showed completely different Ig/TCR rearrangements. Three additional clonal rearrangements were found in T2 relapse sample. By backtracking the rearrangements, the relapse clone was detected in about 1% of T2 diagnostic cells, but not in T1 diagnosis sample.
Several twin-specific abnormalities were detected: both had a deletion involving the q arm of chromosome 10, with different extent: del(10)(q23.33) in T1 and del(10)(q23.1) in T2. Moreover, T1 carried hemizygous deletions on chromosome 9 and a focal loss of 6q15 locus, containing the B-cell specific transcriptional regulator BACH2. Genomic analysis of T2 revealed a gain in the telomeric region of chromosome 2 and heterozygous losses in 14q32.13 and 15q21.3 regions, involving BX247990 (human full-length cDNA clone of B cells) and TCF12 (transcription factor 12) genes, respectively. The T2 relapse showed LOH 17q and del(15)(q21.3), as found in her diagnostic sample, and several additional CNAs, including the heterozygous deletions of IKZF1 (7p12.2), ETV6 (12p13.2) and C20orf94 (20p12.2) genes, trisomy of chromosome 21 and LOH of chromosome 20.

Discussion:
This is the first study that describes the natural history of events in NF1 patients (with 17q LOH) who developed ALL.
The unique feature common to both twins in diagnosis and relapse samples was 17q LOH, that probably arose prenatally in a common hematopoietic progenitor (before somatic recombination) of one twin and spread to the other twin through intraplacental circulation. After birth, an independent clonal evolution is sustained by different Ig/TCR rearrangements as well as specific and different oncogenetic lesions. This model support a preleukemic state followed by at least-two-step mechanism for progression to leukemia. The absence of monosomy 7 at relapse indicates an independent and postnatal acquisition of this lesion; and that relapse in T2 occurs from a pre-leukemic clone before monosomy 7, which further evolved through acquisition of additional abnormailities, commom in BCP-ALL (i.e. ETV6 and Ikaros deletions).
The role of BACH2 and TCF12 gene deletions in NF1 and ALL must be further explored. BACH2 is a human B-cell specific transcriptional repressor already described as tumor suppressor gene, involved in IgH@ translocations. TCF12 (also named HEB) is a basic helix-loop-helix transcription factor involved in B and T cell commitment. The altered expression of these genes could contribute to the differentiation arrest and the uncontrolled proliferation of leukemic blasts.

Disclosures:
No relevant conflicts of interest to declare.