Unique Familial MLL-rearranged Precursor B Cell Infant Acute Lymphoblastic Leukemia (ALL) in Non-Twin Siblings

Abstract 2417

Leukemia is the commonest malignancy in infants, the most frequently occurring form of which is infant ALL. When ALL occurs in infants the disease is clinically aggressive and associated with poor outcome. MLL gene rearrangements producing transforming fusion oncoproteins are found in 75% of infant ALL and they are adverse prognostic factors. Infant ALL has never occurred in families except in monozygous twins, where concordance in leukemia occurrence is nearly 100%. Identical but non-germline genomic breakpoint junction sequences have pointed to an in utero origin of MLL gene rearrangements in these twin cases, where it is believed that metastasis of cells with the rearrangement occurs from one twin to the other via the placental circulation. Here we describe two highly novel siblings both deceased from precursor B cell infant ALL (ages at diagnosis: proband 160 d, sibling 121 d). Remarkably, the second of these two decedents is survived by a now 3 year-old monozygous twin who is as yet unaffected.

MLLrearrangements in the leukemia blasts of both affected siblings were characterized by conventional cytogenetics and/or FISH, M-FISH, high resolution Illumina 610K Bead Chip SNP array and Southern blot analysis. MLL genomic breakpoint junction sequences and fusion transcripts were defined using panhandle PCR approaches, PCR with gene-specific primers and reverse transcriptase PCR. The twins were confirmed to be monozygous using the genotype calls from SNP array analysis of the peripheral blood and bone marrow of the unaffected and affected twins, respectively. Quantitative real-time PCR analysis of leukemia DNA was used to determine the allele specific sequences of the NQO1 (NADPH quinone oxidorecutase 1) gene, an inactivating polymorphism in which previously was implicated as a risk factor for MLL-rearranged infant ALL.

The complex karyotype in the leukemia cells of the proband was 46, XX, der(2) t(2;3) (q3?;?), der(3) ?t(3;4;11), del(4) (q21), der(11) ?del(11) (p11.2) t(3;11) (?;q23).ish der(3) (5'MLL+), der(11) (3'MLL+) [14]/46, XX[8], suggesting extensive damage to the genome. Consistent with a three-way t(3;4;11) translocation, two alternately spliced 5'-MLL-MLLT2(AF-4)-3' fusion transcripts were identified, indicating disruption of the chromosome band 4q21 partner gene MLLT2. Also consistent with the three-way translocation, reverse panhandle PCR detected a 5'-partner-MLL-3' genomic breakpoint junction fusing 3' MLL to the upstream region of a highly novel chromosome 3 gene encoding a nucleotidyltransferase fold protein C3ORF31 (Accession no. NM_138807; Kuchta, 2009). Unlike in the proband, the ALL of the affected twin exhibited a simple t(4;11)(q21;q23) translocation, the reciprocal genomic breakpoint junctions of which fusing MLL and MLLT2 also have been characterized. Similar to non-familial infant ALL, the genomic breakpoint junctions in both infants contained sequence features of nonhomologous end joining DNA repair. The different MLL gene rearrangements in the leukemia cells in the affected siblings indicate that the translocations were not hereditary. Even though the NQO1 inactivating polymorphism is one genetic risk factor for infant ALL, the NQO1 genotype was wild-type in both affected siblings.

Further studies in this uniquely afflicted family with two siblings who succumbed to infant ALL and a monozygous twin of one of the decedents surviving beyond infancy unaffected, will provide a one-time opportunity to capture novel mutations predisposing to the development of, and cooperating with, MLL translocations in infant ALL. The MLLT2 involvement in both cases has even further implications for the knowledge to be gained because the MLL-MLLT2 rearrangement occurs in 50% of infant ALL and adversely impacts outcome.