B cell precursor (BCP) ALL is usually a monoclonal disease in which the number of IGH rearrangements per cell does not exceed the number of the IGH alleles on chromosome 14. Consequently, a clone with disomy 14 can have a maximum of two unique rearrangements. In contrast, monoclonal high hyperdiploid (HeH) cases with a trisomy 14 can harbor either a maximum of three unique or two unique rearrangements together with a third that may share particular sequences with the one or the other The pattern of IGH rearrangements in cases with trisomy 14 may therefore be misinterpreted to be oligoclonal if the chromosome 14 copy number is not known. Since oligoclonal IGH rearrangements may be instable at relapse, they generally are not used for minimal residual disease (MRD) analysis. Thus, in HeH patients seemingly oligoclonal IGH rearrangements may undeserved be skipped as MRD target. We investigated the association between IGH allele copy numbers and the IGH rearrangement patterns in 90 consecutively recruited HeH BCP ALL. This cohort was used for assessing overall frequencies. To enrich the number of small subgroups, 40 selected HeH cases were added. Cytogenetic and FISH analyses were performed according to standard procedures. IGH rearrangements were determined according to standardized ESG-MRD protocols. Even though the majority of HeH cases (78/90, 87%) had an extra chromosome 14, there was a small but distinct subgroup comprising 13% (12/90) of HeH cases with a disomy 14. Overall, IGH rearrangements were present in about 95% of leukemias representing incomplete DJH rearrangements in about 40% of cases. More than two IGH rearrangements and/or related rearrangements were found in 44% of the same HeH cohort with an overall frequency of 16% “true” oligoclonality after correction for the actual number of chromosomes 14. Of note, leukemias with only two copies of chromosome 14 revealed a significantly higher frequency of apparent oligoclonality compared to those with three copies of chromosome 14 (36% versus 13%). Monoclonal HeH leukemias with trisomy 14 could neither be distinguished from their oligoclonal counterparts nor from oligoclonal TEL-AML1 positive and “not further genetically discriminated” BCP ALLs based on the number of IGH rearrangements per cases and the type of secondary rearrangements (VH or DH to DJH or VH replacement). However, the patterns of secondary rearrangements had shifted from a predominantly VH to DJH recombination in the former towards VH replacement in the latter two groups. Our data have implications for MRD analysis, since oligoclonal patterns of IGH rearrangements account for about 25–30% of childhood BCP ALL. Hence, the interpretation of whether a particular IGH rearrangement pattern is really clonal or not may be crucial in some of these cases and may be better defined by taking into account at least the genetic subtype of the respective leukemia (i.e. hyperdiploid versus those with various fusion genes). If necessary the quality of this information can be further refined by enumerating chromosomes 14 with karyotyping or IGH alleles with interphase FISH. The data provide also insights into the biology of HeH leukemia suggesting that nondisjunction of chromosomes - leading to a HeH karyotype - affects a cell at the beginning of IGH recombination, which is a more undifferentiated B progenitor cell than the cell of origin of the TEL-AML1 positive leukemias and the group of other BCP ALLs.

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