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Cases of B-ALL with chromosome counts of >60 and tetrasomic for chromosomes 6, 18,
21 and X are suspicious for “masked hypodiploidy” in that the theoretic hypodiploid
stemline is present as a doubled clone only. To characterize and define these cases
when FISH and chromosomes show no evidence of the hypodiploid clone, our laboratory
utilizes the IlluminaⓇ Cyto-SNP 850K microarray to recognize the loss of heterozygosity (LOH) of the disomic
chromosomes. We report an 18yo female with a known diagnosis of Li Fraumeni syndrome,
referred to Cytogenetics with pancytopenia and circulating blasts. Cytogenetic results
at diagnosis revealed three hyperdiploid clones with counts of 62-64, tetrasomy for
X, 1, 6, 18, and 21, with additional trisomies and structural rearrangements including
a derivative 17p with 11q. No evidence for monosomy was observed by FISH or chromosomes.
Microarray results confirmed loss of heterozygosity (LOH) for those chromosomes present
in 2 copies. Genes associated with low hypodiploid B-ALL include TP53, IKZF1 and RB1 which were present in 2 copies on the isodisomic chromosomes in this specimen. This
patient was in remission for several months, but with reemergence of B-ALL, underwent
CD19 directed CAR-T therapy which resulted in deep remission (by NGS-MRD). Recent
surveillance studies showed a karyotype change to loss of 5q and TP53 by FISH and
a dic(5;17)(q11.2;p11.2) consistent with MDS. Microarray studies confirm the MDS genetic
findings and were negative for the masked hypodiploid findings present at diagnosis.
This strategy has been useful to report chromosome results with confidence for cases
of masked hypodiploidy that may have been otherwise misinterpreted.
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