Widespread Loss of Heterozygosity and Endoreduplication in Odontogenic Myxoma: Expanding the Clinicopathologic Spectrum of An Enigmatic Odontogenic Neoplasm.

Odontogenic myxoma (OM) is an uncommon, locally aggressive odontogenic neoplasm with characteristic histologic and clinico-radiographic features but with potential for histologic overlap with other odontogenic and non-odontogenic entities and a non-specific immunoprofile. Widespread loss of heterozygosity (LOH) has been recently described in rare cases of OM. The aim of this study was to determine whether widespread LOH represents a recurrent molecular signature that can be leveraged for clinical decision-making. Allele-specific copy number variation data from chromosomal microarray were generated from 7 OM, comprising a combined prospective and retrospective cohort. Four tumors arose in the mandible and 3 in the maxilla in patients ranging in age from 18 to 94 years (median: 44), with tumor size ranging from 2.2 to 13.0 cm. Variable amounts of fibrous stroma (odontogenic "fibromyxoma") were present in 4/7 OM, and hypercellularity not typically appreciated in conventional OM was present in 3/7 cases. All OM (7/7) demonstrated widespread LOH, with 5 cases showing a near-haploid/low hypodiploid genomes (multiple monosomies) and 2 cases showing evidence of pseudo-hyperdiploidy due to probable endoreduplication. Both pseudo-hyperdiploid cases were ≥10 cm in size; 1 represented local recurrence. Chromosomes 1 to 3, 6, 9, 11, 13, 15, and 22 demonstrated LOH in ≥75% of cases (chromosomes 1 to 3, 6, and 9 in 100% of cases), while chromosomes 5, 12, 19, and 20 universally retained heterozygosity. Altogether, widespread LOH is a recurrent event in OM and a novel finding in odontogenic pathology, and allele-specific copy number variation analysis can serve as a diagnostic adjunct in challenging cases.