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Interstitial deletion of 13q and a 13;X chromosome translocation results in partial trisomy 13 and bilateral retinoblastoma

Background: Abnormalities of chromosome 13 have been associated with bilateral retinoblastoma. Deletion of a retinoblastoma gene is a common primary mechanism. Other abnormalities are more rare. To our knowledge, a balanced translocation of the long arms of the X and 13 chromosomes associated with bilateral retinoblastoma has been reported five times. We report an unbalanced X;13 translocation resulting in partial trisomy 13 and an interstitial deletion of an RB locus. Methods: Case report. Results: A 19-month-old child presented with seizures to the emergency department. A CT scan revealed bilateral intraocular calcification, and retinoblastoma (RB) was confirmed with an ophthalmic exam. Abnormal facies and developmental delay were noted. A partial trisomy derived from the translocation of X;13 was observed in both bone marrow and peripheral blood cells. Fluorescence in-situ hybridization (FISH) studies confirmed triplication of a region on the q arm of chromosome 13 spanning the RB locus. One of the normal chromosome 13 homologues had an interstitial deletion of the RB locus since no signal was observed for the RB-1 probe despite the visible presence of the 13q14 region. Additional evidence of the interstitial deletion is supported by the typical facial features and developmental delay found. Presumably, the translocated X underwent X inactivation precluding systemic features typically observed in trisomy 13. Parental karyotypes were normal. The chromosomal abnormality was a de-novo constitutional event. Conclusions: Only two RB loci were present in this patient despite triplication of 13q. The third locus was deleted. We believe that the second locus was not expressed due to X inactivation of the RB gene on the der(X)t(Xq:13q) chromosome. The emergence of bilateral retinoblastoma points towards lack of heterozygosity at the third and last remaining RB loci in tumor cells. To our knowledge, an unbalanced translocation resulting in partial trisomy 13 with retinoblastoma has not been previously reported.