Partial Monosomy 21q Due to De Novo t(15;21)(q26.3;q22.11): A Case Report with Clinical and Molecular Findings.

Background: Partial monosomy of chromosome 21q is a rare genetic disorder characterized by a wide spectrum of clinical manifestations including intellectual disability, developmental delay, and distinctive craniofacial features. Concurrent deletions involving chromosome 15q26 are also infrequent and typically benign. Objective: This study reports a rare case of de novo unbalanced translocation between chromosomes 15q26.3 and 21q22.11, resulting in partial monosomy 21q and a benign deletion of 15q26.3, highlighting the importance of comprehensive cytogenetic and molecular analysis. Methods: Peripheral blood samples from the proband and her parents were analyzed using GTG-banding karyotype, fluorescence in situ hybridization (FISH), and whole-genome oligo-array comparative genomic hybridization (array CGH). Results: The proband, a 36-year-old woman with intellectual disability and developmental delay, exhibited a karyotype of 45,XX,der(15)t(15;21)(q26.3;q22.11),-21. Array CGH revealed a 17.32 Mb deletion at 21q11.2q22.11 encompassing 37 genes, and a benign 673 kb deletion at 15q26.3 involving 13 genes. Clinical features included multiple craniofacial dysmorphisms, low birth weight, short stature, and dental anomalies. Conclusion: This case represents the first reported instance from Iran of a pathogenic partial monosomy 21q due to an unbalanced translocation with chromosome 15q26.3. The findings underscore the critical role of integrated cytogenetic and molecular diagnostics in identifying complex chromosomal rearrangements and contribute to the understanding of genotype-phenotype correlations in partial monosomy 21q.