Clinical Characterization and Cytogenetic-Molecular Study of a Patient with a Ring Chromosome 12.

Introduction: Ring chromosomes generally result from the fusion of breaks at the ends of both arms of a chromosome, typically leading to the loss of genetic material from these ends.

Case presentation: We report the case of a four-year-old patient who exhibits multiple café-au-lait spots, hypochromic spots, bitemporal narrowing, bilateral epicanthus, patent foramen ovale, and multiple brain abnormalities identified through magnetic resonance imaging. Karyotyping revealed a ring chromosome: r(12). Chromosomal microarray analysis revealed a ∼3.6 Mb deletion in the short arm and a ∼1.2 Mb deletion in the long arm of chromosome 12. The patient's phenotype is consistent with these genetic imbalances. To investigate ring chromosome instability and mosaicism, 200 metaphases were analyzed using G-banding and FISH with a whole chromosome painting probe, identifying 166 cells with a 46,XX,r(12) karyotype, 20 cells with monosomy 12, eight with a dicentric ring chromosome, three with two monocentric rings, two with two interlocked dicentrics, and one with an open ring chromosome.

Conclusion: This study provided a detailed characterization of the ring rearrangement of chromosome 12, the first with SNP array, enhancing the understanding of its genetic and phenotypic implications and contributing to expanding knowledge about this condition. Additionally, the findings can aid in better understanding the patient's prognosis, clinical follow-up, and genetic counseling.