A child with intellectual disability and dysmorphism due to complex ring chromosome 6: identification of molecular mechanism with review of literature

Background: Ring chromosome 6 (r(6)) is a rare disorder that mainly occurs as a 'de novo' event. Nonetheless, a wide phenotypic spectrum has been reported in r(6) cases, depending on breakpoints, size of involved region, copy number alterations and mosaicism of cells with r(6) and/or monosomy 6 due to loss of r(6). Case presentation: An 11-year-old male was referred with developmental delay, intellectual disability and microcephaly. Physical examination revealed additionally short stature and multiple facial dysmorphisms. Banding cytogenetic studies revealed a karyotype of mos 46,XY,r(6)(p25.3q27)[54]/45,XY,-6[13]/46,XY,r(6)(::p25.3 -> q27::p25.3 -> q27::)[13]/46,XY[6]/47,XY,r(6)(p25.3q27)x2[2]dn. Additionally, molecular karyotyping and molecular cytogenetics confirmed the breakpoints and characterized a 1.3Mb contiguous duplication at 6p25.3. Conclusion: The present study has accurately identified copy number alterations caused by ring chromosome formation. A review of the literature suggests that hemizygous expression of TBP gene in 6q27 similar to qter, is likely to be the underlying cause of the phenotype. The phenotypic correlation and clinical severity in r(6) cases continue to remain widely diverse in spite of numerous reports of genomic variations.