Aneurysmal bone cysts and pathologic fracture associated with supernumerary ring chromosome 6 in two unrelated patients

Supernumerary chromosome 6 marker associated with paternal uniparental isodisomy of chromosome 6 in a patient with a syndromic disorder of insulin secretion

The association of both uniparental disomy and small supernumerary marker chromosomes is rare. Clinical impact depends on the presence of imprinted genes and/or the unmasking of a recessive mutation of the chromosome involved in the uniparental disomy and the euchromatic content of the sSMC. Here, we report on the second case of a patient harbouring a de novo supernumerary marker chromosome 6 causing partial trisomy 6p12.3p11.1 associated with a paternal uniparental isodisomy of chromosome 6. Our patient presented with intrauterine growth retardation, macroglossia, initial developmental delay and transient neonatal diabetes mellitus followed by a congenital hyperinsulinism. Diabetes and intrauterine growth retardation can be linked to the paternal isodisomy of the imprinted locus on chromosome 6q24 whereas developmental delay is probably due to the small supernumerary marker chromosome. However, the clinical impact of partial trisomy 6p is difficult to address due to a limited number of patients. The careful clinical examination and the molecular characterization of additional patients with trisomy 6p are needed to further predict the phenotype for genetic counselling. Finally, uniparental disomy should be considered when a sSMC involving a chromosome containing imprinted regions is detected, especially in the prenatal setting.

Fusion of the Lumican (LUM) Gene With the Ubiquitin Specific Peptidase 6 (USP6) Gene in an Aneurysmal Bone Cyst Carrying a t(12;17)(q21;p13) Chromosome Translocation

BACKGROUND/AIM

Aneurysmal bone cyst is a benign bone lesion with a strong tendency to recur. The rearrangement of chromosome band 17p13/USP6 gene is now considered a characteristic genetic feature of aneurysmal bone cyst, with t(16;17)(q22;p13)/CDH11-USP6 as the most frequent chromosomal aberration/fusion gene. We report a novel variant translocation leading to a new fusion gene in an aneurysmal bone cyst.

MATERIALS AND METHODS

Genetic analyses were performed on an aneurysmal bone cyst found in the tibia of a child.

RESULTS

G-banding chromosome analysis yielded the karyotype 46,XX,t(12;17)(q21;p13)[5]/46,XX[2]. FISH analysis with a USP6 break-apart probe showed rearrangement of USP6. RNA sequencing detected LUM-USP6 and USP6-LUM fusion transcripts which were subsequently verified by RT-PCR/Sanger sequencing. The two genes exchanged 5'- non-coding exons. Thus, promoter swapping between USP6 and LUM had taken place.

CONCLUSION

We report a novel t(12;17)(q21;p13) chromosome translocation which gave rise to a LUM-USP6 fusion in an aneurysmal bone cyst.

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)×2[2]dn. Additionally, molecular karyotyping and molecular cytogenetics confirmed the breakpoints and characterized a 1.3 Mb 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~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.