Chromosome whose ends have fused together to form a ring
Representative karyotype from a well differentiated transitional cell carcinoma of the bladder. The chromosome indicated by "mar" represents unidentified marker, "r" represents ring chromosome. Arrowheads indicate breakpoints. Also evident are monosomy 9, 18, and X and trisomy 7.
Aring chromosome is an aberrantchromosome whose ends have fused together to form a ring. Ring chromosomes were first observed inDrosphila byLilian Vaughan Morgan in 1926 and inmaize byBarbara McClintock in 1931.[1][2] A ring chromosome is denoted by the symbolr in human genetics andR inDrosophila genetics. Ring chromosomes may form in cells following genetic damage bymutagens like radiation, but they may also arise spontaneously during development.
In order for a chromosome to form a ring, both ends of the chromosome are usually missing, enabling the broken ends to fuse together. In rare cases, thetelomeres at the ends of a chromosome fuse without any loss of genetic material, which results in a normalphenotype.[3]
Complex rearrangements, including segmentalmicrodeletions and microduplications, have been seen in numerous ring chromosomes, providing important clues regarding the mechanisms of their formation.[4]
Small supernumerary rings can also form, resulting in a partial trisomy.[5]
Ring chromosomes are unstable during cell division and can form interlocking or fused rings.[5]
Humangenetic disorders can be caused by ring chromosome formation. Although ring chromosomes are very rare, they have been found in all human chromosomes. Symptoms seen in patients carrying ring chromosomes are more likely to be caused by the deletion of genes in thetelomeric regions of affected chromosomes, rather than by the formation of a ring structure itself.[6] Almost all ring chromosome syndromes feature marked growth delay.[5]
Ring chromosomes can be inherited or sporadic.Mosaicism is common and affects the severity of the condition.[6] Location of fusion also affects severity due to loss of differing amounts of genetic material from the ends of chromosomes.
Disorders arising from the formation of a ring chromosome include:
