Ring chromosome


A ring chromosome is an aberrant chromosome whose ends have fused together to form a ring. Ring chromosomes were first discovered by Lilian Vaughan Morgan in 1926. A ring chromosome is denoted by the symbol r in human genetics and R in Drosophila genetics. Ring chromosomes may form in cells following genetic damage by mutagens like radiation, but they may also arise spontaneously during development.

Formation

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, the telomeres at the ends of a chromosome fuse without any loss of genetic material, which results in a normal phenotype.
Complex rearrangements, including segmental microdeletions and microduplications, have been seen in numerous ring chromosomes, providing important clues regarding the mechanisms of their formation.
Small supernumary rings can also form, resulting in a partial trisomy.
Ring chromosomes are unstable during cell division and can form interlocking or fused rings.

Associated syndromes

Human genetic 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 the telomeric regions of affected chromosomes, rather than by the formation of a ring structure itself. Almost all ring chromosome syndromes feature marked growth delay.
Ring chromosomes can be inherited or sporadic. Mosaicism is common and affects the severity of the condition. 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:
ChromosomeTypical features
Ring chromosome 1Mental retardation, microcephaly, facial abnormalities
Ring chromosome 2Small stature
Ring chromosome 3
Ring chromosome 4Craniofacial abnormalities
Ring chromosome 5
Ring chromosome 6Microcephaly, facial abnormalities, hand abnormalities
Ring chromosome 7Craniofacial abnormalities, speech deficits
Ring chromosome 8Craniofacial abnormalities, hydronephrosis, hand abnormalities
Ring chromosome 9Delayed growth, abnormal facial features, low muscle tone
Ring chromosome 10Mental retardation, delayed growth, facial dysmorphia, reproductive abnormalities
Ring chromosome 11
Ring chromosome 12Delayed growth, abnormal facial features, microcephaly
Ring chromosome 13Microcephaly, delayed growth, reproductive abnormalities
Ring chromosome 14Epilepsy, mental retardation
Ring chromosome 15Growth delay, mental retardation, microcephaly, speech delay
Ring chromosome 16Mental retardation, microcephaly, growth delay, facial abnormalities
Ring chromosome 17
Ring chromosome 18Mental retardation, growth delay, facial abnormalities
Ring chromosome 19
Ring chromosome 20Epilepsy, mental retardation, abnormal facial features, growth delay
Ring chromosome 21Short stature, microcephaly, reproductive abnormalities
Ring chromosome 22Mental retardation, hypotonia, autistic-like behavior
Ring chromosome XTurner syndrome
Ring chromosome Y