Marker chromosome
Marker chromosomes are small chromosomes that may occur in addition to the normal chromosomes in an individual. That International System for Human Cytogenetics " defined in 1995 as a marker chromosome, an abnormal chromosome, in which no part can be identified. The inability to identify, referring to the technique of G- banding, with the larger sections of normal chromosomes can be unambiguously determined (see figure). For marker chromosomes this is not possible, usually because of the small size. A characterization is now possible, however, for example, by high-resolution fluorescence in situ hybridization. Marker chromosomes, which are smaller than the human chromosome 20 are ( for: small supernumerary marker chromosomes ) supernumerary marker chromosomes as small or with the appropriate acronym sSMC called.
Marker chromosomes contain a centromere and (mitosis ) are therefore normally passed on to the daughter cells during cell division. Marker chromosomes can also have the form of a ring chromosome. In contrast to B- chromosomes, which have developed active mechanisms in order to accumulate in the germ line, marker chromosomes are passed on to the offspring random pattern. While B- chromosomes do not occur in humans marker chromosomes have been described in a number of cases. The rate of occurrence of marker chromosomes in newborns is given as 0.043 %. This results in estimates result worldwide from 2.5 million people affected and 35,000 in Germany. Human marker chromosomes may arise from each of the 24 human chromosomes. Approximately 60% of cases occur in new, 40 % are inherited family. In addition, marker chromosomes can also occur in tumor cells ( see figure).
Clinical Significance
Whether a marker chromosome has clinical significance depends on whether, and if so, what chromosomal genes it contains and whether that is created by the presence of marker chromosomes is an imbalance in these genes. In question here is particularly partial trisomy, ie a partial triple presence of chromosomal sections instead of the normal two-time execution. It is important, therefore, the chromosomal origin and thus the genetic composition of the marker chromosome, but also whether other molecular genetic abnormalities occur in parallel, eg uniparental disomy.
About 70 % of people with a sSMC are clinically normal, 30 % are clinically apparent in varying degrees. Clinical abnormality starts out from slight physical problems or disabilities, mental retardation easier to severe physical limitations / disabilities with or without mental and / or mental disability / abnormality, such as Autism. With about 30% of marker chromosomes are made of material of chromosome 15 most often. With 11% follow isochromosomes of the short arm of chromosome 12, the Pallister - Killian syndrome. The cat-eye syndrome is another example.
The problem is the diagnosis of a marker chromosome, particularly in the prenatal diagnosis of newly occurring cases. Also, until recently, was the statement about the possible clinical implications of sSMC rather inaccurate. Currently, research on a genotype -phenotype correlation of the sSMC underway.
In the cytogenetic shorthand marker chromosomes are referred to as mar. While a man is quoted at the normal 46 chromosomes (excluding chromosome markers ) as 46, XY, a male karyotype with marker chromosomes as 47, XY, mar written, a female as 47, XX, mar.
Swell
For a short introduction see German. An English review by 2004.
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