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20P Deletions FTNS Support and Information Rare Chromosome Disorder Support Group, G1, The Stables, Station Road West, Oxted, Surrey RH8 9EE, United Kingdom Tel/Fax: +44(0)1883 723356 [email protected] III www.rarechromo.org Join Unique for family links, information and support. Unique is a charity without government funding, existing entirely on donations and grants. If you can, please make a donation via our website at www.rarechromo.org Please help us to help you! 20p deletions Chromosome 20 and beyond is a Facebook community for anyone affected by a disorder of chromosome 20 www.facebook.com/groups/507113779406575 Unique lists external message boards and websites in order to be helpful to families looking for information and support. This does not imply that we endorse their content or have any responsibility for it. This information guide is not a substitute for personal medical advice. Families should consult a medically qualified clinician in all matters relating to genetic diagnosis, management and health. Information on genetic changes is a very fast-moving field and while the information in this guide is believed to be the best available at the time of publication, some facts may later change. Unique does its best to keep abreast of changing information and to review its published guides as needed. The guide was compiled by Unique and reviewed by Dr Jaime Garcia-Heras, Director of Cytogenetics, Center for Medical Genetics, Houston, Texas, US 2007, and by Professor Maj Hultén BSc PhD MD FRCPath, Professor of Reproductive Genetics, University of Warwick, UK 2008. (PM) Copyright © Unique 2008 Rare Chromosome Disorder Support Group Charity Number 1110661 Registered in England and Wales Company Number 5460413 rrraraaarrrreeeecccchhhhrrrroooommmmoooo....oooorrrrgggg 12 Deletions from chromosome 20p Behaviour One child was diagnosed with autism at four years, another with Asperger A chromosome 20p deletion is a rare genetic condition caused by the loss of material syndrome at nine years, suggesting the possibility of a specific difficulty with social from one of the body’s 46 chromosomes. The material has been lost from the short communication. arm (the top part in the diagram on the next page) of chromosome 20. How did this happen? Chromosomes are the structures in the nucleus of the body’s cells that carry the A blood test to check both parents’ chromosomes is needed to find out why the 20p genetic information that controls development and function. In total every human deletion occurred. Some 20p deletions occur when both parents have normal individual normally has 46 chromosomes. Of these, two are a pair of sex chromosomes. The term geneticists use for this is de novo (dn ). De novo 20p deletions chromosomes, XX (a pair of X chromosomes) in females and XY (one X chromosome are thought to be caused by a change that occurred when the parents’ sperm or egg and one Y chromosome) in males. The remaining 44 chromosomes are grouped in cells were formed or possibly just after fertilisation. We know that chromosomes must pairs. One chromosome from each pair is inherited from the mother while the other break and rejoin in quite a complex process when egg and sperm cells are formed but one is inherited from the father. Each chromosome has a short arm (called p) and a this only occasionally leads to problems. This is part of a natural process of inheritance long arm (called q). and as a parent there is nothing you could have done to change it. Children from all Chromosome 20 is one of the smallest chromosomes in man. At present it is known to parts of the world and from all types of background have chromosome 20p deletions. contain 737 genes out of the total of 20,000 to 25,000 genes in the human genome. No environmental, dietary or lifestyle factors are known to cause them. So there is You can’t see chromosomes with the naked eye, but if you stain them and magnify their nothing you did before you were pregnant or during pregnancy that caused this to image enough - about 850 times - you can see that each one has a distinctive pattern of occur and there is nothing you could have done to prevent it. light and dark bands. The diagram on the next page shows the bands of chromosome Other 20p deletions are the result of a rearrangement in one parent’s chromosomes. 20. These bands are numbered outwards starting from the point where the short and This is usually a rearrangement known as a balanced translocation in which material has long arms meet (the centromere ). A low number, as in p11 in the short arm, is close swapped places between chromosomes. As no genetically relevant material has been to the centromere. Regions closer to the centromere are called proximal . A higher lost or gained, the parent usually has no clinical or developmental problems, although number, as in p13, is closer to the end of the chromosome. Regions closer to the end they may have difficulties with fertility or childbearing. Balanced translocations involving of the chromosome are called distal . The tips of the chromosomes are called one or more chromosomes are not rare: one person in 500 has one, making a total telomeres . The areas close to the tips are the subtelomeric regions. world population of over 13 million balanced translocation carriers. Under the microscope, the missing chromosome material (deletion) can usually be One individual occasionally has two cell types, one with normal chromosomes and one identified if it is large enough. Sometimes, however, the deletion is so tiny that the with a rearrangement. This is known as mosaicism . If a parent has a mosaicism chromosomes look normal under a microscope. This type of deletion is called a (normal cells and cells with the same abnormal rearrangement as the child), such a microdeletion and the loss can only be revealed by special techniques such as FISH or parent is most often more mildly affected than the child, and may not be affected at all. microarrays (array-CGH). Occasionally exactly the same deletion is found in one parent as in the child. The Your geneticist or genetic counsellor will tell you more about how much chromosome parent may be affected too, sometimes more mildly than their child. Sometimes the material has been lost from the chromosome with the deletion. You will almost parent has the same deletion as the child but has no developmental, learning, medical certainly be given a shorthand ISCN 2005 code, called a karyotype , for your child’s or behaviour problems. When this happens, a geneticist will usually decide that the 20p chromosome make-up, describing the points where the short arm of chromosome 20 deletion is not the cause of the child’s difficulties. has broken and re-joined or you will be given the results of your child’s molecular analysis. Comparing your child’s results with others, both from the medical literature Can it happen again? and within Unique , can help to build up a general picture of what to expect. But there When both parents have normal chromosomes, it is unlikely that they will have another will still be differences, sometimes quite marked, between your child and others with child with a 20p deletion. When one parent has a rearrangement of their chromosomes apparently similar deletions. These differences can be due to minute differences in the or has mosaicism, however, the risk of having another affected child is increased. If the size of the deletions, to the genes your child has on other chromosomes and to his or same deletion is present in the parent and the child, the risk of it happening again in each future pregnancy can be as high as 50 per cent. her environment from conception onwards, which act together to make him or her into an individual. In future pregnancies, higher-risk families will be offered prenatal testing to study the chromosomes of the coming baby by chorionic villus sampling, amniocentesis or both, and high-level ultrasound screening to monitor growth and identify any anomalies. In a family where one member has already been diagnosed with a 20p deletion, these tests will show whether the pregnancy is affected or not. 2 11 centromere (Garcia-Heras 2005; Michaelis1999; Unique ). For this reason it is difficult to Sources and references anticipate the likely effects of a deletion from this area. The recorded deletions, which are different from each other, are 20p11.22-p11.23, 20p11.2, and 20p11.21-p11.23. The information in this leaflet is drawn partly from published medical research papers. The following are some of the clinically significant features: We have focused on articles describing people with a ‘pure’ 20p deletion, without the Abnormalities in growth Babies whose deleted 20p comes from their mother may be involvement of other chromosome material that is likely to contribute to the effects, unusually long and large and grow into tall adults. By contrast, babies whose deleted and on articles that include a review of 20p deletions. The first-named author and 20p comes from their father may be unusually small. This suggestion is supported by publication date are given to allow you to look for the abstracts or original articles on evidence that children who inherit the two chromosome 20s from their mother (and the internet in PubMed (http://www.ncbi.nlm.gov/pubmed/). If you wish, you can none from their father) have had prenatal growth failure, have been very short at birth obtain abstracts and articles from Unique . and continued in the smallest three per cent of the population for height (Salafsky 2001; The leaflet also draws on Unique ’s database.
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