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Mosaic Trisomy 16 Please Help Us to Help You! Support and Information Rare Chromosome Disorder Support Group, The Stables, Station Road West, Oxted, Surrey RH8 9EE, United Kingdom Tel: +44(0)1883 723356 [email protected] I www.rarechromo.org Unique is a charity without government funding, existing entirely on donations and grants. If you are able to support our work in any way, however small, please make a donation via our website at www.rarechromo.org/donate Mosaic Trisomy 16 Please help us to help you! Disorders of Chromosome 16 Foundation www.trisomy16.org Unique mentions other organisations’ message boards and websites to help families looking for information. This does not imply that we endorse their content or have any responsibility for it. 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 medical content was verified by Dr Wendy Robinson, Associate Professor of Medical Genetics, University of British Columbia and Monica Pearson BSc 2004. Version 2 2014 (PM) Copyright © Unique 2019 Rare Chromosome Disorder Support Group Charity Number 1110661 rarechromo.org Registered in England and Wales Company Number 5460413 8 How common is MT16? Why did this happen? What is Mosaic Full trisomy 16 with an extra chromosome 16 in Almost all MT16 pregnancies start as a pregnancy with a complete extra every cell is believed to be the most common of chromosome 16. During the formation of the egg cells in the mother, an error Trisomy 16? all chromosome abnormalities at conception occurs in the natural process by which chromosomes separate. This leaves an (when a baby is made), and is found in 1 to 2 per extra chromosome 16 in the egg cell. Much more rarely the same mistake There are usually 46 cent of all confirmed pregnancies. occurs when the father’s sperm cells are forming. After conception, there are chromosomes in the However, babies with full trisomy 16 are almost three chromosome 16s, usually two from the mother and one from the father. cells of the body. always lost in very early pregnancy and virtually A natural attempt to correct the mistake after conception is then partly Chromosomes come in none have been born. Even babies with MT16 successful, leaving two cell lines, one with the extra chromosome 16, the other different sizes and are without. This process is called trisomy correction or trisomy rescue. are very rare. Researchers who surveyed every numbered from largest MT16 pregnancy published in the medical to smallest according to Trisomy rescue and skewed X-inactivation literature gathered 162 pregnancies worldwide their size, from number by 2003. Among these pregnancies they The mechanism that results in trisomy rescue also leads to a situation known as 1 to number 22, in confirmed that mosaic trisomy 16 appears to be 100% skewed X-chromosome inactivation. This means that a baby transcribes addition to the sex slightly more common in girls than in boys genes from either a maternal X chromosome or the paternal X chromosome, but chromosomes, X and Y. (Yong 2003; Benn 1998). not both. Research suggests that this seems to be linked with poorer outcomes We have two copies of at birth but it should have no important effects afterwards (Penaherrera 2000). each of the The only likely effect is that in a girl, the risk of rare X-linked recessive disorders chromosomes (23 Chromosome A note (such as muscular dystrophy or haemophilia) might rise to that of a typical boy. pairs), one inherited This leaflet describes what is 16 from the father and one known from published medical Can it happen again? inherited from research and from the There is no evidence that couples are likely to have another baby affected by the mother but experience of Unique and of the MT16. However, if the original trisomy was caused by a failure of the mother’s someone with mosaic Disorders of Chromosome 16 chromosomes to separate when her egg cells were forming and she is an older trisomy 16 (MT16) Foundation support group. mother (over 35), she may have a raised chance of having another pregnancy has some cells with a Although recent research into affected by trisomy. third chromosome 16, MT16 is of a very high quality, making 47 children who come to doctors’ Research chromosomes in attention and feature in research There is a research group at the University of British Columbia that is interested those cells. reports are likely to be the ones in two aspects of pregnancies affected by confined placental mosaicism: Cells with 46 and 47 who have experienced effects on They study the placenta to try to understand what makes it not function as chromosomes exist their health or development from well to nourish the baby. alongside each other their unusual chromosomes. in different body tissues Those who experience few or no They are interested in how the babies develop through childhood and if and organs, exerting health problems will not be there is any long-term impact on health. different effects. described. This means that the The situation changes, picture we paint may appear To find out more, look at these websites: with the number of more negative than it really is. http://mosaicism.cfri.ca/specific.htm cells with the extra References to medical research http://robinsonresearch.ca/ chromosome usually are shown in brackets and a full falling in time, reference list is available especially in blood. from Unique. 2 7 Will my baby survive? How is mosaic trisomy 16 diagnosed in pregnancy? The outlook for mosaic trisomy 16 pregnancies used to be thought to be bleak. Mosaic trisomy 16 is usually first suspected and diagnosed during pregnancy. But a large survey of 162 pregnancies showed a much more optimistic picture. The first tests are non-specific. Usually either ultrasound scans or blood tests Two out of three pregnancies ended in the birth of a baby and the great majority for certain chemicals (maternal serum screening) suggest there may be a of these babies survived the newborn period. Seventeen per cent of babies died problem and a general test for the most common chromosome abnormalities is before birth or in the newborn period (Yong 2003). No long term studies of the performed either on the developing placenta (with chorionic villus sampling, outcomes have been published yet. However, the short term outcomes for CVS) or on amniotic fluid (from an amniocentesis). If this shows trisomy 16 in babies looked very reassuring. Of the babies born with MT16, 93 per cent some cells, the pregnancy has mosaic trisomy 16. survived the newborn period and so long as babies are in good health at birth There are three possible outcomes for the baby: they are believed to usually progress quite well. You may find it helpful to read • Either the only effects are on the baby’s growth rate the stories on the website of the DOC16 Foundation. • Or the baby will be apparently unaffected Common effects on babies and children • Or the baby will be affected. Two surveys showed that most babies with MT16 are born small for their You will be offered tests to try to clarify which is the case for you. gestational age but are otherwise healthy. Isolated medical conditions have been The tests that show that mosaic trisomy 16 is present cannot unfortunately found in a minority of babies, but most of these were no more common than show directly how slightly or severely affected your baby will be. However, when among babies without MT16. Just two conditions did occur more commonly – the tests are put together with information from ultrasound scans, a clearer although they were still unusual: a hole in the heart (between the left and right picture will emerge. chambers) and in boys, hypospadias, a condition where the hole normally at the end of the penis is on the underside instead (Yong 2003; Benn 1998). When your baby is examined, the clinicians will be particularly alert to the body CVS (chorionic villus sampling) tissues and systems listed below. This is because they have been most often Two types of early pregnancy CVS test can be done “In the end a affected in the minority of babies with MT16 who have been found to have a on the cells from the developing placenta. couple has to clinical condition. A rapid test (called direct sampling) gives brace themselves • Heart • Lungs • Hands • Kidneys, urinary and genital systems a result within a day or two. for the possible The placenta cells are cultured (grown), with • Digestion and intestines • Head and face • Joints and bones • Skin worst case the results in a week or two. scenario where Growth You should wait for both results. Most often, they the baby would The typical growth delay before birth is generally attributed to inadequate show that all the cells in the developing placenta placental functioning. This means that growth after birth may be normal but we have the extra chromosome 16. Less often, they have undetected don’t know yet whether children completely catch up in height. show a mixture of trisomy 16 cells and ordinary birth defects, cells. This means that the placenta does contain developmental Two copies of the mother’s chromosome 16 trisomy 16 cells but usually none are found in delay or learning amniotic fluid and your baby does not have any. This Children without mosaic trisomy have two chromosome 16s, one from their is called confined placental mosaicism.
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