There are a number of genetic abnormalities associated with chromosome 16, as there are with each of the 23 pairs of chromosomes in the human body. While the field of genetic research has grown tremendously in recent years, much of the research is still in its earliest stages. The disorders caused by chromosome 16 abnormalities have just begun to be researched and more work needs to be done to help affected individuals.
A: The most common disorder of chromosome 16 is trisomy 16, in which there are three copies of this chromosome instead of the usual pair. Trisomy 16 is responsible for well over 100,000 pregnancy losses a year, representing almost 10% of miscarriages in the US.
Although full trisomy 16 is not compatible with life, there are a number of related abnormalities with which a small, but growing, number of children are born. (Additionally, some adults may have a chromosome 16 abnormality without being aware of it until they have trouble during pregnancy.)
- mosaic trisomy 16, in which some, but not all, of an individual's cells have three copies of chromosome 16.
- mosaic trisomy 16-confined to the placenta, in which the chromosome 16 abnormality is believed to be present only in the placental tissues
- uniparental disomy of chromosome 16, in which the chromosomes appear normal but both copies have originated from just one of the two parents (this is most often found in association with mosaic trisomy 16).
- partial trisomy 16 (16p+ or 16q+), in which there is an extra piece of part of chromosome 16.
- partial trisomy 16 (16p- or 16q-) in which there is a missing piece of chromosome 16.
- translocations involving chromosome 16, in which a piece of chromosome 16 is switched with a piece of another chromosome
Depending on the diagnosed disorder, a variety of medical problems may be caused by chromosome 16 abnormalities. These include: heart and other vascular problems, delayed growth, speech and physical development, reproductive disorders and kidney problems. This list is by no means complete and much more research needs to be done to determine the problems that chromosome 16 abnormalities can cause.
For more information see the page on this site called "What Are Chromosome 16 Disorders".
A: A medical geneticist or genetic counselor can explain in detail how chromosomes are inherited, the ways in which chromosome abnormality can arise, the potential clinical significance of your specific findings, and can also discuss with you the subsequent clinical management of your pregnancy. These individuals are trained to present the information in a non-directive manner.
A: Your obstetrician should be able to provide you with the name of a medical geneticist or counselor that practices in your community. Generally, they can be found at major medical centers and teaching hospitals and are often based in Genetics or Pediatrics departments.
A: Disorders of chromosome 16 (mosaicism and other anomalies) are extremely rare and few physicians have direct experience with these cases. In addition, there is very little long-term follow-up of cases detected prenatally. It is therefore likely that you will have some questions that cannot be fully answered.
A: Not necessarily. Although full trisomy 16 is incompatible with life and is a relatively common cause of first trimester spontaneous abortion, CVS (chorionic villous sample) can confirm only cells present in the placenta and not the baby. It is very unlikely that a pregnancy would survive to the time of CVS (-12 weeks gestation) if there were full trisomy 16. It is currently believed that in most cases showing 100 percent (or less) trisomy 16 in a CVS, the baby will have only or predominantly normal diploid cells, a situation often referred to as 'confined placental mosaicism'. However, there are increased risks associated with such pregnancies (see below) and it is important to follow up such findings with amniocentesis, ultrasound, and careful monitoring of maternal health.
A: Ultrasound examinations of the fetus (sonograms) can be extremely helpful for both the detection of some of the possible fetal abnormalities that can be present and also for monitoring the growth of the fetus. As the fetus grows larger, increasingly more detail can be seen by ultrasound and therefore your obstetrician may suggest more than one follow-up sonogram. Chromosome analysis of amniotic fluid cells is considered to be a highly accurate test and there generally is no advantage to repeating this test.
A: It is difficult to evaluate risks associated with a prenatal finding of trisomy 16 since it is relatively rare and cases with an abnormal outcome are more likely to be reported in scientific/medical journals than those with a normal outcome. There is also no study yet which has attempted to compile outcomes in a large series of prospectively ascertained cases. To the best of our knowledge based on information in published results as well as our own experiences (unpublished findings) we suggest the following (For a detailed summary of published reports see Benn et al. 1998 Am J Medical Genetics Sept'98):
- Situation 1. Trisomy 16
detected on CVS, normal cells only on amniocentesis: Birth weight
of such cases is almost always below average and often below the
5th percentile. However, follow-up of cases with no abnormalities at
term other than low birth weight suggests that post-natal mental
development is completely normal and some catch-up growth may
occur after birth. Maternal hypertension is reported in some cases
be severe enough to force induction of labor prior to term. Occasionally
minor anomalies, for example, minor heart defects, club foot, and
umbilical hernia may be found. There have been a few cases in
which in-utero fetal death occurred. However, in these cases, abnormalities
and/or severe delay in growth were typically noted on ultrasound.
Such abnormalities are thought to be caused by the presence of
cells confined to one or a few fetal tissues but which could not
be detected by amniocentesis.
- Situation 2: Trisomy 16 detected at amniocentesis: When trisomy 16 is detected at amniocentesis the risks are higher than when the trisomy is found only in CVS. Nonetheless, there have been babies born with such a finding with a completely normal outcome or only minor abnormalities. The percentage of trisomic cells found in the amniocentesis seems not to be useful to predict outcome and survival to this stage of pregnancy (17-18weeks) is unlikely unless the trisomy 16 is confined to only a few fetal tissues.
A: When trisomy 16 mosaicism is detected, it is expected that roughly one-third of the time maternal uniparental disomy (both chromosomes 16 inherited from the mother) in the normal diploid (46 chromosome) cell line. Currently, there is no evidence to suggest that babies with uniparental disomy have a significantly worse outcome than those that have inherited one chromosome 16 from each parent. It is therefore not normally offered to couples as a prenatal test when trisomy 16 mosaicism has been diagnosed.
A: Not necessarily. It is very important to remember that the published literature usually over-emphasizes the bad outcomes because these cases are of the most newsworthy and publishable. These articles do provide guidance as to the range of different outcomes possible but do not provide a clear measure of the relative risk of abnormality.
A: Typically a carrier of a balanced translocation can expect roughly 50:50 odds for whether the translocation will be transmitted to their germ cells in balanced (normal) or unbalanced form. In some cases the unbalanced products may not be compatible with life and result in spontaneous abortion. In other cases, there may be risk of birth defects. Each case is different and depends on the involved 'breakpoints'. Such individuals should be referred for genetic counselling for information relating to their particular abnormality.