Interpretation of the karyotype results
Chorionic Villus Sampling (CVS) results with short term and long term cultures show that the foetus has an added copy of chromosomes more than the normal 23 pairs in the examined metaphases. From the two, it is evident that the karyotype is due to the addition of an extra copy of chromosome 16 in the examined cells.
The 46, XY karyotype detected in amniocentesis shows a disparity between the 47, XY karyotype in the examined metaphases. This is an indication that the three copies of chromosome 16 are evident in some cells of the foetus whereas some cells exhibit a normal karyotype with two copies. From the results, the fetus is of male sex.
Discussion of the likely genetic mechanism that would produce the results
The likely genetic mechanism that would produce disparity in the karyotype (chromosome 16 triplication) when assessing different cells of the 37 year old’s fetus is trisomy 16 mosaicism. In this condition, there is an error of cell division in maternal meiosis I during early foetal development that affects some cells while others are perfectly normal.
These errors occur at random, and, therefore, there is no specific pattern in which the errors occur. In most cases, full trisomy 16 normally undergoes rescue to residual trisomy 16 mosaicism. This then becomes confined to the foetus or placenta of the surviving foetus (Johnson 1990, p575).
Clinical significance of these results and outcomes for the foetus
Many studies have determined that many cases of trisomy 16 mosaic pattern have noteworthy postnatal outcome. However, many pregnancies of trisomy 16 mosaicism that has been detected prenatally either through Chorionic Villus Sampling or amniocentesis have an enhanced risk of Intrauterine Growth Restriction (IUGR), maternal preeclampsia, intrauterine death, neonatal death and many fetal malformations.
A variety of these foetal malformations include hypospadias and Cardiac Septal Defects, which all cause developmental delay to the growing foetus (Leschot 1991, p404).
In the 37 year old female who was diagnosed prenatally, follow up amniocentesis that occurred 5 weeks after conducting CVS revealed IUGR on ultrasound. This is an indication that the effects of trisomy 16 mosaicism had started to manifest as early as the 16th week of gestation.
Therefore, it would only be a matter of time before the other classic signs started setting into the fetus, and, hence, clinicians should come up with control measures before many malformations would set up. It would also be important for clinicians to perform genetic counseling to the pregnant woman so that she would be aware of the potential danger that her foetus would potentially be in (Wolstenholme 1995, p115).
The severity of potential malformations to the foetus is another important factor as it would determine the survival rates of the foetus. For example, the severity of the detected IUGR would have a serious implication on the developmental milestones of the foetus. This would also affect the age of gestation and the survival rates of the fetus upon birth.
Moreover, the extent at which the pregnant mother is affected by this genetic condition would also determine the survival rate and condition of the foetus. For example, mothers with severe preeclampsia are advised by healthcare experts to terminate their pregnancy so that their survival rates are enhanced.
It would, therefore, be difficult for the pregnant mother to survive with the foetus with the severe condition. Termination of the pregnancy, therefore, becomes the best alternative (Sanchez 1997, p778).
Development of a foetus is very important and parents are expected to monitor their infants to check whether they achieve developmental milestones in the appropriate time. This is important because, in case there is an abnormality, interventions are done early. This is the same with cases of trisomy 16 mosaicism under study. This condition would have an outcome on the infancy of the fetus in terms of development.
The parent would, therefore, be expected to closely monitor the monitor to see the effect of the condition on development. Some infants would start to walk earlier than others while others would show delayed sitting and speech.
Especially in the speech aspect, early intervention through speech therapy becomes important as it enables neuro-developmental progress to be triggered early in the life of the infant before it becomes too late (Tantravahi 1996, p752).
However, the most dangerous possible outcome for a foetus with trisomy 16 mosaicism would be malformations in the cardiovascular system. This genetic condition predisposes an infant to cardiac congenital malformations such as Ventricular Septal Defect (VSD), endocardial cushion defect, Patent Ductus Arteriousus (PDA) and various aortic valve malformations (Johnson 1993, p107).
The severity of these conditions is of value to the clinician because it determines the survival rate of the foetus since the cardiovascular system is one of the most important systems of the body. A normal cardiovascular system for a foetus is important as a foetus needs sufficient oxygen for its life operations.
If these malformations would be severe, oxygen circulation is affected and this would lead to fetus death if clinicians do not undertake the necessary measures early. Therefore, the extent and severity of all malformations of the mosaicism in the foetus would determine the survival rate of the foetus and the survival rate in subsequent years (Yong 2003, p176).
Follow up tests/ screening tests that should be offered
A variety of follow up tests need to be conducted by clinicians in the pregnant woman so as to observe the extent of foetal and maternal malformations and, hence, the necessary measures that need to be conducted to assure the mother of her safety and that of the foetus.
From the follow up studies also, it would be possible for clinicians to decide whether to terminate the foetus so as to keep the mother safe incase the foetus would have very severe malformations incompatible with life. Serum studies of the mother would assist clinicians determine malformations of the foetus and that of the mother as they share a common circulation.
For example, increased maternal serum hCG is indicative of Atrial Septal Defect in the foetus. Ultrasound screening would also assist the clinician determine the extent of IUGR so that it would be possible to determine survival rates of the foetus.
Computerized tomography scans would be another excellent screening test in the pregnant woman so as to detect the foetal malformations as a result of trisomy 16 mosaicism. Neonatal blood lymphocyte cytogenetic studies may also be used to determine the number of cells affected by the genetic malformation, hence, determines the nature and severity of the condition (Johnson 2000, p420)).
References
Johnson A, W. R. (1990). Mosaicism in Chorionic Villus Sampling: An Association With poor perinatal Outcome. Obstet Gynecol 75 , 573-577.
Johnson MP, C. M. (1993). Viable Pregnancies After Diagnosis of trisomy 16 by CVS: Lethal Aneuploidy Comparmentalized to the Trophoblast. Fetal Diagn Ther 8 , 102-108.
Johnson P, D. K. (2000). Apparent Confined placental Mosaicism of Trisomy 16 and multiple Fetal Anomalies. Prenat Diag , 417-421.
Leschot NJ, W. H. (1991). Is Placental Mosaicism Associated with Poor Perinatal Outcome? Prenat Diag 11 , 403-404.
Sanchez JM, L. D. (1997). Severe fetal malformations Associated With Trisomy 16. Prenat Diag 17 , 777-779.
Tantravahi U, M. C. (1996). Trisomy 16 mosaicism in amniotic fluid cell cultures. Prenat Diag 16 , 749-754.
Wolstenholme, J. (1995). An audit of trisomy 16 in man. Prenat Diag 15 , 109-121.
Yong PJ, B. I. (2003). Clinical Aspects, Prenatal Diagnopsis, and pathogenesis of Trisomy 16 Mosaicism. J Med Genet 40 , 175-182.