Uniparental isodisomy for chromosome 16 in a growth-retarded infant with congenital heart disease

Recurrent Trisomies: Chance or Inherited Predisposition?

Two patients experiencing recurring trisomic pregnancies involving a different chromosome each time are presented. Mechanisms to explain recurrent trisomies include a gene or genes predisposing to nondisjunction in general or to nondisjunction of the acrocentric chromosomes, maternal age effects, and germ-line mosaicism. Genetic counseling is complicated by the lack of a clear explanation for the recurrences, difficulty in quoting a specific recurrence risk, concern regarding the risk for uniparental disomy, and the frustration, grief and guilt reactions of the patients.

Variable outcomes in mosaic trisomy 16: five case reports and literature analysis

OBJECTIVES

To report five cases of mosaic trisomy 16 with variable outcomes in the context of the literature on mosaic trisomy 16. Complications in these cases include preeclampsia, IUGR, fetal anomalies, and death, with no predictable pattern.

METHODS

Observation of five new cases and statistical analysis of 125 reported cases of mosaic trisomy 16 with prenatal detection and outcome data.

RESULTS

(1) IUGR, premature delivery, and/or physical anomalies are observed commonly, even when the trisomy is thought to be confined to the placenta; (2) Level II mosaicism for trisomy 16 in amniotic fluid may reflect a true mosaic state with phenotypic consequences; (3) FISH is more sensitive than traditional cytogenetics in detecting mosaicism in all tissue types examined; (4) hCG levels can be extremely elevated, and MS-AFP levels are often elevated; and (5) Uniparental disomy (UPD) increases the rates of IUGR and physical anomalies in CPM cases.

CONCLUSION

While there is no obvious mosaic trisomy 16 syndrome, IUGR and heart defects commonly occur, even if the mosaicism appears to be confined to the placenta. A completely normal outcome occurs only in about 20% of the cases; however, complications can often be limited to prematurity, small-for-gestational-age infants, and/or minor or surgically reparable birth defects.

Sex-specific chromosome instability in early human development

The predominance of females segregating chromosome aberrations to their offspring has been explained mostly by selection disadvantage of unbalanced products of spermatogenesis. However, analysis of data from the literature supports the idea that somatic cells of early female embryos are similar to female germ cells in that they are prone to malsegregation. The goal of this study was to compare the sex ratio (male to female ratio) of carriers of presumably mitotic-occurring chromosome abnormalities to identify any sex biases. In examining the literature, we found a female prevalence in cases of mosaicism associated with uniparental disomy (UPD) (26 male individuals/conceptions and 45 female individuals/conceptions, sex ratio is 0.58, significantly different from 1.06 in newborn population, P = 0.0292). This predominance was highest at gestational age <16 week (8 male and 22 female conceptuses, sex ratio is 0.36, significantly different from expected figure of 1.28, P = 0.0025), which diminished at later stages of fetal development indicating potential correction of trisomies predominantly in females. There is a threefold prevalence of 46,XX/45,X mosaics over 46,XY/45,X mosaics in prenatally diagnosed cases, which also suggests a gender-specific postzygotic chromosome loss. The male prevalence in Prader-Willi syndrome with maternal UPD of chromosome 15 also can be explained by sex-specific trisomy correction, with predominant loss of a maternal chromosome causing biparental inheritance and therefore, complete correction of trisomy in females (without UPD). Finally, there is a female predominance in carriers of chromosome rearrangement with pericentromere break (mosaicism for Robertsonian translocation/isochromosome, centric fission, nonacrocentric isochromosome, and whole arm rearrangement), in both prenatal (21 males and 36 females, sex ratio is 0.58, P < 0.0184) and postnatal ill-defined cases (14 males and 35 females, sex ratio is 0.40, P = 0.001). Thus, the findings presented in this paper suggest that, in addition to reduction in male fertility, and to probable selection against abnormal cell line(s), there are two mechanisms that contribute to female preponderance among carriers of mosaicism: sex-specific chromosome loss and sex-specific centromere instability. The data obtained suggest that females may have gonadal mosaicism for aneuploidies and structural rearrangements more often than males. This may lead to the maternal origin bias in offspring with trisomies or structural rearrangements.

Uniparental disomy (UPD) other than 15: Phenotypes and bibliography updated

Uniparental disomy (UPD) describes the inheritance of a pair of chromosomes from only one parent. The concept was introduced in Medical Genetics by Engel (1980); Am J Med Genet 6:137-143. Aside UPD 15, which is the most frequent one, up to now (February 2005) 197 cases with whole chromosome maternal UPD other than 15 (124 X heterodisomy, 59 X isodisomy, and 14 cases without information of the mode of UPD) and 68 cases with whole chromosome paternal UPD other than 15 (13 X heterdisomy, 53 X isodisomy, and 2 cases without information of the mode of UPD) have been reported. In this review we discuss briefly the problems associated with UPD and provide a comprehensive clinical summary with a bibliography for each UPD other than 15 as a guide for genetic counseling.

Evidence for imprinting on chromosome 16: The effect of uniparental disomy on the outcome of mosaic trisomy 16 pregnancies

Although a number of infants with maternal uniparental disomy of chromosome 16 (upd(16)mat) have been reported, the evidence for imprinting on chromosome 16 is not yet conclusive. To test the hypothesis that upd(16)mat has a distinct phenotype, which would support the existence of imprinted gene(s) on chromosome 16, statistical analysis was performed on a large series (n = 83) of mosaic trisomy 16 cases with molecular determination of uniparental disomy status. The incidence of upd(16)mat was 40%, which is consistent with the expected one third from random chromosome loss during trisomy rescue (P = 0.262). In pairwise comparisons, upd(16)mat was found to be associated with fetal growth restriction (P = 0.029) and with increased risk of major malformation (RR = 1.43; P = 0.053). Regression modeling showed that the effect of upd(16)mat on fetal/neonatal weight and malformation is independent of the degree of trisomy detected in the fetus. Regression modeling to control for the degree of trisomy detected in the placenta was not possible due to limited sample size. We conclude that upd(16)mat is associated with more severe growth restriction, and possibly, with higher risk of malformation. Our hypothesis is that imprinted gene(s) exist on chromosome 16 and that abnormal expression of these gene(s) in upd(16)mat cells during development results in decreased cell proliferation. Although we do not advocate prenatal testing for upd(16), studies on the long-term outcome of upd(16)mat neonates is necessary for counseling purposes.

Apparent confined placental mosaicism of trisomy 16 and multiple fetal anomalies: case report

Trisomy 16 is frequently found confined to the placenta (confined placental mosaicism (CPM)), with a structurally normal fetus. In some cases of trisomy 16, the fetus has uniparental disomy for chromosome 16 (UPD16) which is associated with intrauterine growth restriction (IUGR) and fetal anomalies. We report a case of apparent confined placental mosaicism for trisomy 16, using standard cytogenetic techniques, but with multiple fetal abnormalities including congenital diaphragmatic hernia in which there was no evidence of UPD in the disomic tissues examined. Subsequent examination of fetal tissues using fluorescent in situ hybridization (FISH) demonstrated low levels of mosaicism for trisomy 16 in all the tissues examined. The use of FISH permits identification of mosaicism which conventional techniques may not identify.

Mosaic trisomy 16 ascertained through amniocentesis: evaluation of 11 new cases

Trisomy 16, once thought to result uniformly in early pregnancy loss, has been detected in chorionic villus samples (CVS) from on-going pregnancies and was initially ascribed to a second, nonviable pregnancy. Prenatally detected trisomy 16 in CVS and its resolution to disomy has led to the reexamination of the viability of trisomy 16. This study evaluates 11 cases of mosaic trisomy 16 detected through second trimester amniocentesis. In 9 of the 11 cases, amniocenteses were performed in women under the age of 35 because of abnormal levels of maternal serum alpha-fetoprotein (MSAFP) or maternal serum human chorionic gonadotropin (MShCG). The other two amniocenteses were performed for advanced maternal age. Five of the 11 pregnancies resulted in liveborn infants, and six pregnancies were electively terminated. The liveborn infants all had some combination of intrauterine growth retardation (IUGR), congenital heart defects (CHD), or minor anomalies. Two of them died neonatally because of complications of severe congenital heart defects. The three surviving children have variable growth retardation, developmental delay, congenital anomalies, and/or minor anomalies. In the terminated pregnancies, the four fetuses evaluated by ultrasound or autopsy demonstrated various congenital anomalies and/or IUGR. Cytogenetic and fluorescent in situ hybridization studies identified true mosaicism in 5 of 10 cases examined, although the abnormal cell line was never seen in more than 1% of cultured lymphocytes. Placental mosaicism was seen in all placentas examined and was associated with IUGR in four of seven cases. Maternal uniparental disomy was identified in three cases. Mosaic trisomy 16 detected through amniocentesis is not a benign finding but associated with a high risk of abnormal outcome, most commonly IUGR, CHD, developmental delay, and minor anomalies. The various outcomes may reflect the diversity of mechanisms involved in the resolution of this abnormality. As 80% of these patients were ascertained because of the presence of abnormal levels of MSAFP or MShCG, the increased use of maternal serum screening should bring more such cases to clinical attention.

Growth effects of uniparental disomies and the conflict theory of genomic imprinting

While numerous theories have been proposed for the evolution of genomic imprinting, few have been tested. The conflict theory proposes that imprinting is an intra-individual manifestation of classical parent-offspring conflict. This theory is unique in predicting that imprinted genes expressed from the paternally derived genome should be enhancers of pre- and post-natal growth, while those expressed from the maternally derived genome should be growth suppressors. We examine this prediction by reviewing the literature on growth of human and mouse progeny that have inherited both copies (or part thereof) of a particular chromosome from only one parent. Perhaps surprisingly, we find that much of the data do not support the hypothesis.

The incidence of uniparental disomy associated with intrauterine growth retardation in a cohort of thirty-five severely affected babies

OBJECTIVE

Our purpose was to screen for uniparental disomy 35 babies with idiopathic intrauterine growth retardation < 5th percentile.

STUDY DESIGN

The placenta and the baby's blood were conventionally karyotyped. Deoxyribonucleic acid from the parents, the baby's blood, and the placenta were then screened for uniparental disomy for 12 candidate chromosomes with use of chromosome-specific polymorphic deoxyribonucleic acid markers.

RESULTS

Two cases of maternal uniparental disomy for chromosome 16 were found associated with confined placental mosaicism for chromosome 16. No other uniparental disomy was found for any of the 12 chromosomes tested. Four structural chromosome abnormalities were also found in this cohort through standard karyotyping.

CONCLUSION

Uniparental disomy for the chromosomes tested does not explain the etiology of the majority of cases of intrauterine growth retardation < 5th percentile. Maternal uniparental disomy for chromosome 16 accounts for 5% of this cohort. Structural chromosomal abnormalities are also much higher than expected at 11%.

Comprehensive 4-year follow-up on a case of maternal heterodisomy for chromosome 16

Uniparental disomy for chromosome 16 has been previously identified in fetal deaths and newborn infants with limited follow-up. Thus there is a lack of information about the long-term effects of maternal uniparental disomy 16 on growth and development. We present a case of maternal heterodisomy for chromosome 16 and a comprehensive 4-year physical and cognitive evaluation. Cytogenetic analysis of chorionic villus obtained at 10 weeks gestation for advanced maternal age showed trisomy 16. At 15 weeks, amniocentesis demonstrated low level mosaicism 47,XY,+16[1]/46,XY[25]. Decreased fetal growth was noted in the last 2 months of pregnancy and the infant was small for gestational age at birth. Molecular studies revealed only maternal alleles for chromosome 16 in a peripheral blood sample from the child, consistent with maternal uniparental heterodisomy 16. Although short stature remains a concern, there appears to be no major cognitive effects of maternal disomy 16. Clinical evaluation and follow-up on additional cases should further clarify the role of placental mosaicism and maternal disomy 16 in intrauterine growth retardation and its effects on long-term growth in childhood.

Molecular-cytogenetic investigations of ten term placentae in cases of prenatally diagnosed mosaicism

Discrepant chromosome findings in the placenta and fetus are detected by additional investigations commonly after chorionic villus sampling (CVS) and occasionally after amniotic fluid cell cultures. In this paper we present the results of molecular-cytogenetic investigations (fluorescence in situ hybridization, FISH) of ten term placentae after prenatally detected mosaicism. In three cases, mosaicism was found after first-trimester CVS and in seven cases, after second-trimester amniotic fluid culture. All three results after CVS represented confined placental mosaicism (CPM). Two of the seven mosaic findings after amniocentesis were not confirmed postnatally. In the remaining five cases, general mosaicism was found. The analyses of six defined areas of the term placentae showed that it is important to investigate the placenta at multiple sites. The frequency of a cell line varied by more than 50 per cent at different analysed sites. FISH on interphase nuclei proved to be a rapid and reliable method of investigating large numbers of biopsies and cells per biopsy.

Case report: uniparental disomy 16 in association with congenital heart disease

Uniparental disomy (UPD) is the inheritance of both copies of a given chromosome from the same parent (Warburton, 1988; Anon., 1991). The exact disease associations of UPD of individual chromosomes have yet to be fully elucidated and the question of whether UPD of some chromosomes may be regarded as a benign finding remains unanswered. We report an infant with uniparental maternal disomy 16, the only such infant identified at King's College Hospital. The infant had intrauterine growth retardation and minor congenital heart disease.

Placental mosaicism is associated with unexplained second-trimester elevation of MShCG levels, but not with elevation of MSAFP levels

In this patient-control study, we examined the impact of placental mosaicism on the concentrations of maternal serum human chorionic gonadotropin (MShCG) and maternal serum alpha-fetoprotein (MSAFP) in the second trimester of pregnancy. Patient and control groups were selected from 2347 women with a singleton pregnancy, who underwent chorionic villous sampling in the first trimester and from whom second-trimester serum samples had been collected. The concentrations of both serum markers, expressed in multiples of the median (MOM), in 35 women with confined placental mosaicism (CPM) were compared with those in 70 controls with uncomplicated pregnancies. Elevated MSAFP or MShCG was defined as a concentration of > or = 2.0 MOM. Of the 35 pregnancies with CPM, none had an elevated MSAFP level, as opposed to two out of the 70 women (2.9 per cent) in the control group (P = NS). Nine women in the placental mosaicism group (26 per cent) had an MShCG level of > or = 2.0 MOM, compared with five in the control group (7.1 per cent; P = 0.0135). Nineteen women in the placental mosaicism group (54 per cent) were screen-positive for Down's syndrome (cut-off 1:250), compared with 17 women (24 per cent) in the control group (P = 0.0042; relative risk = 2.3). The three highest MShCG levels were found in pregnancies with CPM that involved trisomy 16; all these women delivered a small-for-gestational age (SGA) infant. CPM, especially with trisomy 16, is associated with elevated levels of MShCG, but not with elevated levels of MSAFP. It is an important cause of false-positive results in serum screening programmes for fetal Down's syndrome. It is possible that abnormal MShCG levels in pregnancies with CPM result from dysfunctional placenta, caused by chromosomally abnormal areas. We therefore recommend increased surveillance of pregnancies with unexplained elevated MShCG levels.

Maternal serum screening and trisomy 16 confined to the placenta

Five cases of trisomy 16 confined to the placenta have been detected by invasive procedures (amniocentesis and chorionic villus sampling) after high-risk results for Down syndrome and neural tube defects in a maternal serum screening programme of 6614 consecutive cases. All five pregnancies displayed unusually elevated levels of human chorionic gonadotropin and four out of five also had raised alpha-fetoprotein values. No structural malformation was present but all five pregnancies were complicated by fetal growth retardation, and one by intrauterine death. From our results, we suggest that both amniocentesis and chorionic villus sampling should be considered in the management of cases with high mid-trimester levels of these analytes.

Trisomy 16 mosaicism in amniotic fluid cell cultures

Trisomy 16 mosaicism was found in amniotic fluid cells in a patient undergoing amniocentesis because of elevated second-trimester maternal serum alpha-fetoprotein (MSAFP) (2.80 MOM), a markedly elevated human chorionic gonadotropin level (hCG) (12.02 MOM), and a Down syndrome risk of 1:55. Ultrasound evaluation of the fetus indicated the presence of an atrial septal defect and clinodactyly. Cytogenetic analyses of various fetal tissues using fluorescence in situ hybridization (FISH) failed to detect substantial numbers of trisomy 16 cells; however, trisomy 16 mosaicism was identified in placental tissue. Molecular genetic analysis at five different loci [four analysed by polymerase chain reaction (PCR) and one by Southern blot analysis] failed to show any evidence for uniparental disomy. Although trisomy 16 cells could not be clearly demonstrated in the fetus, the presence of a clinically significant proportion of aneuploid cells early in development could not be excluded and it therefore cannot be assumed that a 'confined placental mosaicism' existed. The markedly elevated hCG and elevated MSAFP levels are consistent with abnormal placental function in trisomy 16 mosaicism. Serial ultrasound evaluation (to detect any late-onset growth retardation) and fetal echocardiography may be indicated for patients with extraordinarily high levels of hCG, especially if MSAFP is also elevated.

A case of maternal uniparental disomy of chromosome 9 in association with confined placental mosaicism for trisomy 9

We describe the first case of maternal uniparental disomy (UPD) of chromosome 9 in a fetus who was shown to have mosaic trisomy 9 in a chorionic villus sample. Karyotyping and molecular studies following termination of the pregnancy confirmed mosaicism in the placenta and maternal UPD(9) in the fetal tissues. This case demonstrates that the mechanism of trisomy correction may result in a fetus with UPD(9).