Uniparental disomy in fetuses diagnosed with balanced Robertsonian translocations: risk estimate

Risk estimation of uniparental disomy of chromosome 14 or 15 in a fetus with a parent carrying a non-homologous Robertsonian translocation. Should we still perform prenatal diagnosis?

OBJECTIVE

Uniparental disomy (UPD) testing is currently recommended during pregnancy in fetuses carrying a balanced Robertsonian translocation (ROB) involving chromosome 14 or 15, both chromosomes containing imprinted genes. The overall risk that such a fetus presents a UPD has been previously estimated to be around ~0.6-0.8%. However, because UPD are rare events and this estimate has been calculated from a number of studies of limited size, we have reevaluated the risk of UPD in fetuses for whom one of the parents was known to carry a nonhomologous ROB (NHROB).

METHOD

We focused our multicentric study on NHROB involving chromosome 14 and/or 15. A total of 1747 UPD testing were performed in fetuses during pregnancy for the presence of UPD(14) and/or UPD(15).

RESULT

All fetuses were negative except one with a UPD(14) associated with a maternally inherited rob(13;14).

CONCLUSION

Considering these data, the risk of UPD following prenatal diagnosis of an inherited ROB involving chromosome 14 and/or 15 could be estimated to be around 0.06%, far less than the previous estimation. Importantly, the risk of miscarriage following an invasive prenatal sampling is higher than the risk of UPD. Therefore, we do not recommend prenatal testing for UPD for these pregnancies and parents should be reassured.

CCMG guidelines: prenatal and postnatal diagnostic testing for uniparental disomy

The aim of this statement is to provide clinicians, cytogeneticists and molecular geneticists of the Canadian College of Medical Geneticists (CCMG) a comprehensive review of the role of UPD in constitutional genetic diagnosis and to provide a guideline as to when investigation for UPD is recommended. Members of the CCMG Cytogenetics, Molecular Genetics, Clinical Practice, and Prenatal Diagnosis committees reviewed the relevant literature on uniparental disomy (UPD) in constitutional genetic diagnosis (May 2010). Guidelines were developed for UPD testing in Canada. The guidelines were circulated for comment to the CCMG members at large and following appropriate modification, approved by the CCMG Board of Directors (July 2010).

Unbalanced and balanced acrocentric rearrangements involving chromosomes other than chromosome 21 at amniocentesis

OBJECTIVE

To investigate unbalanced and balanced acrocentric rearrangements involving chromosomes other than chromosome 21 at amniocentesis.

MATERIALS AND METHODS

From January 1987 to September 2009, 31,194 amniocenteses were performed at Mackay Memorial Hospital, Taipei, Taiwan. Two cases with unbalanced acrocentric rearrangements involving chromosomes other than chromosome 21 from two families, and 24 cases with balanced acrocentric rearrangements involving chromosomes other than chromosome 21 from 21 families were diagnosed and investigated.

RESULTS

We detected i(13q13q), +13 (one case), rob(13q14q), +13 (one case), rob(13q14q) (16 cases), rob(14q15q) (five cases), rob(13q15q) (one case), rob(15q22q) (one case), and mosaic rob(14q22q) (one case). Of the 25 cases that underwent parental cytogenetic investigation, six arose de novo and 19 were inherited (10 maternal and nine paternal). The 16 families with an inherited Robertsonian translocation included rob(13q14q) (11 families), rob(14q15q) (four families), and rob(15q22q) (one family). Of these 16 families, only two had known parental carrier status prior to the first amniocentesis, while the other 14 were aware of a parental carrier status only after prenatal diagnosis of a fetus with a heterologous Robertsonian translocation. The 18 fetuses with balanced heterologous Robertsonian translocations inherited them from six maternal carriers of rob(13q14q), four paternal carriers of rob(13q14q), four paternal carriers of rob(14q15q), and one maternal carrier of rob(15q22q). Neither UPD14 nor UPD15 was detected in any of the 16 cases tested for UPD.

CONCLUSION

Concerning acrocentric rearrangements involving chromosomes other than chromosome 21, we found a frequency of 0.0064% for unbalanced rearrangements and 0.0769% for balanced rearrangements at amniocentesis in this study. rob(13q14q) was the most common and rob(14q15q) the second most common rearrangement. Of the families with an inherited translocation, 87.5% were aware of parental carrier status only after prenatal diagnosis of a fetus with a translocation by amniocentesis.

Genetic screening in 2,710 infertile candidate couples for assisted reproductive techniques: results of application of Italian guidelines for the appropriate use of genetic tests

OBJECTIVE

To report the results of the routine application of Italian guidelines that apply to infertile patient candidates for any assisted reproduction technique (ART). The guidelines recommend performing a karyotype analysis in each couple and the screening test for mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) in one of the two partners.

DESIGN

Case series.

SETTING

Infertility unit.

PATIENT(S)

Two thousand seven hundred ten consecutive infertile couple candidates for ART.

INTERVENTION(S)

Peripheral blood evaluation of karyotype and CFTR gene.

MAIN OUTCOME MEASURE(S)

Frequency of aberrant karyotype and mutated CFTR gene.

RESULT(S)

A total of 74 aberrant karyotypes were diagnosed, corresponding to 1.3% (95% confidence interval [CI], 0.9%-1.7%) in women and to 1.5% (95% CI, 1.0%-2.0%) in men. In men, the frequency of chromosomal abnormalities differed according to the treatment group (0.3%, 1.1%, and 2.2% in IUI, IVF, and ICSI, respectively). The same was not observed in women. Excluding the 5T variant, 3.8% of the screened patients showed a mutated CFTR gene (95% CI, 3.1%-4.5%), and the mutation was found in both partners in 0.2% of the couples (95% CI, 0.0-0.4%).

CONCLUSION(S)

The frequency of aberrant karyotypes is higher in infertile couples than in the general population, whereas the frequency of a mutation of the CFTR gene is similar.

Meiotic segregation of rare Robertsonian translocations: sperm analysis of three t(14q;22q) cases

BACKGROUND

The t(14;22) remains one of the rare Robertsonian translocations observed in human, with an occurrence estimated at 1.2%. Three cases of rare Robertsonian translocation t(14;22) were investigated for meiotic segregation in sperm samples from male carriers using the fluorescent in situ hybridization (FISH) procedure. The three carriers included two men with an abnormal semen analysis and one with normal semen parameters.

METHODS

Both locus-specific probes and whole-chromosome painting probes, specific for chromosomes 14 and 22, were used in this study. The number of spermatozoa scored for each probe set ranged from 3279 to 10,024.

RESULTS

In the three carriers, similar frequencies, ranging from 78.53 to 81.76%, were found for normal and balanced spermatozoa resulting from alternate segregation. The total proportion of unbalanced spermatozoa resulting from adjacent modes of segregation ranged from 17.59 to 20.94%.

CONCLUSION

This finding confirmed the predominance of alternate segregation over other segregation types in all Robertsonian translocations and indicates a higher production of imbalances in the t(14;22) than in most of the Robertsonian translocations previously analysed. This could be related to the variable location of breakpoints in Robertsonian translocations. This breakpoint diversity could also play a role in the differences in reproductive status observed in male carriers of Robertsonian translocations.

Is there a higher incidence of maternal uniparental disomy 14 [upd(14)mat]? Detection of 10 new patients by methylation-specific PCR

Maternal uniparental disomy for chromosome 14 [upd(14)mat] is associated with a characteristic phenotype including pre- and postnatal growth retardation, muscular hypotonia, feeding problems, motor delay, small hands and feet, precocious puberty and truncal obesity. Patients with upd(14)mat show features overlapping with Prader-Willi syndrome (PWS) and are probably underdiagnosed. Maternal upd(14) is frequently described in carriers of a Robertsonian translocation involving chromosome 14, but is also found in patients with a normal karyotype. Based on the above mentioned criteria we have identified six patients with upd(14)mat including two patients with a normal karyotype, one patient with a de novo Robertsonian translocation (14;21), one patient with a familial Robertsonian translocation (13;14) and two patients with a marker chromosome. In addition, we analyzed a cohort of 33 patients with low birth weight, feeding difficulties and consecutive obesity in whom PWS had been excluded by methylation analysis of SNRPN. In four of these patients (12%) we detected upd(14)mat. For rapid testing of upd(14)mat we analyzed the methylation status of the imprinted MEG3 locus. In conclusion, we recommend considering upd(14)mat in patients with low birth weight, growth retardation, neonatal feeding problems, muscular hypotonia, motor delay, precocious puberty and truncal obesity as well as in patients with a PWS like phenotype presenting with low birth weight, feeding difficulties and obesity.

Risk estimates for uniparental disomy following prenatal detection of a nonhomologous Robertsonian translocation

Carriers of nonhomologous Robertsonian translocations (ROB) are at risk for having offspring with uniparental disomy (UPD). Although risk estimates have been calculated in several independent studies, the estimates have not been optimal because most studies are not of sufficient size and UPD events are rare. However, these collective data have provided the opportunity to derive an overall risk estimate for UPD in the fetus after the prenatal identification of a ROB.

Mouse genetic models for aneuploidy induction in germ cells

Rodents have been successfully used as models to identify risks of chemical exposures or age to aneuploidy induction in germ cells, which may be transmitted to the progeny. For this administration in vivo as well as exposures to in vitro maturing germ cells have been useful. Genetic models involving mice with structural chromosomal rearrangements and transgenic animals have the potential to model conditions predisposing to aneuploidy in one or both sexes, and in this way to identify potential targets for aneugens and gender-effects. The review provides an overview of mouse genetic models for aneuploidy induction in mammalian germ cells and discusses perspectives for combining genetic with experimental approaches in aneuploidy research.

Prenatal search for UPD 14 and UPD 15 in 83 cases of familial and de novo heterologous Robertsonian translocations

OBJECTIVES

The presence in the conceptus of a Robertsonian translocation predisposes to UPD formation, mainly by post-zygotic events of chromosome abnormality rescue. This is due to the increased risk of generating aneuploid zygotes because the rearranged chromosome and the respective homologues are prone to non-disjunction errors. Given this, carriers and karyotypically normal individuals conceived from a parent with a Robertsonian translocation are at risk for UPD. Abnormal phenotypes due to an imprinting effect have been found to be associated with UPD 14 and 15. The aim of the study was to refine, at the time of prenatal diagnosis, the risk for UPD 14 and 15 in a population with Robertsonian translocations involving these chromosomes.

METHODS

Sixty-five cases of familial and de novo heterologous Robertsonian translocations involving chromosomes 14 and 15 and 18 fetuses with a normal karyotype, but conceived by a Robertsonian translocation carrier were prenatally studied to investigate the presence of UPD for chromosomes 14 and 15.

RESULTS

Of the 65 Robertsonian translocation carriers, one fetus with a de novo der(14;21) showed maternal UPD 14. None of the 18 fetuses with a normal karyotype had UPD.

CONCLUSION

Our data, combined with other previous prenatal investigations provide a general risk estimate for UPD 14 and 15 of 0.6%. Nevertheless, combining our data and those previously reported, all three fetuses with UPD had a de novo Robertsonian translocation, thus suggesting a risk of UPD formation of about 3% for this specific group of translocation carriers.

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.

Fetal phenotype of Prader-Willi syndrome due to maternal disomy for chromosome 15

Prader-Willi syndrome (PWS) results from either paternal deletion of 15q11-q13, or maternal uniparental disomy (UPD) of chromosome 15 or imprinting center mutation. Prenatal diagnosis of PWS is currently indicated for chromosomal parental translocation involving chromosome 15 and for decreased fetal movements during the third trimester of gestation. Here we present the prenatal diagnosis of PWS during the first trimester of gestation and autopsy findings. Chorionic villus sampling (CVS) was performed for advanced maternal age at 13 weeks' gestation. CVS showed mosaicism including cells with a normal karyotype and cells with trisomy 15. Amniocentesis showed cells with a normal karyotype. Molecular analysis demonstrated that the fetus had a typical PWS abnormal methylation profile and maternal disomy for chromosome 15. Fetal ultrasound examination showed slightly enlarged lateral ventricles and hypoplasic male external genitalia without intra-uterine growth retardation. The autopsy showed a eutrophic male fetus with facial dysmorphy, hypoplasic genitalia, abnormal position of both feet and posterior hypoplasia of the corpus callosum. This report points out that in a karyotypically normal fetus with ambiguous male external genitalia and cerebral anomalies, extensive cytogenetic and molecular biology studies are strongly recommended because of risk of PWS.

Uniparental disomy and Robertsonian translocations: risk estimation and prenatal testing

BACKGROUND

Uniparental disomy (UPD) is defined by the inheritance of both homologous chromosomes from only one parent, resulting in an imbalance of the expression of imprinted genes. With the recent identification of several diseases associated with UPD, the diagnostic significance of this molecular finding is a focus of interest. Acrocentric chromosomes involved in Robertsonian translocations (RTs) are particularly prone to being affected by mis-segregation events, possibly resulting in UPD. While UPDs of chromosomes 13, 21, and 22 have no clinical consequences, and therefore have no diagnostic impact despite of homozygosity of recessive alleles, prenatal testing for UPDs 14 or 15 is becoming increasingly asked for.

METHODS

Thirty-one fetuses with nonhomologous balanced RTs involving chromosome 14 were tested for UPD14 by microsatellite typing.

RESULTS

No cases of maternal UPD14 were detected among the 31 fetuses analyzed.

CONCLUSIONS

Based on our own data from molecular testing in 31 prenatal RT cases and findings in the published literature, we delineated a risk of 0.3% for a UPD with clinical consequences for prenatally detected carriers of a nonhomologous RT. Prenatal UPD testing is not associated with any additional risk to the pregnancy once invasive prenatal testing has been carried out. However, the possibly conflicting consequences in the case of a prenatal UPD identification should be discussed in advance. Furthermore, risk figures in specific clinical cohorts, such as couples prior to intracytoplasmic sperm injection, as well as questions of prenatal diagnostic management, will be discussed.