Maternal uniparental disomy of chromosome 21 in a normal child

Mosaic trisomy 21 at amniocentesis in a twin pregnancy associated with a favorable fetal outcome, maternal uniparental disomy 21 and postnatal decrease of the trisomy 21 cell line

OBJECTIVE

We present mosaic trisomy 21 at amniocentesis in a twin pregnancy associated with a favorable fetal outcome, maternal uniparental disomy (UPD) 21 and postnatal decrease of the trisomy 21 cell line.

CASE REPORT

A 36-year-old woman underwent elective amniocentesis at 16 weeks of gestation because of advanced maternal age, and an abnormal non-invasive prenatal testing (NIPT) result suggesting trisomy 21. Amniocentesis revealed the karyotype of 46, XX in co-twin A and the karyotype of 47,XY,+21[12]/46,XY[21] in co-twin B in the cultured amniocytes by in situ culture method. Simultaneous array comparative genomic hybridization (aCGH) analysis on uncultured amniocytes revealed the result of arr (21) × 3 [0.40] in co-twin B, consistent with 40% mosaicism for trisomy 21. Prenatal ultrasound was unremarkable, and the parental karyotypes were normal. Following genetic counseling, the parents decided to continue the pregnancy. At 36 weeks of gestation, a 2140-g female co-twin A and a 1800-g male co-twin B were delivered without any phenotypical abnormality. The karyotypes of the umbilical cord and placenta of co-twin B were 47,XY,+21[16]/46,XY[24] and 47,XY,+21 (40/40 cells), respectively. Quantitative fluorescence polymerase chain reaction (QF-PCR) analysis on the DNA extracted from parental bloods and umbilical cord, cord blood and placenta and peripheral blood at age five months of co-twin B confirmed a maternal origin of trisomy 21 and maternal uniparental isodisomy 21. aCGH analysis on the cord blood revealed the result of arr 21q11.2q22.3 × 2.25 consistent with 20%-25% (log₂ ratio = 0.15-0.2) mosaicism for trisomy 21. When follow-up at age five months, the co-twin B was phenotypically normal. His peripheral blood had a karyotype of 47,XY,+21[3]/46,XY[37]. Interphase fluorescence in situ hybridization (FISH) on 100 buccal mucosal cells detected no trisomy 21 signals. The peripheral blood had uniparental isodisomy 21.

CONCLUSION

Mosaic trisomy 21 at amniocentesis can be a transient and benign condition and should alert the possibility of UPD 21. The abnormal trisomy 21 cell line in mosaic trisomy 21 at amniocentesis may decrease and disappear after birth.

Prenatal diagnosis of maternal uniparental disomy 21 in association with low-level mosaic trisomy 21 at amniocentesis in a pregnancy associated with intrauterine growth restriction and a favorable outcome

OBJECTIVE

We present prenatal diagnosis of maternal uniparental disomy (UPD) 21 in association with low-level mosaic trisomy 21 at amniocentesis in a pregnancy associated with intrauterine growth restriction (IUGR) and a favorable outcome.

CASE REPORT

A 42-year-old, gravida 2, para 0, woman underwent amniocentesis at 17 weeks of gestation because of advanced maternal age. Amniocentesis initially revealed a karyotype of 46,XX in 20/20 colonies of cultured amniocytes. Simultaneous array comparative genomic hybridization (aCGH) analysis on uncultured amniocytes revealed a result of arr [GRCh37] (21) × 3 [0.16], (X) × 2, compatible with mosaic trisomy 21. After extensive investigation, the final result of conventional cytogenetic analysis of cultured amniocytes was 47,XX,+21[1]/46,XX[40]. The parental karyotypes were normal. Repeat amniocentesis was performed at 21 weeks of gestation. The cultured amniocytes had a karyotype of 47,XX,+21[3]/46,XX[27] and the uncultured amniocytes had a mosaic trisomy 21 level of 8.8% (10/114 cells) by interphase fluorescence in situ hybridization (FISH), a mosaic trisomy 21 level of 10% (log₂ ratio = 0.08) by aCGH, and maternal UPD 21 by polymorphic DNA marker analysis. Prenatal ultrasound revealed IUGR. At 38 weeks of gestation, a phenotypically normal 2695-g baby was delivered. The cord blood and umbilical cord had the karyotype of 46,XX and maternal UPD 21. The placenta had a karyotype of 47,XX,+21[8]/46,XX[32] and a maternal origin of trisomy 21. Postnatal FISH analysis on 101 buccal mucosal cells showed 6.9% (7/101 cells) mosaicism compared with 2% (2/100 cells) in the normal control. The baby was doing well at age four months.

CONCLUSION

Pregnancy with low-level mosaic trisomy 21 and maternal UPD 21 at amniocentesis can be associated with IUGR and a favorable outcome. Fetuses with maternal UPD 21 can be associated with mosaic trisomy 21 at amniocentesis.

Prenatal diagnosis of low-level mosaicism for trisomy 21 by amniocentesis in a pregnancy associated with maternal uniparental disomy of chromosome 21 in the fetus and a favorable outcome

OBJECTIVE

We present perinatal molecular cytogenetic analysis of low-level mosaicism for trisomy 21 in a pregnancy with maternal uniparental disomy (UPD) of chromosome 21 in the fetus.

CASE REPORT

A 39-year-old woman underwent amniocentesis at 17 weeks of gestation because of advanced maternal age. Amniocentesis revealed a karyotype of 47,XX,+21[6]/46,XX[25]. Simultaneous array comparative genomic hybridization (aCGH) analysis on the DNA extracted from uncultured amniocytes revealed arr (21) × 2-3, (X) × 2 with about 18% gene dosage increase in chromosome 21 consistent with mosaic trisomy 21. Cordocentesis was performed at 20 weeks of gestation, and the cord blood lymphocytes had a karyotype of 47,XX,+21[3]/46,XX[72]. Prenatal ultrasound findings were unremarkable. After genetic counseling, the parents decided to continue the pregnancy. At 39 weeks of gestation, a 3,494-g phenotypically normal female baby was delivered without phenotypic features of Down syndrome. There was no dysplasia of middle phalanx of the fifth fingers of both hands. The cord blood had a karyotype of 47,XX,+21[2]/46,XX[48]. The placenta had a karyotype of 47,XX,+21[37]/46,XX[3]. The umbilical cord had a karyotype of 47,XX,+21[1]/46,XX[39]. aCGH analysis on the DNA extracted from cord blood revealed no genomic imbalance. Polymorphic DNA marker analysis on the DNAs extracted from cord blood and parental bloods revealed maternal uniparental heterodisomy 21 in the baby. Interphase fluorescence in situ hybridization analysis on buccal mucosal cells revealed trisomy 21 signals in 15/101 (14.9%) buccal cells at birth and in 1/122 (0.82%) buccal cells at age 45 days.

CONCLUSION

Low-level mosaicism for trisomy 21 at amniocentesis associated with maternal UPD 21 in the fetus can have a favorable outcome.

Monosomy chromosome 21 compensated by 21q22.11q22.3 duplication in a case with small size and minor anomalies

Background

Partial monosomy 21 is a rare finding with variable sizes and deletion breakpoints, presenting with a broad spectrum of phenotypes.

Case presentation

We report a 10-month-old boy with short stature, minor anomalies and mild motor delay. The patient had a monosomy 21 and duplication of the 21q22.11q22.3 region on the remaining derivative chromosome 21 which represents a partial 21q uniparental disomy of paternal origin, upd(21q22.11q22.3)pat. The abnormalities were characterized by karyotyping, FISH, chromosomal microarray, and genotyping.

Conclusions

This is the first case showing a monosomy 21 compensated by upd(21q22.11q22.3) as a mechanism of genomic rescue. Because there is no strong evidence showing imprinting on chromosome 21, the uniparental disomy itself is not associated with abnormal phenotype but has reduced phenotype severity of monosomy 21. We reviewed the previously published cases with isolated 21q deletions and identified a common deletion of 5.7 Mb associated with low birth weight, length and head circumference in the 21q21.2 region.

A Pair of Maternal Chromosomes Derived from Meiotic Nondisjunction in Trisomy 21 Affects Nuclear Architecture and Transcriptional Regulation

Eukaryotic genomes are organised into complex higher-order structures within the nucleus, and the three-dimensional arrangement of chromosomes is functionally important for global gene regulation. The existence of supernumerary chromosome 21 in Down syndrome may perturb the nuclear architecture at different levels, which is normally optimised to maintain the physiological balance of gene expression. However, it has not been clearly elucidated whether and how aberrant configuration of chromosomes affects gene activities. To investigate the effects of trisomy 21 on nuclear organisation and gene expression, we performed three-dimensional fluorescent imaging analysis of chromosome-edited human induced pluripotent stem cells (iPSCs), which enabled identification of the parental origin of the three copies of chromosome 21. We found that two copies of maternal chromosomes resulting from meiotic nondisjunction had a higher tendency to form an adjacent pair and were located relatively distant from the nuclear membrane, suggesting the conserved interaction between these homologous chromosomes. Transcriptional profiling of parental-origin-specific corrected disomy 21 iPSC lines indicated upregulated expression of the maternal alleles for a group of genes, which was accompanied by a fluctuating expression pattern. These results suggest the unique effects of a pair of maternal chromosomes in trisomy 21, which may contribute to the pathological phenotype.

Trisomy 21 Alters DNA Methylation in Parent-of-Origin-Dependent and -Independent Manners

The supernumerary chromosome 21 in Down syndrome differentially affects the methylation statuses at CpG dinucleotide sites and creates genome-wide transcriptional dysregulation of parental alleles, ultimately causing diverse pathologies. At present, it is unknown whether those effects are dependent or independent of the parental origin of the nondisjoined chromosome 21. Linkage analysis is a standard method for the determination of the parental origin of this aneuploidy, although it is inadequate in cases with deficiency of samples from the progenitors. Here, we assessed the reliability of the epigenetic 5^mCpG imprints resulting in the maternally (oocyte)-derived allele methylation at a differentially methylated region (DMR) of the candidate imprinted WRB gene for asserting the parental origin of chromosome 21. We developed a methylation-sensitive restriction enzyme-specific PCR assay, based on the WRB DMR, across single nucleotide polymorphisms (SNPs) to examine the methylation statuses in the parental alleles. In genomic DNA from blood cells of either disomic or trisomic subjects, the maternal alleles were consistently methylated, while the paternal alleles were unmethylated. However, the supernumerary chromosome 21 did alter the methylation patterns at the RUNX1 (chromosome 21) and TMEM131 (chromosome 2) CpG sites in a parent-of-origin-independent manner. To evaluate the 5^mCpG imprints, we conducted a computational comparative epigenomic analysis of transcriptome RNA sequencing (RNA-Seq) and histone modification expression patterns. We found allele fractions consistent with the transcriptional biallelic expression of WRB and ten neighboring genes, despite the similarities in the confluence of both a 17-histone modification activation backbone module and a 5-histone modification repressive module between the WRB DMR and the DMRs of six imprinted genes. We concluded that the maternally inherited 5^mCpG imprints at the WRB DMR are uncoupled from the parental allele expression of WRB and ten neighboring genes in several tissues and that trisomy 21 alters DNA methylation in parent-of-origin-dependent and -independent manners.

Mosaicism for trisomy 21: a review

The clinical and cytogenetic findings associated with mosaicism for trisomy 21/Down syndrome are the focus of this review. The primary topics discussed in this overview of the extant literature include the history of this condition and its diagnosis, the incidence of mosaicism, the meiotic and/or mitotic chromosomal malsegregation events resulting in mosaicism, the observation of mosaicism in the parents of children with the non-mosaic form of Down syndrome, and the variation in phenotypic outcome for both constitutional and acquired traits present in people with mosaicism for trisomy 21/Down syndrome, including cognition, fertility, and overall phenotypic findings. Additional topics reviewed include the social conditions of people with mosaicism, as well as age-related and epigenetic alterations observed in people with mosaicism for trisomy 21/Down syndrome. .

Discordant results between fetal karyotyping and non-invasive prenatal testing by maternal plasma sequencing in a case of uniparental disomy 21 due to trisomic rescue

Uniparental disomy (UPD) is an uncommon chromosome condition, but UPD involving chromosome 21 is rarely reported. We reported here a case who had first trimester screening test for Down syndrome, chorionic villus sampling for fetal karyotyping, quantitative fluorescence polymerase chain reaction (QF-PCR), as well as non-invasive prenatal testing (NIPT) by maternal plasma sequencing. There were discordant results between fetal karyotyping and NIPT due to UPD 21combined with confined placental mosaicism of trisomy 21. This demonstrated that it is possible to detect placental mosaicism by NIPT, but further studies are required to confirm its sensitivity. Therefore, all positive NIPT results must be confirmed by conventional invasive test and karyotyping. QF-PCR has the additional benefit in diagnosing UPD.

The consequences of uniparental disomy and copy number neutral loss-of-heterozygosity during human development and cancer

UPD (uniparental disomy) describes the inheritance of a pair of chromosomes from only one parent. Mechanisms that lead to UPD include trisomy rescue, gamete complementation, monosomy rescue and somatic recombination. Most of these mechanisms can involve aberrant chromosomes, particularly isochromosomes and Robertsonian translocations. In the last decade, the number of UPD cases reported in the literature has increased exponentially. This is partly due to the advances in genomic technologies that have allowed for high-resolution SNP (single nucleotide polymorphism) studies, which have complemented traditional methods relying on polymorphic microsatellite markers. In this review, we discuss aberrant cellular mechanisms leading to UPD and their impact on gene expression. Special emphasis is placed on the unmasking of mutant recessive alleles and the disruption of imprinted gene dosage, which give rise to specific and recurrent imprinting phenotypes. Finally, we discuss how copy number maps determined from SNP array datasets have helped identify not only deletions and duplications but also recurrent copy number neutral regions of loss-of-heterozygosity, which have been reported in many cancer types and that may constitute an important driving force in cancer. These tiny regions of UPD also alter imprinted gene dosage, which may have cumulative tumourgenic effects in addition to that of unmasking homozygous cancer-associated mutations.

Positive selection at codon 38 of the human KCNE1 (= minK) gene and sporadic absence of 38Ser-coding mRNAs in Gly38Ser heterozygotes

Background

KCNE1 represents the regulatory beta-subunit of the slowly activating delayed rectifier potassium channel (IKs). Variants of KCNE1 have repeatedly been linked to the long-QT syndrome (LQTS), a disorder which predisposes to deafness, ventricular tachyarrhythmia, syncope, and sudden cardiac death.

Results

We here analyze the evolution of the common Gly38Ser variant (rs1805127), using genomic DNAs, complementary DNAs, and HEK293-expressed variants of altogether 19 mammalian species. The between species comparison reveals that the human-specific Gly38Ser polymorphism evolved under strong positive Darwinian selection, probably in adaptation to specific challenges in the fine-tuning of IKs channels. The involved amino acid exchanges (Asp > Gly, Gly > Ser) are moderately radical and do not induce apparent changes in posttranslational modification. According to population genetic analyses (HapMap phase II) a heterozygote advantage accounts for the maintenance of the Gly38Ser polymorphism in humans. On the other hand, the expression of the 38Ser allele seems to be disadvantageous under certain conditions, as suggested by the sporadic deficiency of 38Ser-coding mRNAs in heterozygote Central Europeans and the depletion of homozygotes 38Ser in the Yoruban sample.

Conclusion

We speculate that individual differences in genomic imprinting or genomic recoding might have contributed to conflicting results of recent association studies between Gly38Ser polymorphism and QT phenotype. The findings thus highlight the relevance of mRNA data in future association studies of genotypes and clinical disorders. To the best of our knowledge, they moreover provide first time evidence for a unique pattern; i.e. coincidence of positive Darwinian selection and polymorphism with a sporadically suppressed expression of one allele.

A paternity case with three genetic incompatibilities between father and child due to maternal uniparental disomy 21 and a mutation at the Y chromosome

A parentage case is described that revealed a potentially erroneous exclusion from paternity in three systems, two on chromosome 21 and one on chromosome Y. Follow-up tests, especially of chromosome 21, were subsequently performed. Actually, the child's chromosome 21 showed alleles of maternal but not of paternal origin being consistent with a maternal uniparental disomy of chromosome 21. The third genetic incompatibility was observed at the Y chromosome and attributed to a usual one-step de novo mutation. This case is emphasizing the (generally adopted) requirement that an exclusion from paternity must not be based on the absence of paternal alleles at genetic systems all located on the same chromosome. In fact, the need for extended typing programmes is demonstrated.

Small supernumerary chromosome marker generating complete and pure trisomy 18p, characterized by molecular cytogenetic techniques and review

Small supernumerary marker chromosomes (sSMC) have been described from all human chromosomes with different sizes and shapes. However, it is difficult to know the clinical manifestations associated with them, because such knowledge depends on the size, presence of euchromatic material, degree of mosaicism and/or uniparental disomy (UPD). Pure trisomy of the whole arm of chromosome 18 (18p), has been described in only a few cases and the general consensus is that there is a mild phenotypic effect. Here we report on a newborn male presenting with an atrial septal defect and a club foot. The high resolution G-band karyotype (550-850 bands) and the molecular cytogenetic techniques revealed in all cells the presence of an sSMC, which was a complex derivative from the short arm of a chromosome 18 (18p) and a centromere of a chromosome 13/21. His healthy mother had the same sSMC in all analyzed cells. With the present case, we support the previous suggestion that this unusual chromosome trisomy 18p has little clinical repercussions.

Nonmosaic balanced homologous translocations of major clinical significance: some may be mosaic

The main mechanism proposed for formation of homologous translocations/isochromosomes is mitotic and if this occurs in a chromosomally normal conceptus, mosaicism would be expected to be seen. The lack of mosaicism in published cases of rearrangement (REA) of mitotic origin might be explained by under-detection due to the low level mosaicism for a normal line. Recently it was reported that sex-specific centromere instability in early embryogenesis leads to a female prevalence among individuals with mosaicism for pericentromeric rearrangements. To determine whether carriers of apparent non-mosaic homologous REA could be mosaics for a normal cell line, the sex ratio (male to female ratio) among carriers of balanced and unbalanced homologous translocations/isochromosomes was studied. This ratio was determined to establish if there is a female predominance similar to that seen in carriers of REA with mosaicism. In reviewing the literature, a female prevalence among fetuses with balanced homologous REA and among carriers of unbalanced homologous REA detected prenatally, postnatally and in miscarriages was found. Overall, there were 48 males and 72 females in the collected sample, and this ratio differed significantly from the expected sex ratio of 1.06 (P = 0.0075). There is not a male prevalence among miscarried fetuses, there is no evidence of selection against males in the collected material of this study. The analysis of sex ratios in different variants of trisomy 13 with respect to ascertainment (prenatal diagnosis, miscarriages, liveborn) also does not support an intrauterine selection against males as a cause of a female prevalence among carriers of homologous REA. Thus the data presented in this paper suggests that a proportion of the carriers of balanced homologous REA may have mosaicism for a normal line. Since low level mosaicism for a normal line in a translocation carrier would alter his/her reproductive options, it can be recommended that molecular polymorphic analysis be applied to these cases. This would allow those resulting from meiotic formation to be distinguished from those resulting from postzygotic formation. This latter mechanism may indicate the presence of a mosaicism for a normal line, making further intensive karyotypic analysis advisable. However, additional studies of healthy carriers of homologous REA of chromosome 14 or 15 should not be done. (c) 2007 Wiley-Liss, Inc.

Specific language impairment as the prominent feature in a patient with a low-level trisomy 21 mosaicism

BACKGROUND

The extent and severity of the disabilities is variable among individuals with Down syndrome, although generally characterized by a range of physical and intellectual conditions, including language impairment. Whether the language deficit is due to the intellectual disability (ID) or associated to the supernumerary or portion of chromosome 21 is still debated.

METHODS

Karyotyping was performed on blood lymphocyte and skin fibroblasts. Fluorescence in situ hybridization analysis was performed on cultured lymphocytes and buccal smear cells.

RESULTS

The trisomy 21 (T21) mosaicism was characterized by 0.7-10% of mosaic cells in the different tissues, in a 14-year-old girl presenting an intellectual development within the normal range and specific language impairment (SLI) as the only prominent feature.

CONCLUSION

This case illustrates the wide range of phenotypical abnormalities possibly associated with T21 mosaicism. We propose that SLI is indeed a phenotypic trait specific to Down syndrome rather than subsequent to the ID most often associated to the syndrome.

Risk factors for nondisjunction of trisomy 21

The leading cause of Down syndrome (DS) is nondisjunction of chromosome 21 occurring during the formation of gametes. In this review, we discuss the progress made to identify risk factors associated with this type of chromosome error occurring in oogenesis and spermatogenesis. For errors occurring in oocytes, the primary risk factors are maternal age and altered recombination. We review the current progress made with respect to these factors and briefly outline the potential environmental and genetic influences that may play a role. Although the studies of paternal nondisjunction are limited due to the relatively small proportion of errors of this type, we review the potential influence of paternal age, recombination and other environmental and genetic factors on susceptibility. Although progress has been made to understand the mechanisms and risk factors that underlie nondisjunction, considerably more research needs to be conducted to dissect this multifactorial trait, one that has a considerable impact on our species.

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.

Linkage disequilibrium mapping in trisomic populations: Analytical approaches and an application to congenital heart defects in Down syndrome

Many of the birth defects associated with trisomy exhibit both variable expressivity and incomplete penetrance. This variability suggests that it is allelic variation and not simply the presence of an additional chromosome that leads to the development of certain trisomy-associated birth defects. With the proper tools, one may use trisomic populations to identify genes involved in the development of specific birth defects. A trisomic population may be advantageous over a normal population if the defect is over-represented in the trisomic population. Alternatively, one can view the trisomic populations as a "model system" to offer insight into aspects of both normal and abnormal embryonic development. Standard disomic linkage disequilibrium mapping approaches need to be adjusted to account for the presence of the additional genetic material in the trisomic individuals. We present an approach for linkage disequilibrium mapping of variable phenotypes in a trisomic population that adequately accounts for the additional alleles and the pattern of non-independent inheritance. We establish the laboratory methods and statistical tools necessary to conduct an association study in a trisomic population. As an example, we have applied these tools to a pilot study of Down syndrome-associated congenital heart defects.

A prenatally diagnosed patient with full monosomy 21: Ultrasound, cytogenetic, clinical, molecular, and necropsy findings

We report on a patient with a full monosomy 21 (FM21) prenatally diagnosed in cord fetal blood, and subsequently confirmed in other tissues. Subtle chromosomal translocations of chromosome 21, were ruled-out by FISH using both painting and 21q telomeric probes. Microsatellites analysis demonstrated the paternal origin of the single chromosome. The propositus showed at 32 weeks of gestation a severe intrauterine growth retardation and microcephaly. He was born with multiple congenital malformations, hypotonia, microcephaly, bilateral microphthalmia (more severe on the left), facial dysmorphism, agenesis of the external auditory meatus, redundant skin in the neck, narrow chest, flat scrotum, cryptorchydism, hypospadias, micropene, camptodactyly, nail hypoplasia, and abnormal palmar and plantar creases. The patient died in the first day of life. At necropsy, micrencephaly, semilobar holoprosencephaly, polimicrogyria, ocular abnormalities, skeletal anomalies, congenital heart disease, and agenesis of right kidney were also observed. To our best knowledge, this case is one of the most completely patient studied with FM21.

Recurrent Trisomy 21 and Uniparental Disomy 21 in a Family

OBJECTIVE

A 32-year-old pregnant woman was referred to our genetic counselling because of recurrent trisomy 21 in the family. Analysis of amniotic fluid cell culture revealed karyotype 47,XY+21 of the fetus.

METHODS

Karyotyping and molecular analysis were undertaken in the fetal and parental samples to determine the origin of the extra chromosome 21.

RESULTS

Both parents had a normal blood karyotype. Microsatellite marker analysis showed maternal origin of the fetal extra chromosome 21. As the mother showed homozygosity for all investigated markers on chromosome 21, we also tested her family. We detected the same homozygosity in some family members which was consistent with isodisomy of the chromosome 21 caused by uniparental disomy (UPD).

CONCLUSIONS

Here we report on a family in which multiple aneuploid conceptions occurred with trisomy 21, and molecular analysis showed that the euploidy of the investigated healthy family members is due to UPD21. This observation stresses the importance of prenatal cytogenetic and molecular analysis in case of parental UPD.

The role of imprinted genes in fetal growth

Genomic imprinting is the phenomenon by which one of the two alleles of a subset of genes is preferentially expressed according to its parental origin. This pattern of inheritance is different from the more frequent mode of Mendelian inheritance, which is not influenced by the parental origin of the allele. The idea that imprinted genes can affect fetal growth is becoming increasingly intriguing as it has been shown that most imprinted genes are expressed in the placenta and some play a role in regulating the interactions between its fetal and maternal interfaces. This article considers genomic imprinting by reviewing recent findings of alterations in fetal growth related to different types of genetic changes affecting the expression of imprinted genes. Among the genetic anomalies, the uniparental disomy (UPD) defines the inheritance of both homologous chromosomes from only one parent. UPDs of a number of chromosomes have been described in association with effects on the phenotype. We reviewed cases of UPD reported till now with particular reference to those associated to growth alterations.

Low incidence of UPD in spontaneous abortions beyond the 5th gestational week

Approximately 15-20% of all clinically recognised pregnancies abort, most commonly between 8-12 gestational weeks. While the majority of early pregnancy losses is attributed to cytogenetic abnormalities, the aetiology of approximately 40% of early abortions remains unclear. To determine additional factors causing spontaneous abortions we retrospectively searched for uniparental disomies (UPD) in 77 cytogenetically normal diploid spontaneous abortions. In all cases an unbalanced chromosome anomaly was ruled out by cytogenetic investigation of chorionic/amniotic membranes and/or chorionic villi. For UPD screening microsatellite analyses were performed on DNA of abortion specimens and parental blood using highly polymorphic markers showing UPD in two cases. The distribution of markers analysed indicated maternal heterodisomy for chromosome 9 (UPhD(9)mat) in case 1 and paternal isodisomy for chromosome 21 (UPiD(21)pat) in case 2. The originating mechanism suggested was monosomy complementation in UPiD(21)pat and trisomy rescue in UPhD(9)mat. In the case of UPhD(9)mat purulent chorioamnionitis was noted and a distinctly growth retarded embryo of 3 cm crown-rump length showing no gross external malformations. Histological analysis in the case of UPiD(21)pat suggested a primary anlage defect. Our results indicate that less than 3% of genetically unexplained pregnancy wastage is associated with total chromosome UPD. UPD may contribute to anlage defects of human conception. Chromosome aneuploidy correction can occur in very early cleavage stages. More research, however, ought to be performed into placental mosaicism to further clarify timing and mechanisms involved in foetal UPD.

Complex and segmental uniparental disomy (UPD): review and lessons from rare chromosomal complements

OBJECTIVE

To review all cases with segmental and/or complex uniparental disomy (UPD), to study aetiology and mechanisms of formation, and to draw conclusions.

DESIGN

Searching published reports in Medline.

RESULTS

The survey found at least nine cases with segmental UPD and a normal karyotype, 22 cases with UPD of a whole chromosome and a simple or a non-homologous Robertsonian translocation, eight cases with UPD and two isochromosomes, one of the short arm and one of the long arm of a non-acrocentric chromosome, 39 cases with UPD and an isochromosome of the long arm of two homologous acrocentric chromosomes, one case of UPD and an isochromosome 8 associated with a homozygous del(8)(p23.3pter), and 21 cases with UPD of a whole or parts of a chromosome associated with a complex karyotype. Segmental UPD is formed by somatic recombination (isodisomy) or by trisomy rescue. In the latter mechanism, a meiosis I error is associated with meiotic recombination and an additional somatic exchange between two non-uniparental chromatids. Subsequently, the chromatid that originated from the disomic gamete is lost (iso- and heterodisomy). In cases of UPD associated with one isochromosome of the short arm and one isochromosome of the long arm of a non-acrocentric chromosome and in cases of UPD associated with a true isochromosome of an acrocentric chromosome, mitotic complementation is assumed. This term describes the formation by misdivision at the centromere during an early mitosis of a monosomic zygote. In cases of UPD associated with an additional marker chromosome, either mitotic formation of the marker chromosome in a trisomic zygote or fertilisation of a gamete with a marker chromosome formed in meiosis by a disomic gamete or by a normal gamete and subsequent duplication are possible.

CONCLUSIONS

Research in the field of segmental and/or complex UPD may help to explain undiagnosed non-Mendelian disorders, to recognise hotspots for meiotic and mitotic recombinations, and to show that chromosomal segregation is more complex than previously thought. It may also be helpful to map autosomal recessively inherited genes, genes/regions of genomic imprinting, and dysmorphic phenotypes. Last but not least it would improve genetic counselling.