New chromosome 11p15 epigenotypes identified in male monozygotic twins with Beckwith-Wiedemann syndrome

Beckwith-Wiedemann syndrome (BWS) is an overgrowth syndrome demonstrating heterogeneous molecular alterations of two imprinted domains on chromosome 11p15. The most common molecular alterations include loss of methylation at the proximal imprinting center, IC2, paternal uniparental disomy (UPD) of chromosome 11p15 and hypermethylation at the distal imprinting center, IC1. An increased incidence of female monozygotic twins discordant for BWS has been reported. The molecular basis for eleven such female twin pairs has been demonstrated to be a loss of methylation at IC2, whereas only one male monozygotic twin pair has been reported with this molecular defect. We report here two new pairs of male monozygotic twins. One pair is discordant for BWS; the affected twin exhibits paternal UPD for chromosome 11p15 whereas the unaffected twin does not. The second male twin pair is concordant for BWS and both twins of the pair demonstrate hypermethylation at IC1. Thus, this report expands the known molecular etiologies for BWS twins. Interestingly, these findings demonstrate a new epigenotype-phenotype correlation in BWS twins. That is, while female monozygotic twins with BWS are likely to show loss of imprinting at IC2, male monozygotic twins with BWS reflect the molecular heterogeneity seen in BWS singletons. These data underscore the need for molecular testing in BWS twins, especially in view of the known differences among 11p15 epigenotypes with respect to tumor risk.     

High frequency of copy number variations (CNVs) in the chromosome 11p15 region in patients with Beckwith–Wiedemann syndrome

Beckwith–Wiedemann syndrome (BWS), an overgrowth and tumor predisposition syndrome is clinically heterogeneous. Its variable presentation makes molecular diagnosis particularly important for appropriate counseling of patients with respect to embyronal tumor risk and recurrence risk. BWS is characterized by macrosomia, omphalocele, and macroglossia. Additional clinical features can include hemihyperplasia, embryonal tumors, umbilical hernia, and ear anomalies. BWS is etiologically heterogeneous arising from dysregulation of one or both of the chromosome 11p15.5 imprinting centers (IC) and/or imprinted growth regulatory genes on chromosome 11p15.5. Most BWS cases are sporadic and result from loss of maternal methylation at imprinting center 2 (IC2), gain of maternal methylation at imprinting center 1 (IC1) or paternal uniparental disomy (UPD). Heritable forms of BWS (15 %) have been attributed mainly to mutations in the growth suppressor gene CDKN1C, but have also infrequently been identified in patients with copy number variations (CNVs) in the chromosome 11p15.5 region. Four hundred and thirty-four unrelated BWS patients referred to the molecular diagnostic laboratory were tested by methylation-specific multiplex ligation-dependent probe amplification. Molecular alterations were detected in 167 patients, where 103 (62 %) showed loss of methylation at IC2, 23 (14 %) had gain of methylation at IC1, and 41 (25 %) showed changes at both ICs usually associated with paternal UPD. In each of the three groups, we identified patients in whom the abnormalities in the chromosome 11p15.5 region were due to CNVs. Surprisingly, 14 patients (9 %) demonstrated either deletions or duplications of the BWS critical region that were confirmed using comparative genomic hybridization array analysis. The majority of these CNVs were associated with a methylation change at IC1. Our results suggest that CNVs in the 11p15.5 region contribute significantly to the etiology of BWS. We highlight the importance of performing deletion/duplication testing in addition to methylation analysis in the molecular investigation of BWS to improve our understanding of the molecular basis of this disorder, and to provide accurate genetic counseling.     

Origin of uniparental disomy 6: presentation of a new case and review on the literature

Paternal uniparental disomy (UPD) of chromosome 6 has been reported several times in patients with (transient) neonatal diabetes mellitus ((T)NDM). Here we present our short tandem repeat typing results in a new patient with NDM, revealing a paternal isodisomy (UPiD). Summarising these data with those published previously on complete paternal (n=13) and maternal (n=2) UPD6, all cases show isodisomy. In general, several modes of UPD formation have been suggested: While a meiotic origin of UPD mainly results in a uniparental heterodisomy (UPhD), UPiD is probably the result of a post-zygotic mitotic error. This mode of formation consists of a mitotic nondisjunction in a disomic zygote, followed by either a trisomic rescue or a reduplication. Endoduplication in a monosomic zygote is another possible but less probable mechanism, taking into consideration that monosomic zygotes are not viable. The exclusive finding of isodisomy in case of chromosome 6 therefore gives strong evidence that segregational errors of this chromosome are mainly influenced by postzygotic factors. This hypothesis is supported by the observation of two cases with partial paternal UPiD6 originating from mitotic recombination events. The influence of mitotic segregational errors in UPD6 formation is in agreement with the results in trisomy/UPD of other chromosomes of the C group (7 and 8), and is in remarkable contrast to the findings in studies on the origin of the frequent aneuploidies. Multiple factors ensure normal segregation and we speculate that they vary in importance for each chromosome.     

Quantitative DNA methylation analysis improves epigenotype-phenotype correlations in Beckwith-Wiedemann syndrome

Beckwith-Wiedemann syndrome (BWS) is a rare disorder characterized by overgrowth and predisposition to embryonal tumors. BWS is caused by various epigenetic and/or genetic alterations that dysregulate the imprinted genes on chromosome region 11p15.5. Molecular analysis is required to reinforce the clinical diagnosis of BWS and to identify BWS patients with cancer susceptibility. This is particularly crucial prenatally because most signs of BWS cannot be recognized in utero. We established a reliable molecular assay by pyrosequencing to quantitatively evaluate the methylation profiles of ICR1 and ICR2. We explored epigenotype-phenotype correlations in 19 patients that fulfilled the clinical diagnostic criteria for BWS, 22 patients with suspected BWS, and three fetuses with omphalocele. Abnormal methylation was observed in one prenatal case and 19 postnatal cases, including seven suspected BWS. Seven cases showed ICR1 hypermethylation, five cases showed ICR2 hypomethylation, and eight cases showed abnormal methylation of ICR1 and ICR2 indicating paternal uniparental disomy (UPD). More cases of ICR1 alterations and UPD were found than expected. This is likely due to the sensitivity of this approach, which can detect slight deviations in methylation from normal levels. There was a significant correlation (p < 0.001) between the percentage of ICR1 methylation and BWS features: severe hypermethylation (range: 75–86%) was associated with macroglossia, macrosomia, and visceromegaly, whereas mild hypermethylation (range: 55–59%) was associated with umbilical hernia and diastasis recti. Evaluation of ICR1 and ICR2 methylation by pyrosequencing in BWS can improve epigenotype-phenotype correlations, detection of methylation alterations in suspected cases, and identification of UPD.     

The impact of imprinting: Prader-Willi syndrome resulting from chromosome translocation, recombination, and nondisjunction.

Prader-Willi syndrome (PWS) is most often the result of a deletion of bands q11.2-q13 of the paternally derived chromosome 15, but it also occurs either because of maternal uniparental disomy (UPD) of this region or, rarely, from a methylation imprinting defect. A significant number of cases are due to structural rearrangements of the pericentromeric region of chromosome 15. We report two cases of PWS with UPD in which there was a meiosis I nondisjunction error involving an altered chromosome 15 produced by both a translocation event between the heteromorphic satellite regions of chromosomes 14 and 15 and recombination. In both cases, high-resolution banding of the long arm was normal, and FISH of probes D15S11, SNRPN, D15S10, and GABRB3 indicated no loss of this material. Chromosome heteromorphism analysis showed that each patient had maternal heterodisomy of the chromosome 15 short arm, whereas PCR of microsatellites demonstrated allele-specific maternal isodisomy and heterodisomy of the long arm. SNRPN gene methylation analysis revealed only a maternal imprint in both patients. We suggest that the chromosome structural rearrangements, combined with recombination in these patients, disrupted normal segregation of an imprinted region, resulting in uniparental disomy and PWS.     

Formation of uniparental disomy 7 delineated from new cases and a UPD7 case after trisomy 7 rescue. Presentation of own results and review of the literature

Maternal uniparental disomy for the entire chromosome 7 (matUPD7) has been reported several times in Silver-Russell syndrome (SRS) and growth-restricted patients. Here we present our results from the analysis of an abortion with confined placental mosaicism (CPM) for trisomy 7 which showed a maternal meiotic origin of the trisomy in the placenta and rescue to maternal UPD7 in foetal membrane. Furthermore, two newly detected SRS cases with maternal UPD7 revealed isodisomy and partial heterodisomy, respectively. Summarising these results with those published previously on the origin of UPD7, similar numbers of isodisomy (n=11) and cases with complete or partial heterodisomy (n=12) have been reported. In respect to the different formation mechanisms of UPD, complete isodisomy should be the result of a post-zygotic mitotic segregation error, whereas heterodisomic UPDs should be caused by trisomic rescue after meiotic non-disjunction events. In maternal UPD7, 50% of cases seem to be caused by post-zygotic mitotic segregation errors, which is similar to the situation in trisomy 7. This result corresponds to the situation in trisomy 8 but is in contrast to observations in the frequent aneuploidies. Thus, the different findings in these aberrations reflect the presence of multiple factors that act to ensure normal segregation, varying in importance for each chromosome.     

A case of segmental paternal isodisomy of chromosome 14

Uniparental disomy of chromosome 14 (UPD 14) results in one of two distinct abnormal phenotypes, depending upon the parent of origin. This discordance may result from the reciprocal over-expression and/or under-expression of one or more imprinted genes. We report a case of segmental paternal isodisomy for chromosome 14 with features similar to those reported in other paternal disomy 14 cases. Microsatellite marker analysis revealed an apparent somatic recombination event in 14q12 leading to proximal biparental inheritance, but segmental paternal uniparental isodisomy distal to this site. Analysis of monochromosomal somatic cell hybrids containing either the paternally inherited or the maternally inherited chromosome 14 revealed no deletion of the maternally inherited chromosome 14 and demonstrated the presence of paternal sequences from D14S121 to the telomere on both chromosomes 14. Thus, the patient has paternal isodisomy for 14q12-14qter. Because the patient shows most of the features associated with paternal disomy 14, this supports the presence of the imprinted domain(s) distal to 14q12 and suggests that the proximal region of chromosome 14 does not contain imprinted genes that contribute significantly to the paternal UPD 14 phenotype.     

Nondisjunction of chromosome 15: origin and recombination.

Thirty-two cases of uniparental disomy (UPD), ascertained from Prader-Willi syndrome patients (N = 27) and Angelman syndrome patients (N = 5), are used to investigate the pattern of recombination associated with nondisjunction of chromosome 15. In addition, the meiotic stage of nondisjunction is inferred by using markers mapping near the centromere. Two basic approaches to the analysis of recombination are utilized. Standard methods of centromere mapping are employed to determine the level of recombination in specific pairwise intervals along the chromosome. This method shows a significant reduction in recombination for two of five intervals examined. Second, the observed frequency of each recombinant class (i.e., zero, one, two, three, or more observable crossovers) is compared with expected values. This is useful for testing whether the reduction in recombination can be attributed solely to a proportion of cases with no recombination at all (because of asynapsis), with the remaining groups showing normal recombination (or even excess recombination), or whether recombination is uniformly reduced. Analysis of maternal UPD(15) data shows a slight reduction in the multiple-recombinant classes, with a corresponding increase in both the zero- and one-recombinant classes over expected values. The majority, more than 82%, of the extra chromosomes in maternal UPD(15) cases are due to meiotic I nondisjunction events. In contrast, most paternal UPD(15) cases so far examined appear to have a postzygotic origin of the extra paternal chromosome.     

A Somatic Origin of Homologous Robertsonian Translocations and Isochromosomes

One t(14q14q), three t(15q15q), two t(21q21q), and two t(22q22q) nonmosaic, apparently balanced, de novo Robertsonian translocation cases were investigated with polymorphic markers to establish the origin of the translocated chromosomes. Four cases had results indicative of an isochromosome: one t(14q14q) case with mild mental retardation and maternal uniparental disomy (UPD) for chromosome 14, one t(15q15q) case with the Prader-Willi syndrome and UPD(15), a phenotypically normal carrier of t(22q22q) with maternal UPD(22), and a phenotypically normal t(21q21q) case of paternal UPD(21). All UPD cases showed complete homozygosity throughout the involved chromosome, which is supportive of a postmeiotic origin. In the remaining four cases, maternal and paternal inheritance of the involved chromosome was found, which unambiguously implies a somatic origin. One t(15q15q) female had a child with a ring chromosome 15, which was also of probable postmeiotic origin as recombination between grandparental haplotypes had occurred prior to ring formation. UPD might be expected to result from de novo Robertsonian translocations of meiotic origin; however, all de novo homologous translocation cases, so far reported, with UPD of chromosomes 14, 15, 21, or 22 have been isochromosomes. These data provide the first direct evidence that nonmosaic Robertsonian translocations, as well as isochromosomes, are commonly the result of a mitotic exchange.     

Mechanisms leading to uniparental disomy and their clinical consequences

Uniparental disomy (UPD) refers to the situation in which both copies of a chromosome pair have originated from one parent. In humans, it can result in clinical conditions by producing either homozygosity for recessive mutations or aberrant patterns of imprinting. Furthermore, UPD is frequently found in conjunction with mosaicism for a chromosomally abnormal cell line, which can also contribute to phenotypic abnormalities. Investigations into the mechanisms by which UPD may arise have helped to expand our general awareness of the impact of chromosomal abnormalities and chromosomal mosaicism in normal human development. Specifically, it appears that errors in the transmission of a chromosome from parent to gamete and during early somatic cell divisions are remarkably common but that embryo and cell selection during early embryogenesis help to ensure the presence of a numerically balanced chromosome complement in the developing fetus. UPD is also likely to occur within a portion of cells in all individuals simply as a consequence of somatic recombination occurring during mitotic cell divisions. This can be an important step in cancer development as well as a contributing factor to other late onset diseases. This review summarizes mechanisms by which UPD may arise and their associated clinical consequences.     

Uniparental disomy analysis in trios using genome-wide SNP array and whole-genome sequencing data imply segmental uniparental isodisomy in general populations

Whole chromosomal and segmental uniparental disomy (UPD) is one of the causes of imprinting disorder and other recessive disorders. Most investigations of UPD were performed only using cases with relevant phenotypic features and included few markers. However, the diagnosis of cases with segmental UPD requires a large number of molecular investigations. Currently, the accurate frequency of whole chromosomal and segmental UPD in a normal developing embryo is not well understood. Here, we present whole chromosome and segmental UPD analysis using single nucleotide polymorphism (SNP) microarray data of 173 mother-father-child trios (519 individuals) from six populations (including 170 HapMap trios). For two of these trios, we also investigated the possibility of shorter segmental UPD as a consequence of homologous recombination repair (HR) for DNA double strand breaks (DSBs) during the early developing stage using high-coverage whole-genome sequencing (WGS) data from 1000 Genomes Project. This could be overlooked by SNP microarray. We identified one obvious segmental paternal uniparental isodisomy (iUPD) (8.2 mega bases) in one HapMap sample from 173 trios using Genome-Wide Human SNP Array 6.0 (SNP6.0 array) data. However, we could not identify shorter segmental iUPD in two trios using WGS data. Finally, we estimated the rate of segmental UPD to be one per 173 births (0.578%) based on the UPD screening for 173 trios in general populations. Based on the autosomal chromosome pairs investigated, we estimate the rate of segmental UPD to be one per 3806 chromosome pairs (0.026%). These data imply the possibility of hidden segmental UPD in normal individuals.     

Epigenetic alterations of H19 and LIT1 distinguish patients with Beckwith-Wiedemann syndrome with cancer and birth defects

Beckwith-Wiedemann syndrome (BWS) is a congenital cancer-predisposition syndrome associated with embryonal cancers, macroglossia, macrosomia, ear pits or ear creases, and midline abdominal-wall defects. The most common constitutional abnormalities in BWS are epigenetic, involving abnormal methylation of either H19 or LIT1, which encode untranslated RNAs on 11p15. We hypothesized that different epigenetic alterations would be associated with specific phenotypes in BWS. To test this hypothesis, we performed a case-cohort study, using the BWS Registry. The cohort consisted of 92 patients with BWS and molecular analysis of both H19 and LIT1, and these patients showed the same frequency of clinical phenotypes as those patients in the Registry from whom biological samples were not available. The frequency of altered DNA methylation of H19 in patients with cancer was significantly higher, 56% (9/16), than the frequency in patients without cancer, 17% (13/76; P=.002), and cancer was not associated with LIT1 alterations. Furthermore, the frequency of altered DNA methylation of LIT1 in patients with midline abdominal-wall defects and macrosomia was significantly higher, 65% (41/63) and 60% (46/77), respectively, than in patients without such defects, 34% (10/29) and 18% (2/11), respectively (P=.012 and P=.02, respectively). Additionally, paternal uniparental disomy (UPD) of 11p15 was associated with hemihypertrophy (P=.003), cancer (P=.03), and hypoglycemia (P=.05). These results define an epigenotype-phenotype relationship in BWS, in which aberrant methylation of H19 and LIT1 and UPD are strongly associated with cancer risk and specific birth defects.     

Uniparentale Disomien und Mosaike

Of the various mechanisms of formation of uniparental disomy (UPD) discussed in the literature, the mechanism of trisomy rescue is mostly prone to mosaicism from a trisomy cell line and from a disomy 46, XN uniparental cell line. Therefore, low level or undetected mosaicism has been assumed for a significant number of UPD cases. The clinical consequences of trisomy/UPD mosaicism probably depend on the chromosome involved and the proportional content in individual tissues. As the trisomy cell line of some mosaics might have a disadvantage in biological selection it might not be detected in routine lymphocyte investigations. For evaluation of the clinical relevance in the case of an imprinted chromosome the associated imprinting disorder must also be considered. In a postnatal setting analysis of UPD is indicated in the case of clinical, cytogenetic and molecular data. In the prenatal setting genetic counseling of the parents should be offered prior to any laboratory testing. In total, the impact of mosaicism associated with UPD has to consider the affected chromosome, the associated phenotype, the mechanism of formation and the laboratory method used.     

Assessment of p57(KIP2) gene mutation in Beckwith-Wiedemann syndrome

Beckwith-Wiedemann syndrome (BWS) is an overgrowth disorder involving developmental anomalies, tissue and organ hyperplasia and an increased risk of embryonic tumours (most commonly Wilms' tumour). This multigenic disorder is caused by dysregulation of the expression of imprinted genes in the 11p15 chromosomal region. It may involve paternal uniparental disomy (UPD), loss of imprinting of the IGF2 gene, maternal inherited translocations and trisomy with paternal duplication. Recently, a small proportion of BWS patients has been shown to have a mutation in the paternal imprinted p57(KIP2) gene, which encodes a cyclin-dependent kinase inhibitor and negatively regulates cell proliferation. We screened for p57(KIP2) gene mutations in 21 BWS patients with no 11p15 UPD in leucocyte DNA. All patients had a phenotype typical of BWS. We analysed the entire coding sequence of p57(KIP2), including intron-exon boundaries, by direct sequencing of five PCR-amplified fragments. No mutation was found in the p57(KIP2) gene. Our results are consistent with those of previous studies showing that mutation of p57(KIP2) is infrequent in BWS. Thus, other mechanisms of p57(KIP2) silencing (imprinting errors) and/or other 11p15 genes are probably involved in the pathogenesis of BWS.     

Multipoint genetic mapping with uniparental disomy data

Uniparental disomy (UPD) refers to the presence of two copies of a chromosome from one parent and none from the other parent. In genetic studies of UPDs, many genetic markers are usually used to identify the stage of nondisjunction that leads to UPD and to uncover the associated unusual patterns of recombinations. However, genetic information in such data has not been fully utilized because of the limitations of the existing statistical methods for UPD data. In the present article, we develop a multilocus statistical approach that has the advantages of being able to simultaneously consider all genetic markers for all individuals in the same analysis and to allow general models for the crossover process to incorporate crossover interference. In particular, for a general crossover-process model that assumes only that there exists in each interval at most one crossover, we describe how to use the expectation-maximization algorithm to examine the probability distribution of the recombination events underlying meioses leading to UPD. We can also use this flexible approach to create genetic maps based on UPD data and to inspect recombination differences between meioses exhibiting UPD and normal meioses. The proposed method has been implemented in a computer program, and we illustrate the proposed approach through its application to a set of UPD15 data.     

Maternal uniparental disomy for human chromosome 14, due to loss of a chromosome 14 from somatic cells with t(13;14) trisomy 14.

Uniparental disomy (UPD) for particular chromosomes is increasingly recognized as a cause of abnormal phenotypes in humans. We recently studied a 9-year-old female with a de novo Robertsonian translocation t(13;14), short stature, mild developmental delay, scoliosis, hyperextensible joints, hydrocephalus that resolved spontaneously during the first year of life, and hypercholesterolemia. To determine the parental origin of chromosomes 13 and 14 in the proband, we have studied the genotypes of DNA polymorphic markers due to (GT)n repeats in the patient and her parents' blood DNA. The genotypes of markers D14S43, D14S45, D14S49, and D14S54 indicated maternal UPD for chromosome 14. There was isodisomy for proximal markers and heterodisomy for distal markers, suggesting a recombination event on maternal chromosomes 14. In addition, DNA analysis first revealed--and subsequent cytogenetic analysis confirmed--that there was mosaic trisomy 14 in 5% of blood lymphocytes. There was normal (biparental) inheritance for chromosome 13, and there was no evidence of false paternity in genotypes of 11 highly polymorphic markers on human chromosome 21. Two cases of maternal UPD for chromosome 14 have previously been reported, one with a familial rob t(13;14) and the other with a t(14;14). There are several similarities among these patients, and a "maternal UPD chromosome 14 syndrome" is emerging; however, the contribution of the mosaic trisomy 14 to the phenotype cannot be evaluated. The study of de novo Robertsonian translocations of the type reported here should reveal both the extent of UPD in these events and the contribution of particular chromosomes involved in certain phenotypes.     

Novel mutations of CDKN1C in Japanese patients with Beckwith-Wiedemann syndrome

Beckwith-Wiedemann syndrome (BWS) is an imprinting-related human disease that is characterized by macrosomia, macroglossia, abdominal wall defects, and variable minor features. BWS is caused by several genetic/epigenetic alterations, such as loss of methylation at KvDMR1, gain of methylation at H19-DMR, paternal uniparental disomy of chromosome 11, CDKN1C mutations, and structural abnormalities of chromosome 11. CDKN1C is an imprinted gene with maternal preferential expression, encoding for a cyclin-dependent kinase (CDK) inhibitor. Mutations in CDKN1C are found in 40 % of familial BWS cases with dominant maternal transmission and in ~5 % of sporadic cases. In this study, we searched for CDKN1C mutations in 37 BWS cases that had no evidence for other alterations. We found five mutations—four novel and one known—from a total of six patients. Four were maternally inherited and one was a de novo mutation. Two frame-shift mutations and one nonsense mutation abolished the QT domain, containing a PCNA-binding domain and a nuclear localization signal. Two missense mutations occurred in the CDK inhibitory domain, diminishing its inhibitory function. The above-mentioned mutations were predicted by in silico analysis to lead to loss of function; therefore, we strongly suspect that such anomalies are causative in the etiology of BWS.     

Copy number alteration and uniparental disomy analysis categorizes Japanese papillary thyroid carcinomas into distinct groups

The aim of the present study was to investigate chromosomal aberrations in sporadic Japanese papillary thyroid carcinomas (PTCs), concomitant with the analysis of oncogene mutational status. Twenty-five PTCs (11 with BRAF(V600E), 4 with RET/PTC1, and 10 without mutation in HRAS, KRAS, NRAS, BRAF, RET/PTC1, or RET/PTC3) were analyzed using Genome-Wide Human SNP Array 6.0 which allows us to detect copy number alteration (CNA) and uniparental disomy (UPD), also referred to as copy neutral loss of heterozygosity, in a single experiment. The Japanese PTCs showed relatively stable karyotypes. Seven cases (28%) showed CNA(s), and 6 (24%) showed UPD(s). Interestingly, CNA and UPD were rarely overlapped in the same tumor; the only one advanced case showed both CNA and UPD with a highly complex karyotype. Thirteen (52%) showed neither CNA nor UPD. Regarding CNA, deletions tended to be more frequent than amplifications. The most frequent and recurrent region was the deletion in chromosome 22; however, it was found in only 4 cases (16%). The degree of genomic instability did not depend on the oncogene status. However, in oncogene-positive cases (BRAF(V600E) and RET/PTC1), tumors with CNA/UPD were less frequent (5/15, 33%), whereas tumors with CNA/UPD were more frequent in oncogene-negative cases (7/10, 70%), suggesting that chromosomal aberrations may play a role in the development of PTC, especially in oncogene-negative tumors. These data suggest that Japanese PTCs may be classified into three distinct groups: CNA(+), UPD(+), and no chromosomal aberrations. BRAF(V600E) mutational status did not correlate with any parameters of chromosomal defects.     

Complex rearrangements of chromosome 15 in two patients with mild/atypical Prader Willi syndrome

Multiple mechanisms are responsible for the development of Prader Willi syndrome (PWS), the most common genetic cause of obesity in childhood. Molecular findings are usually deletions and uniparental disomy (UPD) of the 15q11-13 region. Rarely, structural rearrangements of the pericentromeric region of chromosome 15 are also detected. Two cases with mild PWS phenotype and complex maternal UPD identified by microsatellite analysis are described: the first patient had uniparental iso and heterodisomy and the second displayed biallelic inheritance and uniparental isodisomy.     

Unexpected Inheritance of a Balanced Homologous translocation t(22q;22q) from father to a phenotypically normal daughter

Rearrangements between homologous chromosomes are extremely rare and manifest mainly as monosomic or trisomic offsprings. There are remarkably few reports of balanced homologous chromosomal translocation t (22q; 22q) and only two cases of transmission of this balanced homohologous rearrangement from mother to normal daughter are reported. Robersonian translocation carriers in non-homologous chromosomes have the ability to have an unaffected child. However, it is not possible to have an unaffected child in cases with Robersonian translocations in homologous chromosomes. Carriers of homologous chromosome 22 translocations with maternal uniparental disomy do not have any impact on their phenotype. We are presenting a family with a history of multiple first trimester miscarriages and an unexpected inheritance of balanced homologous translocation of chromosome 22 with paternal uniparental disomy. There are no data available regarding the impact of paternal UPD 22 on the phenotype. We claim this to be the first report explaining that paternal UPD 22 does not impact the phenotype.