Identification of uniparental disomy in phenotypically abnormal carriers of isochromosomes or Robertsonian translocations

A Male Case of Kagami-Ogata Syndrome Caused by Paternal Unipaternal Disomy 14 as a Result of a Robertsonian Translocation

Kagami-Ogata syndrome (KOS) is a rare imprinting disorder characterized by skeletal abnormalities, dysmorphic facial features, growth retardation and developmental delay. The genetic etiology of KOS includes paternal uniparental disomy 14 [upd(14)pat], epimutations and microdeletions affecting the maternally derived imprinted region of chromosome 14q32.2. More than seventy KOS cases have been reported thus far; however, only 10, including two familial, are associated with upd(14)pat harboring Robertsonian translocation (ROB). Here, we reported a male infant with clinical manifestations of facial dysmorphism, bell-shaped small thorax, and omphalocele. Karyotype analyses identify a balanced ROB involving the long arms of chromosomes 13 and 14 both in the patient and his father. We further confirm the pattern of upd(14)pat utilizing DNA polymorphic markers. In conclusion, our case report provides a new male KOS case caused by upd(14)pat with paternally inherited Robertsonian translocation, which represents the second male case officially reported. Notably, a KOS case due to upd(14)pat and ROB is rare. An accurate diagnosis requires not only the identification of the characteristic clinical features but also systemic cytogenetic and molecular studies.

The Frequency and Spectrum of Chromosomal Translocations in a Cohort of Sri Lankans

Translocations are the most common type of structural chromosomal abnormalities. Unbalanced translocations are usually found in children who present with congenital abnormalities, developmental delay, or intellectual disability. Balanced translocations are usually found in adults who frequently present with reproductive failure; either subfertility, or recurrent pregnancy loss. Herein, we report the spectrum and frequency of translocations in a Sri Lankan cohort. A database of patients undergoing cytogenetic testing was maintained prospectively from January 2007 to December 2016 and analyzed, retrospectively. A total of 15,864 individuals were tested. Among them, 277 (1.7%) had translocations. There were 142 (51.3%) unbalanced translocations and 135 (48.7%) balanced translocations. Majority (160; 57.8%) were Robertsonian translocations. There were 145 (52.3%) children and adolescents aged less than 18 years with translocations, and 142 (97.9%) were unbalanced translocations. Majority [138 (95.2%)] were referred due to congenital abnormalities, developmental delay, or intellectual disability, and 91 were children with translocation Down syndrome. All adults aged 18 years or above (132) had balanced translocations. Subfertility and recurrent pregnancy loss [84 (63.6%)] and offspring(s) with congenital abnormalities [48 (36.4%)] were the most common indications in this group. Majority (68.2%) in this group were females with reciprocal translocations (55.3%). Chromosomes 21, 14, and 13 were the most commonly involved with rob(14q21q) [72 (26%)], rob(21q21q) [30 (13.7%)], and rob(13q14q) [34 (12.3%)] accounting for 52% of the translocations. Chromosomes 1, 8, 11, and 18 were most commonly involved in reciprocal translocations. The observed high frequency of chromosomal translocations in our cohort highlights the importance of undertaking cytogenetic evaluation and providing appropriate genetic counseling for individuals with the phenotypes associated with these translocations.

Uniparental disomy and prenatal phenotype: Two case reports and review

RATIONALE

Uniparental disomy (UPD) gives a description of the inheritance of both homologues of a chromosome pair from the same parent. The consequences of UPD depend on the specific chromosome/segment involved and its parental origin.

PATIENT CONCERNS

We report prenatal phenotypes of 2 rare cases of UPD.

DIAGNOSES

The prenatal phenotype of case 1 included sonographic markers such as enlarged nuchal translucency (NT), absent nasal bone, short femur and humerus length, and several structural malformations involving Dandy-Walker malformation and congenital heart defects. The prenatal phenotype of Case 2 are sonographic markers, including enlarged NT, thickened nuchal fold, ascites, and polyhydramnios without apparent structural malformations.

INTERVENTIONS

Conventional G-band karyotype appears normal in case 1, while it shows normal chromosomes with a small supernumerary marker chromosome (sSMC) in case 2. Genetic etiology was left unknown until single-nucleotide polymorphism-based array (SNP-array) was performed, and segmental paternal UPD 22 was identified in case 1 and segmental paternal UPD 14 was found in case 2.

OUTCOMES

The parents of case 1 chose termination of pregnancy. The neonate of case 2 was born prematurely with a bellshaped small thorax and died within a day.

LESSONS

UPD cases are rare and the phenotypes are different, which depend on the origin and affected chromosomal part. If a fetus shows multiple anomalies that cannot be attributed to a common aneuploidy or a genetic syndrome, or manifests some features possibly related to an UPD syndrome, such as detection of sSMC, SNP-array should be considered.

Robertsonian translocations: an overview of 872 Robertsonian translocations identified in a diagnostic laboratory in China

Robertsonian translocations (ROBs) have an estimated incidence rate of 1/1000 births, making this type of rearrangement the most common structural chromosomal abnormalities seen in the general population. In this study, we reports 872 cases of ROBs from 205,001 specimens karyotyped postnatally in a single accredited laboratory in China, including 583 balanced ROBs, 264 unbalanced ROBs, 9 mosaic ROBs, and 18 complex ROBs. Ninety-three percent of the balanced ROBs observed were adults with infertility, miscarriage, or offspring(s) with known chromosomal abnormalities. Significant excess of females were found to be carriers of balanced ROBs with an adjusted male/female ratio of 0.77. Ninety-eight percent of the unbalanced ROBs observed were children with variable referral reasons. Almost all of the unbalanced ROBs involved chromosome 21 except a single ROB with [46,XX,der(13;14),+13] identified in a newborn girl with multiple congenital anomalies. Multiple novel ROB karyotypes were reported in this report. This study represents the largest collections of ROBs in Chinese population.

Uniparental disomies 7 and 14

Normally, one inherits one chromosome of each pair from one parent and the second chromosome from the other parent. Uniparental disomy (UPD) describes the inheritance of both homologues of a chromosome pair from the same parent. The biological basis of UPD syndromes is disturbed genomic imprinting. The consequences of UPD depend on the specific chromosome/segment involved and its parental origin. Phenotypes range from unapparent to unmasking of an autosomal-recessive disease to presentation as a syndromic imprinting disorder. Whilst paternal UPD(7) is clinically unapparent, maternal UPD(7) is one of several causes of Silver-Russell syndrome. Presentation of paternal UPD(14) ("Kagami syndrome") is a thoracic dysplasia syndrome with mental retardation and limited survival. Findings in maternal UPD(14) ("Temple") syndrome show an age-dependent overlap with the well-known maternal UPD(15) (Prader-Willi) syndrome and are dominated by initial failure to thrive followed by obesity, learning difficulties and precocious puberty. Diagnostic strategies to tackle the genetic heterogeneity of UPD(7) and UPD(14) syndromes will be explained. Management issues in UPD(7) and UPD(14) patients will be discussed, and finally areas requiring further research will be outlined.

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.

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.

Allele-specific methylation of a functional CTCF binding site upstream of MEG3 in the human imprinted domain of 14q32

The gene MEG3 is located in the imprinted human chromosomal region on 14q32. Imprinting of a structurally homologous region IGF2/H19 on 11p15 is mediated through cytosine methylation-controlled binding of the protein CTCF to target sites upstream of H19. We identified five new CTCF binding sites around the promoter of MEG3. Using an electrophoretic mobility shift assay, we showed that these sites bind CTCF in vitro. Using one of these sites, chromatin immunoprecipitation (ChIP) analysis confirmed CTCF binding in-vivo, and differential allele-specific methylation was demonstrated in seven individuals with either maternal or paternal uniparental disomy 14 (UPD14). The site was unmethylated on the maternally inherited chromosomes 14 and methylated on the paternally inherited chromosomes 14, suggesting parent-specific methylation of sequences upstream of MEG3. We speculate that this CTCF-binding region may provide a mechanism for the transcriptional regulation of MEG3 and DLK1.

Maternal uniparental disomy chromosome 14: case report and literature review

Uniparental disomy is a genetic cause of disease implicated in a wide variety of neurologic disorders. A recently identified condition is maternal uniparental disomy for chromosome 14 (mUPD14) syndrome. A child with hypotonia and developmental delay was found to have mUPD14 after identification of a balanced karyotypic rearrangement involving both chromosomes 14. We explore the genetic mechanisms by which uniparental disomy can cause clinical abnormalities and karyotypic findings that should raise suspicion for uniparental disomy, review the literature on the mUPD14, and discuss clinical indications on which to suspect this diagnosis. Although it is more difficult to establish a diagnosis in the absence of visible karyotypic abnormalities involving chromosome 14, a distinct phenotype exists in mUPD14 syndrome: in utero growth restriction, congenital hypotonia, gross motor delay, arrested hydrocephalus, mild to moderate mental retardation, joint hyperextensibility, short stature, and precocious puberty. Testing for mUPD14 should be considered in infants with generalized hypotonia who have a history of in utero growth restriction.

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.

Paternal uniparental disomy of chromosome 14: confirmation of a clinically-recognizable phenotype

We report on a girl with a dicentric chromosome 14 [45,XX,inv(9)(p11q13),dic(14;14)(p11.1;p11.1)] with paternal uniparental disomy (UPD) for chromosome 14. Clinical findings include severe hypotonia, thoracic dystrophy, diastasis recti, swallowing difficulties with aspiration, developmental delay, and multiple minor anomalies. UPD for chromosome 14 has been documented with paternal UPD much less commonly than with maternal UPD. There have been ten cases of paternal UPD for chromosome 14 and one case of segmental paternal isodisomy of chromosome 14. Many of the findings are nonspecific, but the radiographic rib findings (referred to as the "coat-hanger" sign) are characteristic for this condition. UPD 14 studies should be performed in children thought to have Jeune asphyxiating thoracic dystrophy or other related osteochondrodysplasias when the diagnosis is in question. Our patient and the previously reported cases support a discrete recognizable phenotype for paternal UPD for chromosome 14.

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.

Under-ascertainment of mosaic carriers of balanced homologous acrocentric translocations and isochromosomes

Acrocentric rearrangements are the most common chromosome abnormalities in humans. Carriers of homologous acrocentric rearrangements (Robertsonian translocations (ROBs) between homologous chromosomes and isochromosomes) are at very high risk of having multiple spontaneous abortions and chromosomally abnormal offspring. Parents of fetuses and children with unbalanced homologous acrocentric rearrangements are rarely found to be carriers or mosaic for the same rearrangement. Even though recurrent miscarriages may indicate a carrier parent, carriers are rarely identified. Comparison of non-chromosome 21 homologous rearrangements to rea(21q21q) culled from the literature revealed a 7-fold decrease in the number of mosaic cases among the parents of non-rea(21q21q) offspring. This under-ascertainment in parents may be due to low level mosaicism confined to the gonads, a true biological difference between chromosome 21 rearrangements and other homologous acrocentric rearrangements, or simply to the lack of rigorous clinical investigation of the parental karyotypes to uncover mosaicism. We recommend that polymorphic marker analysis be applied to apparently de novo acrocentric rearrangements to distinguish those resulting from biparental postzygotic formation from those resulting from meiotic formation; the latter of which may indicate a potential carrier parent. Parental chromosomal constitutions could then be screened in a large number of cells and in more than one tissue type to identify mosaicism. Identification of mosaicism allows for accurate genetic counseling and discussion of reproductive options. However, given that mosaicism may be restricted to the gonads, prenatal testing is likely to be desired by the family whether or not mosaicism is found.

Robertsonian translocations: mechanisms of formation, aneuploidy, and uniparental disomy and diagnostic considerations

Robertsonian translocations (ROBs) are rearrangements of the acrocentric chromosomes 13-15 and 21-22. Cytologically, ROBs between homologous chromosomes cannot be distinguished from isochromosomes that originate through duplication of a single homologue. Both types of rearrangements can be involved in aneuploidy. A conceptus with a trisomy or a monosomy can be rescued, and in a proportion of cases, a uniparental disomy (UPD) would result. If there are regions of genome imprinting on a uniparental chromosome pair, phenotypic consequences can result. Chromosomes 14 and 15 are imprinted, and UPD of these are known to result in abnormalities. Thus, prenatal testing should be considered in all pregnancies when one of the parents is a balanced carrier of a ROB because of the risk for aneuploidy, and UPD testing should be considered in fetuses found to carry a balanced ROB or isochromosome that involves chromosomes 14 or 15. Additionally, infants or children with congenital anomalies who carry a ROB should also be considered for UPD testing.