Cytogenetic and molecular analysis of a de novo tandem duplication of chromosome 21

Mosaicism for structural non-centromeric autosomal rearrangements in disease-defined carriers: sex differences in the rearrangements profile and maternal age distributions

Background

Mosaicism for an autosomal structural rearrangement (Rea) associated with clinical manifestation of chromosomal imbalance is rare. Consequently, there is a lack of basic epidemiological characterization of this kind of mosaicism, such as population rate, cytogenetic profile of Reas involved, maternal age distribution, and sex (male to female) ratio among Rea carriers. The objectives of the present study were: (i) determination of the Rea profile in clinically affected individuals, (ii) comparative analysis of the cytogenetic profile and involvement of single chromosomes to rearrangements in affected and previously reported asymptomatic carriers, (iii) analysis of the male/female ratio in carriers of various types of Rea, and, (iv) examination of parental ages distributions according to carriers’ sex.

Results

Two hundred and forty six disease-defined cases of mosaicism for autosomal non-centromeric Rea with a normal cell line of known sex were identified from the literature. There was a significant difference in single chromosome involvements compared to structural rearrangements between affected and asymptomatic carriers of unbalanced Rea, p =0.0030. In affected carriers, chromosome 18 was most frequently involved in structural rearrangements (12.6% of 246 instances). The least frequently rearranged were chromosomes 16 and 21 (0.8% and 1.2%, respectively). In asymptomatic carriers, the most frequently rearranged were chromosomes 5 and 21 (13% of 51 instances each). Among carriers of “loss” or “gain/loss” of genomic material, a female predominance was observed (50 M/89 F, different from population ratio of 1.06 at p = 0.0002). Carriers of either “gain” or balanced Rea demonstrated typical male predominance (41 M/30 F and 18 M/16 F), not different from 1.06. Maternal and paternal ages were reported in 129 and in 109 cases, respectively. There was a significant difference in maternal age distribution between male and female carriers, with mean maternal age of 25.2 years vs 28.3 years (p = 0.032). However, there was no difference in paternal age, with mean paternal age of 29.4 in both groups.

Conclusion

The data suggested that structural rearrangements of certain chromosomes involved in mosaicism may not be tolerated by the embryo, while others have higher survival prospects. Maternal age appears to be a risk factor for somatic mosaicism of structural Rea in female offspring or might cause an adverse effect on male embryo viability.

Electronic supplementary material

The online version of this article (doi:10.1186/s13039-017-0321-9) contains supplementary material, which is available to authorized users.

Meiotic errors followed by two parallel postzygotic trisomy rescue events are a frequent cause of constitutional segmental mosaicism

Structural copy number variation (CNV) is a frequent cause of human variation and disease. Evidence is mounting that somatic acquired CNVs are prevalent, with mosaicisms of large segmental CNVs in blood found in up to one percent of both the healthy and patient populations. It is generally accepted that such constitutional mosaicisms are derived from postzygotic somatic mutations. However, few studies have tested this assumption. Here we determined the origin of CNVs which coexist with a normal cell line in nine individuals. We show that in 2/9 the CNV originated during meiosis. The existence of two cell lines with 46 chromosomes thus resulted from two parallel trisomy rescue events during postzygotic mitoses.

A girl with 15q overgrowth syndrome and dup(15)(q24q26.3) that included telomeric sequences

Distal 15q trisomy or tetrasomy is associated with a characteristic phenotype that includes mild to moderate intellectual disability, abnormal behavior, speech impairment, overgrowth, hyperlaxity, long face, prominent nose, puffy cheeks, pointed chin, small ears, and hand anomalies (mainly arachno- and camptodactyly). We present the case of a 13-yr-old girl with the main clinical features of 15q overgrowth syndrome and a 46,XX,dup(15)(q24q26.3)[117]/46,XX[3].ish dup(15)(q24q26.3) (SNPRN+,PML+,subtel++,tel++) de novo karyotype. The findings in this case are consistent with those in the previous distal 15q trisomy cases that presented with overgrowth and mental retardation. Further, the rearranged chromosome had a double set of directly oriented telomeric and subtelomeric sequences.

Genotype-phenotype correlations in Down syndrome identified by array CGH in 30 cases of partial trisomy and partial monosomy chromosome 21

Down syndrome (DS) is one of the most frequent congenital birth defects, and the most common genetic cause of mental retardation. In most cases, DS results from the presence of an extra copy of chromosome 21. DS has a complex phenotype, and a major goal of DS research is to identify genotype-phenotype correlations. Cases of partial trisomy 21 and other HSA21 rearrangements associated with DS features could identify genomic regions associated with specific phenotypes. We have developed a BAC array spanning HSA21q and used array comparative genome hybridization (aCGH) to enable high-resolution mapping of pathogenic partial aneuploidies and unbalanced translocations involving HSA21. We report the identification and mapping of 30 pathogenic chromosomal aberrations of HSA21 consisting of 19 partial trisomies and 11 partial monosomies for different segments of HSA21. The breakpoints have been mapped to within approximately 85 kb. The majority of the breakpoints (26 of 30) for the partial aneuploidies map within a 10-Mb region. Our data argue against a single DS critical region. We identify susceptibility regions for 25 phenotypes for DS and 27 regions for monosomy 21. However, most of these regions are still broad, and more cases are needed to narrow down the phenotypic maps to a reasonable number of candidate genomic elements per phenotype.

Prader-Willi syndrome phenocopy due to duplication of Xq21.1-q21.31, with array CGH of the critical region

We report on a 4-year-old male with an interstitial tandem duplication of Xq21.1-q21.31 who presented with clinical features of Prader-Willi syndrome (PWS). The duplication was maternally inherited. Abnormalities of the X chromosome have previously been reported in association with a PWS phenotype, but to date, specific duplications of Xq21.1-q21.31 have not. We refined the chromosomal breakpoints seen on initial G-banded karyotyping in our case with comparative genomic hybridization by microarray (array CGH). The duplication was between 11.1 and 14.4 Mb in length and overlaps with three loci to which mental retardation with PWS-like features have been previously mapped, showing the utility of array CGH in helping to identify candidate genes. We conclude that duplication of chromosomal region Xq21.1-q21.31 potentially results in a PWS-like phenotype. Reviewing the literature on similar duplications, we further conclude that distal Xq duplications can result in features typically seen in infants with PWS, while proximal duplications can result in features typically seen in older children and adults with PWS. Duplications of chromosome Xq should be considered in the differential diagnosis of PWS, especially in males.

Tandem duplication mosaicism: characterization of a mosaic dup(5q) and review

Mosaicism for tandem duplications is rare. Most patients reported had abnormal phenotypes of varying severity, depending on the chromosomal imbalance involved and the level of mosaicism. Post-zygotic unequal sister-chromatid exchange has been proposed as the main mechanism for tandem duplication mosaicism. However, previous molecular analyses have implicated both meiotic and post-zygotic origins for the duplication. We describe a newborn male who was originally diagnosed in utero with arrhythmia and tetralogy of Fallot. He had multiple dysmorphic features including telecanthus, blepharophimosis, high broad nasal bridge with a square-shaped nose, flat philtrum, thin upper lip, down-turned corners of the mouth, high-arched palate, micrognathia, asymmetric ears, and long, thin fingers and toes. Karyotyping of peripheral blood lymphocytes showed mosaicism for a tandem duplication of part of the long arm of one chromosome 5: mos46,XY,dup(5)(q13q33)[6]/46,XY[45]. Fibroblast cultures had the same mosaic karyotype with a higher frequency of the dup(5) clone: mos46,XY,dup(5)(q13q33)[9]/46,XY[21]. Fluorescence in situ hybridization analysis with a wcp5 confirmed the chromosome 5 origin of the additional material. Parental karyotypes were normal indicating a de novo origin of the dup(5) in the proband. Molecular analyses of chromosome 5 sequence-tagged-site (STS) markers in our family were consistent with a post-zygotic origin for the duplication. Therefore, mosaicism for tandem duplications can arise both through meiotic or mitotic errors, as a result of unequal crossing over or unequal sister-chromatid exchange, respectively. Our review indicates that mosaicism for tandem duplications is likely under-ascertained and that parental karyotyping of probands with non-mosaic tandem duplications should be performed.

Parental origin and mechanisms of formation of cytogenetically recognisable de novo direct and inverted duplications

Cytogenetic, FISH, and molecular results of 20 cases with de novo tandem duplications of 18 different autosomal chromosome segments are reported. There were 12 cases with direct duplications, three cases with inverted duplications, and five in whom determination of direction was not possible. In seven cases a rearrangement between non-sister chromatids (N-SCR) was found, whereas in the remaining 13 cases sister chromatids (SCR) were involved. Paternal and maternal origin (7:7) was found almost equally in cases with SCR (3:4) and N-SCR (4:3). In the cases with proven inversion, there was maternal and paternal origin in one case each. Twenty three out of 43 cytogenetically determined breakpoints correlated with common or rare fragile sites. In five cases, including all those with proven inverse orientation, all breakpoints corresponded to common or rare fragile sites. In at least two cases, one with an interstitial duplication (dup(19)(q11q13)) and one with a terminal duplication (dup(8) (p10p23)), concomitant deletions (del(8) (p23p23.3) and del(19)(q13q13)) were found.

Mosaicism for a tandem duplication dup(1)(q12q22) in an 18 year old female

The clinical features and cytogenetic results of an 18 year old mentally handicapped female found to be a mosaic for a tandem duplication of chromosome 1 (46,XX,dup(1)(q12q22)/46,XX) are reported. The case is compared with the three previously described cases and possible mechanisms for the origin of the duplication are discussed. This patient was not found to have features of Proteus syndrome which was previously reported in a subject mosaic for a tandem duplication involving chromosome (1)(q11q25).

Prenatal detection and molecular characterization of a de novo duplication of the distal long arm of chromosome 19

A tandem duplication of the distal long arm of chromosome 19 was identified in a 10 week fetus by analysis of chorionic villi. The fetal karyotype from two primary cultures was 46,XY,dir dup(19)(q13.2q13.4). The origin of the extra material was confirmed by fluorescence in situ hybridization using a chromosome 19 whole chromosome probe. Parental chromosomes were normal, indicating a de novo origin of the extra chromosome material. This is the first case of dup(19q) detected by prenatal diagnosis. Molecular studies demonstrated that the duplication involved a maternal chromosome 19.

Mosaicism with a normal cell line and an autosomal structural rearrangement

Over three decades, 12 cases of mosaicism for an autosomal rearrangement were recognised in the major cytogenetics laboratories in New Zealand, eight of which were studied between 1990 and 1992. One case inferentially involved the gonad, eight the soma, and three both gonad and soma. This mosaicism could have arisen as a postzygotic event either in a conceptus that was initially normal, with the generation of an abnormal cell line, or in a conceptus having a supernumerary chromosome which was lost at a subsequent mitosis, thereby restoring a normal cell line. Three of the 12 cases involved a presumed direct duplication, an otherwise very uncommon rearrangement. This may indicate a propensity for direct duplications to arise at mitosis rather than at meiosis; unequal sister chromatid exchange is a plausible mechanism. Mosaicism has clinical relevance for genetic counselling, as an intragonadal cell line carrying a rearrangement could generate multiple unbalanced gametes. Mosaicism for an autosomal rearrangement my be very much more common that is, or ever could be, recognised.

Mapping of the Down syndrome phenotype on chromosome 21 at the molecular level

Phenotypic and molecular analysis of individuals with partial trisomy 21 can be used to determine which regions of chromosome 21 are involved in the pathogenesis of specific features of Down's Syndrome. Using dosage analysis of 27 sequences we defined, at the molecular level, the extent of the chromosome 21 duplication in ten individuals with partial trisomy 21. Phenotype-genotype correlations led to the definition of minimal regions, the duplications of which are linked to the expression of 23 clinical features of Down's Syndrome. The D21S55 region or Down's Syndrome Chromosome Region 1 (DCR1) (1/20 of the long arm), on 21q22.2-21q22.3 proximal, is involved in four cardinal features of the disease: mental retardation, growth retardation, muscular hypotonia and joint hyperlaxity, and in eight of the 18 more common morphological anomalies of the face, hands and feet. Overlapping the DCR1, the D21S55-MX1 region or DCR2 (1/10 of the long arm), spanning 21q21.2 down to the 1/4th proximal part of 21q22.3, is involved in the features defined by the DCR1 plus congenital heart defect and five additional morphological anomalies. Thus, our results indicate that duplication of a relatively small region of chromosome 21 plays a critical role in the pathogenesis of the Down's phenotype.