Two Robertsonian translocations in a boy with mental retardation

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.

The conundrum of a jumping translocation (JT) in CVS from twins and review of JTs

Jumping translocations (JTs) are rare constitutional or acquired rearrangements involving a donor and several receiver chromosomes. They may be inherited or de novo. JTs can be found as a cultural artifact, in normal individuals or in pathological conditions. The clinical consequences range from spontaneous abortion, loss of fetus, chromosome syndrome, congenital abnormalities, and infertility to malignancy. Considering the breakpoints of JTs, they are localized predominantly in repeat regions such as pericentromeric, centromeric, subtelomeric, telomeric, and occasionally interstitial regions that may be in a low copy repeats (LCR) or in a telomere like sequence. Differences between the constitutional and acquired JTs donor breakpoints suggest an independent mechanism in their formation. In this review, a new JT involving a donor chromosome 18p10qter and recipients 17q25qter or 1q25qter found by CVS of a twin pregnancy is described. This case illustrates the diagnostic challenges posed by JTs.In this study, our knowledge on JTs is consolidated to improve identification, management, and counseling.

Jumping translocation in a phenotypically normal male: A study of mosaicism in spermatozoa, lymphocytes, and fibroblasts

Both Robertsonian translocations, rob(13;13) and rob(13;15), (in the present case defined as dic(13;15)), are rare chromosomal rearrangements and there is scarce information regarding their behavior during meiosis. In this report we describe a man with mosaicism for two cell lines, each cell line containing a different de novo Robertsonian translocation with the common breakpoint in the centromeric region on chromosome 13. The karyotype was finally defined as: 45,XY,rob(13;13)(q10;q10)[29]/45,XY,dic(13;15)(p11.2;p12)[22], a phenomenon referred to as jumping translocation. The relative occurrence of the two clones in lymphocytes and fibroblasts as well as the meiotic segregation in spermatozoa and the mechanism of formation were studied using karyotype analysis, fluorescence in situ hybridization (FISH), and quantitative fluorescence-PCR. Karyotype analysis of cultured lymphocytes revealed 57% rob(13;13) cells and 43% dic(13;15) cells and for cultured skin fibroblasts the figures were almost identical (56% and 44%, respectively). FISH analysis showed 55% balanced nuclei for unselected spermatozoa and after swim-up selection the number of balanced spermatozoa decreased to 41%. In addition, 16% of the unselected spermatozoa and 27% of the spermatozoa after swim-up selection carried an additional chromosome 13, indicating a high risk for a trisomy 13 offspring. Swim-up selection did not increase the number of balanced spermatozoa.

Jumping translocations

Jumping translocations (JT) are uncommon constitutional or acquired chromosome rearrangements involving one donor and several recipient chromosomes. They occur in various pathologic conditions and the mechanism of their formation remains elusive. A review of the literature showed that the major localizations of the breakpoints of JTs in human samples are nonrandomly located in pericentromeric and telomeric regions of chromosomes. Interestingly, comparison of the localization of the chromosomal breakpoints and of presence of interstitial DNA repeats showed differences between constitutional and acquired JTs suggesting differences in the mechanisms for the genesis of JTs and their consequences.

Unstable Robertsonian translocations der(13;15)(q10;q10): heritable chromosome fission without phenotypic effect in two kindreds

Robertsonian translocations (RTs) are amongst the most common chromosome abnormalities, but being essentially balanced are not usually associated with phenotypic abnormality. Despite being dicentric, RTs are almost always transmitted stably through cell division without chromosome breakage. We have investigated spontaneous fission of der(13;15)(q10;q10) chromosomes in eight individuals from two unrelated kindreds with a view to assessing clinical significance and to seek an explanation for the peculiar heritable instability displayed by these chromosomes. In Family 1, fission products were observed in five members in three generations. The instability was observed in cells derived from chorionic villus and lymphocytes. In Family 2, the same phenomenon was observed in amniocytes from two separate pregnancies and maternal blood lymphocytes. Detailed FISH analysis of these RTs showed them to be dicentric with an unremarkable pericentromeric structure. Notably, combined immunofluoresence and FISH analysis showed the presence of the centromere-specific proteins CENP-A and CENP-E, consistent with functional dicentricity in >75% of cells analyzed. The fission products are, therefore, presumed to be the result of sporadic, bipolar kinetochore attachment, anaphase bridging with resultant inter-centromeric breakage in a small proportion of mitoses. None of the eight carriers shows phenotypic abnormality and therefore, for prenatal counseling purposes, there appears to be no increased specific risk associated with this phenomenon.

Sex-specific chromosome instability in early human development

The predominance of females segregating chromosome aberrations to their offspring has been explained mostly by selection disadvantage of unbalanced products of spermatogenesis. However, analysis of data from the literature supports the idea that somatic cells of early female embryos are similar to female germ cells in that they are prone to malsegregation. The goal of this study was to compare the sex ratio (male to female ratio) of carriers of presumably mitotic-occurring chromosome abnormalities to identify any sex biases. In examining the literature, we found a female prevalence in cases of mosaicism associated with uniparental disomy (UPD) (26 male individuals/conceptions and 45 female individuals/conceptions, sex ratio is 0.58, significantly different from 1.06 in newborn population, P = 0.0292). This predominance was highest at gestational age <16 week (8 male and 22 female conceptuses, sex ratio is 0.36, significantly different from expected figure of 1.28, P = 0.0025), which diminished at later stages of fetal development indicating potential correction of trisomies predominantly in females. There is a threefold prevalence of 46,XX/45,X mosaics over 46,XY/45,X mosaics in prenatally diagnosed cases, which also suggests a gender-specific postzygotic chromosome loss. The male prevalence in Prader-Willi syndrome with maternal UPD of chromosome 15 also can be explained by sex-specific trisomy correction, with predominant loss of a maternal chromosome causing biparental inheritance and therefore, complete correction of trisomy in females (without UPD). Finally, there is a female predominance in carriers of chromosome rearrangement with pericentromere break (mosaicism for Robertsonian translocation/isochromosome, centric fission, nonacrocentric isochromosome, and whole arm rearrangement), in both prenatal (21 males and 36 females, sex ratio is 0.58, P < 0.0184) and postnatal ill-defined cases (14 males and 35 females, sex ratio is 0.40, P = 0.001). Thus, the findings presented in this paper suggest that, in addition to reduction in male fertility, and to probable selection against abnormal cell line(s), there are two mechanisms that contribute to female preponderance among carriers of mosaicism: sex-specific chromosome loss and sex-specific centromere instability. The data obtained suggest that females may have gonadal mosaicism for aneuploidies and structural rearrangements more often than males. This may lead to the maternal origin bias in offspring with trisomies or structural rearrangements.

Acute myelogeneous leukemia (M5a) that demonstrated chromosomal abnormality of robertsonian 13;21 translocation at onset

A 27-year-old woman had congenital lissencephaly syndrome and mental retardation. She had a fever of unknown origin and visited her local physician. Blood test indicated leukocytosis, so she was referred to our hospital for detailed examination. She was diagnosed to have acute myelogeneous leukemia (M5a). The chromosome analysis in blast cells revealed Robertsonian 13;21 translocation. Complete remission was obtained by induction chemotherapy. As normal karyotype (46, XX) was observed in the chromosome analysis of bone marrow cells after remission, it was considered that the patient had acquired Robertsonian 13;21 translocation complicated by acute myelogeneous leukemia.

Mosaicism in a patient with Down syndrome reveals post-fertilization formation of a Robertsonian translocation and isochromosome

It has been estimated that a few hundred children are born each year in the United States with translocation Down syndrome. About 5% of the cases with Down syndrome carry a Robertsonian translocation involving chromosome 21. The case described here is a patient with Down syndrome who showed mosaicism for two cell lines. Each cell line contains a different, de novo acrocentric rearrangement. We constructed somatic cell hybrids from the patient's cells and determined the parental origins of the rearrangements by molecular and fluorescence in situ hybridization (FISH) analyses. The analysis showed that the rob(14q21q) formed between a paternally inherited chromosome 21 and a maternally inherited chromosome 14, indicating that this rearrangement formed post-zygotically. Further molecular analysis also determined that the rea(21q21q) is an isochromosome of paternal origin. The cell line containing the isochromosome is unbalanced, resulting in trisomy 21. Because the same paternal chromosome 21 was involved in both the isochromosome and the Robertsonian translocation, we speculate that an unstable chromosome 21 was stabilized either through formation of a rob(14q21q) or through formation of an isochromosome. The mechanism proposed for the formation of the rob(14q21q) in this case is different from that for most de novo rob(14q21q), but similar to a previously reported mosaic case of Down syndrome.

Multicolour spectral karyotyping for complex chromosomal rearrangements in repeated abortion or congenital anomalies

Advances in molecular cytogenetics, especially the technique of fluorescence in situ hybridization (FISH), have allowed more precise definition of chromosomal structures, which are difficult to identify using conventional G-banding. Recently, a novel approach based on hybridization of 24 fluorescent-labelled chromosome painting probes was developed, termed spectral karyotyping (SKY), which allows the simultaneous and differential colour display of all human chromosomes. We have used SKY to identify not only five parental complex translocation carriers but also minute chromosome rearrangements in the fetus. Here, we concentrate attention on the clinical application of SKY for prenatal diagnosis.

Molecular analysis of mosaicism for two different de novo acrocentric rearrangements demonstrates diversity in Robertsonian translocation formation

Robertsonian translocations (ROBs) involving chromosome 21 occur in about 5% of individuals with Down syndrome. ROBs are the most common chromosomal rearrangements in humans and are formed through whole arm exchanges of any two acrocentric chromosomes. The de novo formation of ROBs occurs at exceptionally high rates. The present case concerns a child with mosaic Down syndrome who has two cell lines that contain two different de novo ROBs: 45,XX,rob(14;21)(q10;q10) and 46,XX,rea(21;21)(q10;q10),+21. To elucidate the mechanisms by which the rearrangements formed, somatic cell hybrids were constructed to allow the parental origins of the chromosomes involved in the ROBs to be distinguished. The analysis of the hybrids showed that the rob(14q21q) must have formed postzygotically because it contained a maternal chromosome 14 and a paternal chromosome 21. Furthermore, hybrid analysis of the rea(21q21q) demonstrated two copies of the same chromosome from the mother and thus, by definition, was an isochromosome [i(21q)]. All free-lying chromosomes 21 isolated in hybrids were of maternal origin. These chromosomes may have originated from either of the patient's cell lines. We present four hypotheses for the formation of the two cell lines of this child. This case is part of an ongoing project to determine the mechanism(s) of de novo ROB formation and the results differ from the other de novo rob(14q21q) studied in our laboratory (n = 7) in that all previously studied translocations were maternally derived, leading to the conclusion that most de novo rob(14q21q) occur in oogenesis. The current case illustrates that other mechanisms may contribute to ROB formation.

Chromosome stability is maintained by short intercentromeric distance in functionally dicentric human Robertsonian translocations

While the formation of a dicentric chromosome often leads to chromosome instability, human dicentric Robertsonian translocations usually remain stable. To investigate the basis for this stability, we have examined the centromeres of 15 structurally dicentric rob(13q14q) Robertsonian translocations using immunofluorescence and fluorescence in situ hybridization (FISH). The immunofluorescence detection of centromere protein C (CENP-C) was used as a marker for centromere function as CENP-C seems to play an essential role in kinetochore structure and stability and was previously shown to be absent from inactive centromeres. In all 15 translocation-containing cell lines, CENP-C was confined to only one of the centromeres of the translocation in a fraction of the cells analyzed. This suggests that centromere inactivation commonly occurs on dicentric Robertsonian translocations and may serve as one mechanism allowing for their stability. However, in the majority of the translocations (12 out of 15), a portion of the cells analyzed displayed CENP-C immunofluorescence at both centromeres, suggesting that both centromeres were active and that the translocation was functionally dicentric. The percentage of cells with CENP-C at both centromeres ranged from 2% to 82%. These results support the hypothesis that the close proximity of two functional centromeres on Robertsonian translocations allows them to remain stable.

Moving satellites and unstable chromosome translocations: clinical and cytogenetic implications

We describe 2 families in which acrocentric short arm material moved from one chromosome to another. In case 1, a meiotic event resulted in movement of an unusually large paternal 21p to chromosome 13 in the fetus. In case 2, a mitotic event resulted in fetal mosaicism. The short arm material from a paternal chromosome 15 moved to chromosome 14 in some of the fetal cells. Movement of acrocentric short arm material resulted from breakage and exchange in centromeric areas of repetitive DNA. We suggest the mechanism may be similar to that of previously reported "jumping" translocations. Failure to recognize movement of the short arms of acrocentric chromosomes can result in erroneous interpretation of prenatal cytogenetic results and of other cytogenetic assays dependent on acrocentric short arm polymorphisms.

An unusual case of mosaic Down's syndrome involving two different Robertsonian translocations

A baby girl with some of the stigmata of Down's syndrome was found to be a mosaic with three different cell lines: 45,XX,-13,-21,+t(13q21q)/(46,XX/46,XX, -21,+t(21q21q). The chromosome rearrangements detected in this patient appear to have arisen de novo. In the normal cell line the terminal end of the p arm of one chromosome 21 is thought to have been damaged. It seems probable that this is related to the other chromosomal anomalies found.

Trisomy 22 in a newborn with multiple malformations

A case of complete trisomy 22 in a live-born female child with multiple malformations is reported. The karyotype of the index patient had 46 chromosomes, with one chromosome 22 missing and one supranumerary metacentric chromosome. Different banding methods and in situ hybridization revealed that the extra chromosome consists of the long arms and a part of the short arms of two chromosomes 22. Our report supplies further proof that a fetus with complete trisomy 22 can occasionally survive to term, but the condition is not compatible with life over a long period.

Partial duplication 8q12----q21.2 in two sibs with maternally derived insertional and reciprocal translocations: case reports and review of partial duplications of chromosome 8

We report on two sibs with duplication of the segment 8q12----8q21.2 resulting from malsegregation of a maternal insertional translocation: [inv ins (5;8)(p13;q12q21.2)]. The mother also carries a reciprocal translocation [t(1;6)(q31;q5)], which was transmitted in the balanced state to the propositi and to a phenotypically normal son and daughter. The literature on two translocations occurring in one individual and on insertional rearrangements is reviewed in terms of reproductive risks to balanced carriers. The two affected infants have a previously undescribed partial duplication of an interstitial segment of 8q and a pattern of abnormalities distinct from those seen in other partial duplications of 8. These infants are reviewed with 78 other cases of partial duplications of chromosome 8 with regard to phenotype-karyotype correlations.

Mosaic Down's syndrome with de novo 45,XX,-21,-22,+t(21q;22q)/46,XX,-21,+t(21q;21q) rearrangement

The occurrence of mosaic Down's syndrome with two independent Robertsonian translocation cell lines is very rare. Such a patient is reported here, in whom an unbalanced Robertsonian translocation between two chromosomes 21 was detected in the majority of cells. The patient also revealed a minor cell line with a second Robertsonian translocation involving a chromosome 21 and a 22. The chromosome translocations detected in this patient were de novo in origin.