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The importance of aneuploidy screening in reciprocal translocation carriers

¹ Departament de Biologia Cel·lular, Fisiologia i Immunologia, Unitat de Biologia, Facultat de Medicina, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Barcelona, Spain, ² Centre for Medical Genetics, Dutch-speaking Brussels Free University, Laarbeeklaan 101, 1090 Brussels, Belgium, ³ Centre for Reproductive Medicine. Dutch-speaking Brussels Free University. Laarbeeklaan 101, 1090 Brussels, Belgium, and ⁴ Laboratori de Medicina Computacional, Unitat de Bioestadística, Facultat de Medicina, Universitat Autònoma de Barcelona

Correspondence should be addressed J N Ferreté; Email: Joaquima.Navarro{at}uab.es

	Abstract

Top Abstract Introduction Materials and Methods Results Discussion Acknowledgements References

 
The purpose of this study is to investigate the aneuploidy rate and the mosaicism of chromosomes not involved in reciprocal translocations. Aneuploidy screening (AS) (13, 16, 18, 21 and 22) was performed as a re-analysis on fixed blastomeres from 126 embryos already analysed in preimplantation genetic diagnosis (PGD) cycles of eight female and five male reciprocal translocation carriers who had not achieved a pregnancy. A successful diagnosis for AS was achieved in 91.3% of embryos; 30.9% were euploid and 60.3% were aneuploid for the five chromosomes analysed. Of the embryos, 8.7% were euploid for AS and normal-balanced for the translocation and 22.2% were euploid for AS but unbalanced for the translocation; 8% of the embryos were aneuploid for AS but normal-balanced for the translocation and 52.4% were aneuploid for AS and also unbalanced for the translocation. At least 58.7% of the embryos were mosaic regarding mosaicism for the chromosomes involved and not involved in the translocations. Six of the 16 embryos transferred in the PGD cycles were aneuploid for the AS study; four of them were also mosaics. AS should be performed in reciprocal translocation carriers after segregation analysis in PGD.

	Introduction

Top Abstract Introduction Materials and Methods Results Discussion Acknowledgements References

 
Reciprocal translocations are produced when there is an exchange of terminal chromosome segments between non-homologous chromosomes. These translocations are a common form of chromosomal abnormality, occurring in about 1 in every 625 newborns (Van Dyke et al. 1983) and they are usually phenotypically neutral because there is a balanced complement of genes. Some kinds of abnormalities can appear if the breakpoints disrupt important genes. However, due to segregation modes, a germ cell with a balanced reciprocal translocation can produce 32 types of gametes, only two of which would result in a chromosomally normal child (Scriven et al. 1998). For a carrier of a balanced reciprocal translocation, the frequency of the different modes of segregation depends on the specific translocation itself. For male carriers and for alternate segregation, the frequency ranges between 18.6% and 80.7% (Benet et al. 2005).

The production of genetically unbalanced gametes in such a high proportion leads translocation carriers to experience difficulty in achieving pregnancy, to frequently suffer spontaneous abortions and to have a high risk of delivering phenotypically abnormal offspring.

Preimplantation genetic diagnosis (PGD) has been used to select chromosomally normal and balanced embryos in carriers of chromosome reorganisations (Sermon et al. 2005). Different strategies have been used for this purpose. (a) The use of translocation-specific fluorescence in-situ hybridisation (FISH) probes on individual blastomeres has been consistently successful for the detection of the products of both Robertsonian (Rob) and reciprocal translocations (Conn et al. 1998, Munné et al. 1998, Pierce et al. 1998, Van Assche et al. 1999). (b) Blastomere biopsy followed by blastomere fusion with mouse zygotes, which has been described as a method for obtaining metaphase chromosomes from individual human blastomeres (Verlinsky & Evsikov 1999, Willadsen et al. 1999). This technique has been used to karyotype embryos from patients with translocations (Verlinsky et al. 2002). (c) Selection of morphologically normal blastocysts (Ménezo et al. 1997) although some studies (Verlinsky & Evsikov 1999, Evsikov et al. 2000) suggest that the presence of an unbalanced form of a specific chromosomal translocation does not affect the embryo’s ability to reach the blastocyst stage in vitro. Consequently, this method can be considered as not applicable for the detection of the unbalanced status of the translocations described. (d) PGD analysing of polar bodies (PBs), which can be used when thefemale is the carrier of a genetic disease orof a balanced chromosomal rearrangement (Durban et al. 2001, Verlinsky et al. 2004) or for aneuploidy screening (AS) in cases of advanced maternal age or in vitro fertilization (IVF) failures (Munné et al. 2000, Verlinsky et al. 2004). This modality of PGD can be used alone or in combination with blastomere biopsy (Magli et al. 2004).

PGD analysing of blastomeres for aneuploidy screening (PGD-AS) is being extensively used in patients of advanced maternal age and repeated IVF or implantation failures (Gianaroli et al. 1999, Kahraman et al. 2000, Munné et al. 2002, 2003). In applying PGD-AS to translocation carriers after the analysis of the chromosomes implicated in the translocation, some authors have found a high degree of numerical aberrations and have pointed out the existence of an interchromosomal effect (ICE). ICE, aneuploidies affecting chromosomes not implicated in the translocation, played a role in Rob translocations, and the relevant contribution of aneuploidy exposed the couple to an additional risk of abnormal pregnancy (Gianaroli et al. 2002, Pujol et al. 2003a).

There is some evidence for considering chromosomal translocations as a risk factor for aneuploidy, and for this reason testing for translocations has to be considered in combination with aneuploidy analysis (Kuliev & Verlinsky 2002). Chromosome alterations related to chromosomes different from those involved in the translocation (Malmgren et al. 2002) could be the cause for the high incidence of arrest and poor embryo development in translocation carriers (Findikli, 2003).

Until now, only the chromosomes involved in translocations have been routinely analysed although, according to data collected by ESHRE (Harper et al. 2006), there is an 18.4% positive heartbeat detection per transfer in PGD in reciprocal translocations and 25.4% positive heartbeat per transfer in Rob translocations.

In the present work the aneuploidy rate of different chromosomes (13, 16, 18, 21 and 22) not involved in reciprocal translocations in female and male carriers with poor PGD outcome, i.e. without pregnancy after a PGD cycle, has been studied. Cytogenetic results have been analysed taking into account the sex of the carrier and the maternal age in order to evaluate their possible effect on the origin of aneuploidy. The degree of mosaicism detected has also been analysed.

	Materials and Methods

Top Abstract Introduction Materials and Methods Results Discussion Acknowledgements References

 
Patients, fixation and FISH procedure
Aneuploidy screening was performed as a re-analysis of fixed blastomeres of 13 reciprocal translocation carriers (eight females and five males) selected at random (Table 1). These patients had undergone a total of 18 PGD cycles for chromosome segregation analysis in the Centre of Genetics and for Reproductive Medicine of the Dutch-Speaking Brussels Free University in 2002–2003. PGD had been performed to analyse the segregation of the chromosomes involved in each reciprocal translocation. From one to three embryos were transferred in six female carrier and three male carrier cycles, but none of the patients included in this study achieved a pregnancy. Four of the seven semen samples used in the male translocation carriers’ cycles (57.1%) were abnormal according to WHO criteria (1999). In female translocation carriers’ cycles, 2 out of 11 semen samples were abnormal (18.2%). Sperm morphology had not been evaluated.

Our group reported a FISH efficiency of 90–100% in the second round of FISH and between 63.6% and 91.6% in the third round of FISH in PGD cycles for the analysis of chromosome segregation and AS (Pujol et al. 2003a).

A total of 126 embryos, 76 of female and 50 of male carriers that had been analysed for the meiotic segregation have been re-analysed. An average of 7 embryos (2–16) has been analysed per cycle, 6.9 and 7.1 in the female and the male carrier groups respectively. Two blastomeres have been analysed per embryo. The mean maternal age was 33.8 years (24–41), 33.1 years (female carriers) and 34.6 years (male carriers) (Table 1).

The AS re-analysis was performed using CEP and LSI probes for the chromosomes implicated in the most frequent autosomal abnormalities (13, 16, 18, 21 and 22) (MultiVysionPB, Vysis, NY, USA). In four female and three male translocation carriers the rearrangement involved one of the chromosomes checked in the AS. The FISH procedure used was the same as in the PGD cases for translocations (Staessen et al. 1999).

Visualisation and analysis was made with a Zeiss microscope equipped with a high-sensitivity camera (Roger Scientific, Photometrics, Tucson, AZ, USA), with filters for the fluorochromes used and connected to a Power Macintosh G3 computer with software for Smartcapture (Digital Scientific, Cambridge, UK).

Scoring criteria and statistical analysis
Two chromosomes were scored when the FISH signals were separated by more than the distance that the diameter of two additional signals would allow. When no diagnosis was achieved in either of the two blastomeres analysed, the corresponding embryo was diagnosed according to the results obtained in the other single blastomere. When the two analysed blastomeres of one embryo showed different FISH results, the corresponding embryo was classified as mosaic and considered to be an abnormal embryo. The results of the AS only included the abnormalities of the analysed chromosomes not involved in each translocation. The aneuploid events observed per embryo (Table 2) include all the aneuploidies identified in each one of the blastomeres. Cytogenetic results were analysed taking into account the gender of the translocation carrier and the maternal age (37 years and >37 years) in each couple.

	Results

Top Abstract Introduction Materials and Methods Results Discussion Acknowledgements References

 
PGD for chromosome segregation analysis had been performed for the mentioned 13 different reciprocal translocation carriers (eight females and five males) in order to select and transfer normal or balanced embryos. Two blastomeres of transferable and non-transferable embryos were re-analysed (Table 3) with the AS probe panel.

Euploid or aneuploid embryos were successfully identified throughout the AS analysis of the blastomeres analysed (Table 4) in a total of 91.3% (115/126) of the embryos, while in 8.7% (11/126) cell material losses or FISH negative results did not allow for a final embryo diagnosis. Of the embryos, 30.9% (39/126) were euploid for the AS-analysed chromosomes and 60.4% (76/126) were aneuploid; 8.7% (11/126) were euploid for AS and normal/balanced for the translocation shown and 22.2% (28/126) were euploid for AS but unbalanced for the translocation. Eight percent (10/126) of the embryos were aneuploid for AS but normal/balanced for the translocation and 52.4% (66/126) of the embryos were aneuploid for AS and also unbalanced for the translocation. No significant differences were observed when the cytogenetic results obtained between genders or between maternal age groups (37 years old and >37 years old) were compared.

Embryo mosaicism for the two chromosomes involved in the translocation and also for those included in the AS analysis (Table 6) was evaluated in 67.5% (85/126) of embryos. In the remaining 32.5% (41/126), FISH results which could be interpreted were available only for one single blastomere, thus mosaicism could not be evaluated.

Analysing aneuploid events in the 76 aneuploid embryos, either mosaic or not, a total of 133 missing chromosomes (55 nullisomies and 78 monosomies) and 42 extra chromosomes (32 trisomies, 9 tetrasomies and 1 pentasomy) were found (Table 2).

In general, there were more missing chromosomes than extra chromosomes. Only in two translocations, t(4;18)(q31.1;p11.2) and t(11;12)(q12;p11.1), were there more extra chromosomes. Non-significant differences were found in the distributions of the aneuploid events among the different translocations.

From the 16 embryos which had been transferred in the PGD cycles (diagnosed as normal/balanced for the corresponding translocation), 10 were euploid for the AS study; 8 of them were non-mosaic and in 2 mosaicism could not be diagnosed. In the remaining six embryos transferred, aneuploidies for one to three of the chromosomes analysed in the AS were observed (Table 5). Four of them were also mosaic, one was non-mosaic and in the other mosaicism could not be diagnosed. This means that during the PGD cycles carried out, six aneuploid embryos had been transferred, and in at least four of them mosaicism was also present.

	Discussion

Top Abstract Introduction Materials and Methods Results Discussion Acknowledgements References

 
In the present study, aneuploidy screening cytogenetic re-analysis of the embryos already diagnosed for PGD chromosome segregation analysis has been performed. A total of 13 different reciprocal translocation carriers are included in this work. Chromosomes 13, 16, 18, 21 and 22 were analysed by FISH in the AS study, since 82% of all abnormal embryos, including most mosaic ones, involve these chromosomes (Abdelhadi et al. 2003). None of the patients included in the AS study had achieved a pregnancy after the PGD cycle for translocation analysis despite a one-, two- or three-embryo transfer having been performed in some of them (Table 1).

Chromosome segregation results, normal, balanced or unbalanced embryos, which had been identified in the PGD cycles in female and male carriers of different chromosome translocation have been summarised in the present study. Most of the embryos analysed, 105/126 (83.3% average: 84.2% for female carriers and 82% for male carriers; Table 4) had, including mosaics, unbalanced chromosome complements, due to the segregation of the chromosomes involved in the translocation that had been detected during the PGD cycles. Similar frequencies of unbalanced embryos in reciprocal translocation carriers have been published. Verlinsky et al.(2002) reported 72.3% of unbalanced embryos, 76.4% being in female carriers and 68.2% in male carriers. Findikli (2003) found 78.3% unbalanced embryos in female carriers and 63.2% in male carriers. According to data collected by ESHRE between 1997 and 2001 (Harper et al. 2006), 615 PGD cycles of reciprocal translocation carriers were made (308 in female carriers and 307 in male carriers) and 78.4% of embryos were diagnosed as non-transferable (78.8% in female carriers and 78% in male carriers).

In the present study, 84.2% of embryos from female carriers were unbalanced. A review on maternally derived reciprocal translocations (Durban et al. 2001) showed that 56% of the oocytes diagnosed through first polar body (1PB) were unbalanced. On the other hand, Findikli (2003) also found a high percentage (69.2%) of unbalanced oocytes in female reciprocal translocation carriers.

In the present study, 82% of embryos from male carriers were unbalanced. The frequencies of unbalanced spermatozoa found in reciprocal translocation carriers using the hamster-test or FISH in decondensed sperm heads, vary between 23% and 81% (Benet et al. 2005). The differences in the frequencies could be a consequence of the size of the samples and the characteristics of each translocation.

Some authors have pointed out the existence of ICE when studying translocation carriers using FISH in decondensed sperm heads (Estop et al. 2000, Pellestor et al. 2001, Oliver-Bonet et al. 2002, Anton et al. 2004). In general, it seems that this phenomenon is basically observed in translocation carriers with abnormal semenograms, as in 57.1% of the male translocation carriers’ cycles included in the present work (Table 1). When the genetic risk is evaluated the importance of the existence of an ICE is the undesirable consequences of non-disjunction more than its frequency, because non-disjunction can lead to the production of spermatozoa with viable aneuploidies which could generate viable abnormal embryos (Oliver-Bonet et al. 2002).

In the present study, 115 (91.3%) abnormal embryos, 28 of them (24.3%) altered only for chromosomes involved in translocations, 66 (57.4%) altered for chromosomes involved and not involved in translocations and 10 (8.7%) altered only for chromosomes not involved in translocations were found (Table 7). Performing PGD in female and male reciprocal translocation carriers and analysing the chromosomes involved and also others not involved in translocations, Gianaroli et al.(2002) found 54 (89%) abnormal embryos, 35 of them (65%) altered only for translocation-chromosomes, 9 (16%) altered for chromosomes involved and not involved in translocations and 3 (6%) altered only in chromosomes not involved in translocations. The rate of abnormalities for the chromosomes not involved in translocations was quite high and, when also studying Rob translocation carriers, that kind of chromosome abnormality was observed much more frequently than in reciprocal translocation carriers (67% vs 22%) and was considered a consequence of ICE. Other authors also found a high rate of aneuploidies in the chromosomes not involved in translocations (90.5% of the abnormal embryos) when performing PGD by analysing 1PB in two Rob t(13;14) female carriers; on the other hand, for a female carrier of a rec t(8;13), the frequency was lower (12.5%) (Pujol et al. 2003a).

In males, some studies indicate that there is a strict checkpoint in gametogenesis which stops meiosis when unbalanced chromosome complements are detected, thus reducing the generation of altered spermatozoa (Roeder & Bailis 2000) although the arrest may be overcome and result in the production of diploid or aneuploid sperm. In female meiosis the anaphase checkpoint is not so stringent and altered oocytes can be produced (LeMarie-Adkins et al. 1997). According to this, it would be expected that the frequency of chromosome abnormalities observed in embryos could be different depending on the sex of the rearrangement carrier. In the present work, no statistically significant differences were obtained in the cytogenetic results of the translocation chromosomes when comparing female and male carriers, and all types of segregations were found, even a 4:0 in one case. This could indicate that a sex-linked selective process against the unbalanced chromosome complements generated due to the presence of a translocation does not exist. This is in agreement with Armstrong et al.(2000) and also with Oliver-Bonet et al.(2001, 2002) who found spermatozoa with all the theoretical unbalanced segregations.

An increase of the aneuploidy rate in embryos affecting chromosomes not involved in a translocation could depend, as already mentioned, on the influence of the quadrivalent on the segregation of chromosomes not involved in the translocation (ICE), but it could also be affected by maternal age. In order to evaluate both mechanisms, a comparison was made between AS results of the chromosomes not implicated in the translocation of patients with a maternal age of 37 years and >37 years. In female carriers, the embryos with aneuploidies in chromosomes not implicated in translocation were 52.8% (37 years) and 60% (>37 years). These percentages were not statistically different. The presence of a quadrivalent could affect the segregation of the other chromosomes increasing the aneuploidy rate, especially in young female carriers where a high aneuploidy rate would not be expected.

In male carriers with a partner 37 years, although aneuploidies related to maternal age were not expected, 77.4% of embryos had aneuploidies of chromosomes not implicated in translocation. Nevertheless, it is not possible to know if this high aneuploidy rate is caused by abnormal segregation that took place during meiotic division or during the first few mitotic divisions. Male carriers with a partner >37 years had fewer aneuploid embryos (52.7%), but it should be taken into account that there was only one patient in that group.

No statistically significant differences were obtained in cytogenetic results when comparing the two established maternal age groups, neither considering female and male carriers separately nor considering both female and male carriers as a single group. The generation of aneuploidies by the studied patients in the chromosomes not involved in translocations is not increased by maternal age.

All chromosomes studied in the AS had high rates of aneuploidy and this result confirms the importance of including them in the AS analysis performed alone or in combination with the translocation analysis.

In female carriers, a high frequency of aneuploidies for the five chromosomes analysed in the AS was found (61.8%, 42/68) (Table 4). The number of embryos altered for one, two, three, four or five chromosomes fits a Poisson with parameter 1.26 with a significance of P=0.079. Then, each embryo would have a probability of 71.6% of being aneuploid and 35.7%, 22.5%, 9.5%, 3% and 0.8% would be aneuploid for one, two, three, four and five chromosomes respectively. In a study made in patients of normal karyotype, using metaphase II (MII) oocytes and 1PBs from IVF cycles, the aneuploidy rate found by FISH analysing chromosomes 1, 13, 15, 16, 17, 18, 21, 22 and X (Pujol et al. 2003b) was 47.5% and an estimation for the 23 chromosomes reported that 57.2% of oocytes would be aneuploid, and 36.3%, 15.4%, 4.4%, 0.9% and 0.2% would be aneuploid for one, two, three, four and five chromosomes respectively. Recently, in analysing the whole chromosome complement of MII oocytes and 1PBs by comparative genomic hybridisation (CGH) (Gutiérrez-Mateo et al. 2004), a very similar frequency of aneuploidy was found (57.1%). Comparing the frequencies of aneuploidy for each of the analysed chromosomes (13, 16, 18, 21 and 22) obtained in the present work with those obtained for oocytes from normal karyotype females (Pujol et al. 2003b), a slightly significant difference (P=0.046) using the Chi-squared test was found.

For the AS analysis made in the female carriers of the present study, each of the five analysed chromosome pairs of the embryos had a 7.48% (4.49–8.76%) risk of being involved in aneuploidy. In oocytes from females with normal karyotype, each of the nine analysed chromosomes of the oocytes had a 0.89% (0.52–1.70%) risk of being involved in aneuploidy. Significant differences (P<0.0005, Chi-squared test) are observed for chromosomes 13, 16, 18, 21 and 22. The high increase in the aneuploidy rate found in embryos from female carriers compared with the rate observed in oocytes from normal karyotype females could be, in part, due to the presence of the translocation during the meiotic segregation process (ICE). Errors in the first few mitotic divisions and in male meiosis could also be responsible.

Chromosome 16 showed an equivalent incidence of extra and missing chromosomes (Table 5). For chromosomes 13, 18, 21 and 22, missing chromosomes were more frequent than extra chromosomes, as also happened in global (133 vs 42). This is in accordance with other studies made in embryos (Munné et al. 2004), in which one or two rounds of FISH were performed.

In the present work at least 58.7% (74/126) of the embryos were mosaic, 37.3% (47/126) were mosaic for the chromosomes involved in translocations and at least 34.9% (44/126) were mosaic for the chromosomes not involved. No statistically significant difference was found between female and male carriers when analysing mosaicism. A high degree of mosaicism has previously been detected in human embryos of patients-with balanced structural chromosome aberrations; the frequencies are as high as 73%, 98% and even 100% (Iwarsson et al. 2000, Malmgren et al. 2002, Emiliani et al. 2003).

Until recently, and according to the results published by the ESHRE PGD Consortium (Harper et al. 2006), PGD in translocation carriers is carried out to study only the segregation pattern of the chromosomes involved in the rearrangement and AS is only performed in other groups of patients (patients of advanced maternal age, recurrent miscarriages or IVF failures). It has been found that the couples with a translocation carrier have an increased risk of having embryos with slow development, arrest and, consequently, IVF failures; this is probably due to the production of gametes with alterations not only in translocation chromosomes, but also in others (Findikli, 2003).

In the present work, AS analysis allowed for the detection of, in at least 60.4% (76/126) of the analysed embryos, aneuploid events affecting chromosomes not involved in translocations (Table 4). Ten (8%) of those embryos had been detected as being normal/balanced for the chromosomes involved in translocation and six of those ten embryos had been transferred in the PGD cycles (Table 5).

As expected, a low frequency of the embryos were normal for all analysed chromosomes (8.7% average: 7.9% in females and 10% in males), while a high frequency had chromosome abnormalities (91.3% average: 92.1% in females and 90% in males) (Table 7).

Although there are also other studies which, in translocation carriers, recommend the analysis of chromosomes not involved in translocation (Gianaroli et al. 2002, Kuliev & Verlinsky 2002, Pujol et al. 2003a), this is not yet extensively used.

The number of transferable embryos would diminish if an AS is performed during PGD cycles in translocation carriers, but aneuploidies would be detected in some of the embryos which are normal or balanced for the translocation. The present study allows for the corroboration of the importance of routinely including AS in the PGD cycles carried out in translocation carriers. There is enough time to perform AS after the analysis of the chromosomes involved in a translocation, although a blastocyst transfer could be made and a more strict selection of embryos would benefit the outcome of chromosome reorganisation carriers.

	Acknowledgements

Top Abstract Introduction Materials and Methods Results Discussion Acknowledgements References

 
This work has been supported by the Ministerio de Sanidad (FIS PI-020168), DURSI (2001 SGR-00104) and the Fundació Catalana Síndrome de Down/Marató de TV3 (1994-98). Aïda Pujol Masana has been a recipient of a grant from DURSI (Generalitat de Catalunya). The authors declare that there is no conflict of interest that would prejudice the impartiality of this scientific work.

	Footnotes

 
Received 7 December 2005
First decision 13 January 2006
Revised manuscript received 12 February 2006
Accepted 16 February 2006

A Pujol is now at Clínica Eugin de Ginecologia i Reproducció, Entença 293-295, baixos 08029, Barcelona, Spain

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