1 Department of Medical Genetics, University of Helsinki and Department of Clinical Genetics, Helsinki University Central Hospital, FIN-00029 Helsinki, Finland, 2 Department of Obstetrics and Gynecology, Institute of Womens and Childrens Health, Sahlgrenska University Hospital, SE-413 45 Göteborg, 3 Department of Obstetrics and Gynecology, Linköping University Hospital, SE-581 85 Linköping, 5 IVF Clinic at Sophiahemmet, SE-114 86 Stockholm,Sweden and 4 Department of Reproductive Medicine, Volvat Medical Center, N-0303 Oslo, Norway
6 To whom correspondence should be addressed at: Department of Clinical Genetics, Helsinki University Central Hospital, PO Box 140, FIN-00029 Helsinki, Finland. e-mail: Kristiina.Aittomaki{at}hus.fi
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Abstract |
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Key words: CFTR/genetic testing/ICSI/male infertility/Y microdeletion
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Introduction |
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Identifying a genetic cause for infertility has several implications. First, identification of the specific cause of infertility is in itself important both to the treating professional and to the patient by providing an explanation as to why the patient is infertile. Secondly, the identification of a specific cause may have clinical value relevant to the choice of treatment for an individual couple (Giltay et al., 1999; Nap et al., 1999
). More importantly, the possible genetic risk for the offspring can only be assessed individually if genetic testing has been performed. Therefore, we suggest, at this point in time, that at least all ICSI candidates with azoospermia or with a sperm count <5 x 106/ml be thoroughly informed about currently available genetic testing procedures, their testing powers and consequences, as explained below. The pre-testing information needs to be structured and concise to enable decision making. Thereafter, the patients should decide whether they would like to go through one or all of these tests before treatment.
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Chromosome analysis |
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The value of chromosome analysis of female partners of ICSI patients in general remains unclear, particularly as a proportion of chromosome aberrations identified in studies so far, such as low level sex chromosome mosaicism, are of uncertain importance (Gekas et al., 2001). However, in cases where the indication for ICSI is a poor reproductive outcome, chromosome analysis for both partners should be considered.
The implications of identifying a chromosome aberration
Certain chromosome aberrations are a highly probable explanation for low sperm counts, although it is not always possible to conclude unequivocally whether a translocation is causal or not.
Numerical sex chromosome aberrations (such as 47,XXY) are not inherited from the patients parents and there is no risk for the same condition for the siblings. Translocations can either be de novo rearrangements or inherited, in which case the information is also relevant to other family members, particularly siblings of the patient.
Carriers of balanced translocations are healthy, but may be infertile.
In most cases, there are three possibilities for the offspring of a translocation carrier: the translocation may not be inherited; it may be inherited in a balanced form (which the carrier parent has); or it can be inherited in an unbalanced form. Unbalanced chromosome translocations usually cause congenital malformations and mental retardation, and there is a high rate of spontaneous abortions in these pregnancies.
When a person is a carrier of a chromosome abnormality, prenatal testing and PGD are an option to study the chromosomes of the fetus or the embryo.
Even after normal results of parental karyotyping, there may be a higher risk of chromosomal rearrangement in a child born after ICSI. Based on fluorescence in situ hybridization (FISH) studies, it has been suggested that some infertile men with a normal lymphocyte karyotype have a high frequency of chromosomally abnormal spermatocytes (Rubio et al., 2001). In addition, sperm retrieved with testicular sperm extraction (TESE) and used for ICSI seem to result in a higher percentage of chromosomally abnormal embryos compared with ejaculated sperm (Silber et al., 2003
). The present studies also suggest that sperm aneuploidy is associated with implantation failure and early fetal loss, thus lowering the success rate of treatment (Bernardini et al., 1998
; Pang et al., 1999
; Burrello et al., 2003
).
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Y chromosome microdeletions |
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The implications of identifying a Y chromosome microdeletion
A highly probable explanation for the low number of sperm.
The deletions are almost always the result of a de novo deletion in the patient and are not inherited from the father of the patient.
The deletion is always inherited by all male offspring of a man who carries the deletion as sons always inherit the Y chromosome from their father. Daughters will not inherit the deletion.
The male offspring who inherit the microdeletion are likely to be infertile/subfertile as adults.
The microdeletions cause spermatogenetic failure due to loss of important genes residing in the AZF regions. It is possible that some infertile men do not have microdeletions but carry mutations in the actual genes within the AZF regions, although only one case has been demonstrated to date (Sun et al., 1999). These mutations would also cause inherited infertility in male offspring, although such mutations cannot be tested for. Based on genetic studies on 621 infertile couples treated with ICSI, Meschede et al. (2000
) state that male factor infertility should be considered a potentially heritable condition. Therefore, it is not possible to exclude entirely inheritance of infertility by male offspring through microdeletion or other present testing, but it is possible to identify families in whom this would be inevitable. For these patients, the inheritance of a microdeletion could be avoided by using donor sperm, if this option is more acceptable to the patient (Nap et al., 1999
). More recently, preimplantation genetic testing and sex selection have also been suggested, as these patients would have to be treated with ICSI anyway.
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Genetic testing for the CFTR gene |
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CF is caused by mutations in the CFTR gene located on chromosome 7. Over 900 mutations have been described in the gene. CBAVD usually results from a combination of a severe mutation with a mild CF mutation or with a specific intronic variant (5T). Unfortunately, routinely used clinical testing identifies far fewer mutations (4580%) than are found in research studies. This, however, varies in different populations due to recurrent mutations, which are more prevalent in some populations (Mak et al., 1999; Claustres et al., 2000
). As genetic testing is not able to identify all causative mutations, it then follows that a negative test does not exclude the existence of an unknown mutation. However, when a patient tests positive, i.e. has CBAVD due to CFTR mutations, there is a risk for both male and female offspring to have CF and for male offspring to have CBAVD. The risk for the offspring depends on whether or not the spouse is a carrier, since one mutated allele will always be inherited from the affected male. As the carrier frequency of CFTR mutations in many Caucasian populations is in the order of 1:2228, it is recommended that genetic testing for CFTR mutations be offered to the partners of men with CBAVD prior to treatment. In performing such testing, it is important to note that the 5T variant frequently found in patients with CBAVD is often not routinely tested for in clinical testing panels.
The implications of identifying CFTR mutations
CFTR mutations would be an explanation for obstructive azoospermia.
One mutated allele will always be inherited from the affected male by the offspring.
Genetic testing for CFTR mutations should be offered to the partner as CFTR mutations have a high carrier frequency (1:2228) in many populations. If the partner of a male with CBAVD is a healthy carrier of a CFTR mutation, there usually is a 25% risk of CF to all children and a 25% risk of CBAVD in male offspring.
If a partner is a carrier, prenatal testing or preimplantation genetic testing is offered.
Different mutations of the CFTR gene cause varying phenotypes, and therefore it is sometimes difficult to predict the phenotype in the offspring. Although previously suggested, mutation testing is not useful in men with idiopathic oligozoospermia or azoospermia (Tuerlings et al., 1998b).
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Discussion and conclusions |
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If testing is performed and an abnormality is identified, professional genetic counselling should be offered. In a genetic counselling session, patients are provided with an explanation of the cause of the genetic defect they have been identified with by an expert in genetics. More importantly, the consequences for the person tested, his future children and his family members are discussed and, if necessary, further counselling and testing for other family members is organized. The options for treatment of infertility considering the genetic aspects, prenatal diagnosis or PGD are also discussed. At all times, it must be clear to all parties that infertility treatment will not be withheld on genetic grounds if the patient choses to go ahead with ICSI treatment despite positive genetic testing. Indeed, Giltay et al. (1999) found that out of 75 patients that tested positive for chromosomal aberrations, 44 decided to proceed with ICSI treatment, while most patients (79%) with microdeletions chose ICSI treatment in the study by Nap et al. (1999
).
It is clear that men with non-obstructive azoospermia should have both karyotyping and Y chromosome microdeletion screening. However, it is not as easy to set criteria on testing when the patient is oligozoospermic. The recent report of the ESHRE consensus meeting suggests chromosome analysis for oligozoospermia with <5 x 106 sperm/ml, but Y chromosome microdeletion testing with 1 x 106 sperm/ml (Land and Evers, 2003
). In a comprehensive review of the literature on Y chromosome microdeletions, Foresta et al. (2001
) state that most studies have been performed on oligozoospermic men with <5 x 106 sperm/ml and the overall prevalence of microdeletions in these studies is 10.5% while prevalence in patients with a higher sperm count is very low. Based on this and on the practicalities of testing, as Y microdeletion testing is much easier than chromosome analysis, we suggest using the same criteria (<5 x 106 sperm/ml) for both although the proportion of positive results is lower when less stringent criteria are used. This could be debated further, as Cruger et al. (2003
) recently studied 392 men referred for ICSI and found that Y chromosome microdeletions were present in only 2% of men with extreme oligozoospermia and 6.5% of those with azoospermia. They suggest that all couples referred for ICSI should be offered chromosome analysis but only males with <1 x 106 sperm/ml should have microdeletion testing.
Although genetic causes of infertility can now be identified in a proportion of patients, it is clear that in the future, genetic causes will be identified in a much larger number of patients than today and we may need to consider testing options other than those available at present. It is also likely that interpreting the results and communicating their significance to the patients will become more difficult as alleles bearing a lower risk are identified. At the same time, new methods of treating infertility are increasingly applied to help families with known genetic diseases. This all means that in the future there will be a much greater overlap between reproductive medicine and genetics and that a close collaboration between professionals working in these two fields is imperative in treating patients with infertility in the best possible way.
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Acknowledgements |
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