1 Liverpool Women's Hospital, Crown Street, Liverpool L8 7SS and 2 University of Aberdeen, Aberdeen Maternity Hospital, Foresterhill, Aberdeen AB25 2ZD, UK
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Abstract |
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Key words: assisted reproduction/cystic fibrosis/mutation/screening
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Introduction |
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The CF transmembrane conductance regulator (CFTR) gene, also called ABCC7, was identified in 1989 (Kerem et al., 1989; Riordan et al., 1989
; Rommens et al., 1989
) and to date over 800 `disease causing' mutations and 70 DNA sequence variants have been identified. Most of these mutations are rare and are present in only a small number of CF patients. The most common mutation responsible for CF is called
F508 and its frequency varies among populations of different geographical and ethnic backgrounds.
Infertility, or at least subfertility, in males with CF was first suspected in the 1960s (Denning et al., 1968). Infertility in males with CF and absence of the vas deferens originates either as a developmental abnormality or an obstructive lesion (Patrizio and Salameh, 1998
). In 9798% of men with CF, a bilateral congenital absence of the vas deferens (CBAVD) blocks the transport of spermatozoa from testicular or epididymal structures to the outer genital tract, resulting in azoospermia.
The inheritance of CBAVD on the CF gene seems to follow simple Mendelian rules. Most male CF carriers do not have CBAVD. To have CBAVD, the patient has to receive two CF mutations, one from each of his parents. CF positive CBAVD males always have the same heterozygous mutation as one of the two parents. Offspring of CBAVD males (with heterozygous mutation) [following surgical sperm retrieval and intracytoplasmic sperm injection (ICSI)] have a 50% chance of having the same CF heterozygous mutation as the CBAVD father.
Abnormalities in semen parameters have also been identified in males with CF. They include azoospermia, reduced volume, increased acidity and absent or low concentrations of fructose (Denning et al., 1968; Kaplan et al., 1968
; Rule et al., 1970
; Holsclaw et al., 1971
). Not all male CF patients are infertile, a small percentage of them having fathered children. It is now believed that 23% of CF males are fertile (Welsh and Smith, 1995
). It was shown that at least some CF individuals carrying the 3849+10kbC
T mutation are fertile and have no CBAVD (Passero, 1995
; Stern et al., 1995
; Dreyfus et al., 1996
). More recently, it was suggested that the frequency of CF mutations in infertile males was significantly higher than the expected CF carrier frequency in the general population (van der Ven et al., 1996
). This increased frequency in otherwise healthy men with reduced sperm quality and in men with azoospermia without CBAVD suggests that the CFTR protein could be involved in the process of spermatogenesis or sperm maturation. It could also play a critical role in the development of the epididymis glands and the vas deferens.
Routine screening methods have been developed for CF carrier detection in the general population. These methods usually take into account the frequency of CFTR mutations particular to the population being studied. Around 10% of obstructive azoospermia is congenital and may be due to mutations in the CF gene. It is caused by either compound heterozygosity with at least one `mild' mutation (10%) (Chillon et al., 1995) or, very occasionally, by homozygosity for two `mild' mutations. If 5-T tract is included as a mutation, up to 60% of CBAVD males are found to be carriers of one CF mutation upon routine screening (DeBraekeleer and Ferec, 1996
) with presumably a rare second mutation that is not included in the routine screening. Up to 20% of CBAVD males are carriers of two different mutations (compound heterozygotes) but in these instances at least one of the CF mutations is thought to have a mild effect on the clinical phenotype.
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Discussion |
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The Human Fertilisation and Embryology Authority (HFEA) suggests that men who have CBAVD as well as men with azoospermia of unknown origin should be considered for CF screening prior to ICSI treatment. This might even extend into the investigation and treatment of severely oligozoospermic men. The HFEA strongly recommends genetic counselling for both partners when CBAVD is diagnosed, but screening was suggested for the male only. These recommendations do not conform to the recommendations provided for the screening of sperm donors and recipients in a donor insemination programme, where both parties are counselled and screened prior to treatment to provide the greatest amount of information as to treatment safety.
In our unit, since the introduction of ICSI, all males with azoospermia due to obstruction (other than that caused by vasectomy) or germ cell failure are routinely screened for CF mutations. Our current protocol on CF screening involves testing for eight locally common mutations (detection rate 90%) with further advanced testing if necessary in two stages up to a final detection level of 98.5% (Table II). Following this practice, we have come to question whether our screening could be targeted more accurately. If the male partner proves to be a carrier of one of the eight CF mutations routinely tested for, then it is an obvious requirement to screen the female partner. If the female partner proves to be negative for these mutations then the risk of an affected child would be 1:960 and treatment could proceed with the appropriate degree of counselling. As long as the female has no factors in her family history that would make her at risk of carrying CF gene mutations, then more intense CF screening is probably not indicated. This decision is based on risk assessment, cost and the manpower required for advanced screening and on consideration of the clinical practice in sperm donation where the female is found to be CF positive and the donor CF negative. Donors are only tested for the eight most common mutations in our local population as they are not at increased risk by definition for CF gene mutations.
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These two clinical scenarios provide us with a set of circumstances whereby we can look more closely at which partner should be given the priority regarding CF mutation screening prior to ICSI treatment, be it at a routine or the advanced level. If the male is found to be CF positive with routine screening, then the female must be screened for common mutations and if she is negative then treatment can proceed with the necessary genetic counselling. If an oligo/azoospermic male is found to be negative, then there is a tendency to suggest that the female does not need screening. This is fundamentally wrong because if the female is screened and is found to be CF positive on routine testing, her partner needs the fullest possible investigation of the CTFR gene.
In conclusion, we suggest that it is crucial to screen both partners for CF mutations in couples with oligozoospermia or CBAVD. However, it may be suggested that if resources are stretched, then only the female needs to be routinely CF screened because if she is negative, then the couple's residual risk of having a CF or CBAVD child will be greatly reduced (maximum 1:960) and hence the treatment can proceed following counselling. Only when the female is found to be a carrier does one have to check the male partner, firstly by routine mutation testing, and then by referral to a specialist centre for full investigation of the rarer CF gene mutations. It must be emphasized, however, that screening only the female will fall short in populations with a broader range of CF mutations (for instance in Southern Europe) where it would still be safer to screen both partners.
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Notes |
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References |
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Submitted on April 14, 2000; accepted on July 14, 2000.