1 Institute of Human Genetics of the University, Vesaliusweg 1214, 2 Institute of Reproductive Medicine of the University and 3 University Women's Hospital, D-48149 Münster, Germany
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Abstract |
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Key words: family history/genetics/intracytoplasmic sperm injection/male infertility
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Introduction |
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Infertility is to be expected in boys who inherit a submicroscopic Y chromosome deletion from their fathers (Chang et al., 1999; Kamischke et al., 1999
; Kleiman et al., 1999
; Page et al., 1999
), a genetic defect found in 35% of men with severe disorders of spermatogenesis (Reijo et al., 1995
; Simoni et al., 1997
). A cytogenetically visible chromosomal abnormality is present in 510% of ICSI-treated couples (Meschede et al., 1998
). Some of the structural chromosome rearrangements in this cohort can be passed on to the offspring generation with a possible negative impact on their fertility.
The vast majority of children born after microassisted reproduction will carry neither of these easily demonstrable genetic abnormalities. They may nevertheless be at increased risk for later infertility as epidemiologic data suggest a significant impact of the genetic background on an individual's fertility, particularly in males (Lilford et al., 1994). To better define the importance of hereditary factors for the fertility of children born after ICSI, we analysed the pedigrees of more than 600 couples who had opted for this form of treatment. We supplemented the family data with an analysis of the patients' medical and reproductive histories and selected laboratory data to estimate what proportion of these couples had a genetically based fertility problem.
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Materials and methods |
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Family size data from 1302 consecutive fertile couples requesting prenatal diagnosis were used for comparison with the ICSI group. They had undergone counselling at the Institute of Human Genetics while the infertile cohort was being recruited for reasons of advanced maternal age or abnormal results upon -fetoprotein serum or triple serum marker screening (Falk, 1995
). Fifty-four other couples considering prenatal diagnosis were excluded from the control group as they reported a history of previous infertility.
Collection and interpretation of pedigree data
In the genetic counselling session the medical and reproductive history of both partners was reviewed and a family pedigree encompassing first- and second-degree relatives was constructed. Fertility problems amongst relatives were recorded according to the patients' account. A narrow definition for a positive family history for infertility was chosen: a first- or second-degree relative who wished to have children and never attained a clinical pregnancy (livebirth, stillbirth or spontaneous abortion). Cases where a relative's childlessness was known to result from a fertility problem in the non-related partner were excluded.
Laboratory procedures
In the ICSI group, chromosome analyses were undertaken for both partners, as was (during the second half of the study) a molecular screen for Y chromosomal microdeletions (Reijo et al., 1995) in men with unexplained severe spermatogenic failure. The set of tested loci on the long arm of the Y chromosome encompassed at least DAZ 254, DAZ 255, SY84 and SY143, as described elsewhere (Simoni et al., 1997
). The cystic fibrosis (CFTR) gene was tested for a minimum of 12 common mutations [R117H, R347P,
I507,
F508, 17171 G
A, G542X, G551D, R553X, 3849 + 10 kB, W1282X, N1303K, IVS8-5T (T5 allele)] in men with bilateral or unilateral vas deferens aplasia (Dörk et al., 1997
) or bilateral ejaculatory duct obstruction (Meschede et al., 1997
). Mutation analysis of the 21-hydroxylase gene was performed for a female patient clinically suspected to have congenital adrenal hyperplasia. One man with a positive family history for Kennedy disease (X-linked spinal and bulbar muscular atrophy) was tested for the pathognomonic CAG repeat expansion in the androgen receptor gene.
Documentation and statistical analysis
The pertinent clinical, family and laboratory data as recorded in the patient files were extracted on standardized documentation sheets and entered into an electronic database. Where necessary, raw data were categorized and encoded for later computerized analysis. Statistical testing was performed with the 2 test for categorical data and with the MannWhitney rank sum test for non-normally distributed discrete data.
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Results |
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Baseline characteristics of the fertile group
Mean (±SD) age of the control subjects was 35.7 (±6.0) years for the men and 34.0 (±4.9) years for the women. For 25.4% of the couples the ongoing pregnancy was the first. Gestational age at the time of counselling ranged from the seventh to the 30th postmenstrual week (mean 15.2 weeks). Both partners were of German ancestry in 88.8% of the group.
Diagnostic procedures
In the ICSI group a chromosome analysis was performed for 97.7% of the individuals. Of the 621 males, 42.2% underwent Y microdeletion screening, and 3.9% of the infertile individuals had a mutation analysis of the CFTR gene.
Family history of infertility
Of the 621 infertile couples, 73 (11.8%) reported permanent involuntary childlessness in at least one first or second degree relative (brother, sister, half-sib, uncle, aunt). Two of these couples had a positive family history for infertility on both the men's and the women's side. Eight men and one woman had two affected relatives. Details are given in Table I. A positive family history was significantly more common on the males' than on the females' side (P = 0.001;
2 test) (Figure 1
). There was a strong preponderance of male over female infertile relatives in the male patients' families (P < 0.001;
2 test).
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Sibship size
Sibship size could be evaluated for 98.7% of the infertile group and 98.5% of the fertile group. Twenty-seven couples with incomplete data (e.g. due to adoption) were excluded from this analysis. The fertile controls had more full siblings than the individuals in the ICSI cohort. Broken down by the sex of the subjects' sibs, the difference between the infertile and fertile groups was more pronounced for the number of sisters than brothers. The group differences reached statistical significance for the sisters of the male and female patients, and for the number of all full siblings (irrespective of their sex) of the female patients. The average number of half siblings was slightly higher in the ICSI cohort. Table III summarizes the data.
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Discussion |
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The understanding of the human genome is still far from complete. For this reason, one has to assume that a major proportion of genomic defects adversely affecting fertility currently evade detection. Even in such a well-characterized gene as CFTR, current standard laboratory procedures fail to detect a significant proportion of those mutations actually present (Mak et al., 1999). Our study also could not take account of genetic factors co-determining multifactorial disorders of the reproductive system such as testicular maldescent or endometriosis. Genetic factors play a well-established role in their pathogenesis, but are difficult to identify at the level of the individual. In view of these limitations, the 6.4% rate found in the present study should be considered a highly conservative minimum estimate of the prevalence of genetically determined infertility among ICSI-treated patients. It should also be mentioned that routine screening for Y chromosomal microdeletions was introduced only during the second half of our study. It is therefore likely that our cohort of infertile couples encompassed several more microdeletion carriers than were actually diagnosed.
Our data demonstrate a considerable familial aggregation of fertility problems. A positive family history was found much more commonly on the male than on the female partners' side, not an unexpected finding in a cohort with predominantly male factor infertility. However, even among the male patients' relatives, male predominated over female factor infertility. Taken together, these results indicate a substantial familial clustering of male infertility. These data were obtained through questioning the infertile couples about their family history. In most cases there was no chance to verify these second-hand reports from medical records of the presumably affected relatives. This potential source of error and bias proved difficult to eliminate, as most infertile patients were unwilling to contact their relatives to inquire about further details or obtain medical records. In addition, we did not document infertility cases among the controls' relatives. This is a limitation of our study. It should be noted, however, that with regard to the familial clustering of infertility, the infertile group contained its own control; we analysed separately (Table I) how common infertility was among the relatives of the males and the females. Only 38.8% of the latter had a fertility problem, but 97.7% of the males. Infertile relatives of the males were much more common than infertile relatives of the females, and this demonstrates (although in a less ideal way than an external control group) the specificity of the familial clustering of infertility.
We found only minimal overlap between the 40 couples with a demonstrable genetic infertility factor and the 73 couples with a positive family history. If the latter is regarded as possible indicator of an underlying genetic aetiology, our investigations failed to identify a major proportion of fertility-affecting genetic aberrations. In the familial cases, non-Mendelian multifactorial inheritance could play a significant role. The recurrence of male infertility in several unclenephew pairs as observed in our study would be compatible with this mode of intrafamilial disease transmission. It is of interest that a recent study of a genetically isolated Hutterite population failed to demonstrate a negative impact of inbreeding on male fecundity (Ober et al., 1999). This finding argues against a major role of recessive mutations in the causation of male reproductive failure, but is compatible with the hypothesis that non-Mendelian genetic factors could be of importance.
Compared with the fertile control group, the infertile couples had fewer siblings. This difference reached statistical significance only for the female subjects and is largely a result of a group difference in the number of sisters. No explanation for these sex specific effects is apparent. The design of our study does not permit an analysis of the reasons for the difference in sibship size. While social factors merit consideration, a suggestive biological explanation would be reduced fertility of the infertile patients' parents. Individuals with mild to moderate degrees of subfertility can obtain spontaneous pregnancies, but on average need a longer period of unprotected intercourse to do so (Bostofte et al., 1982a,b
; Collins et al., 1983
; Bonde et al., 1998
). This interpretation of the family size data concurs with other evidence from our study and those of others (Lilford et al., 1994
; Auger et al., 1995
) demonstrating the importance of familial factors in male fertility and infertility.
Interestingly, a trend towards a lower number of siblings was also observable in the females from the infertile cohort. This could point to a more important role of heritable factors in female infertility than currently appreciated. Unfortunately, the further investigation of this hypothesis is hampered by the scant knowledge about the chromosomal and molecular basis of most gynaecological disorders.
In summary, our results indicate that genetic factors play a substantial role in the pathogenesis of human infertility. In particular, male factor infertility appears to have a major familial component and should be regarded as a potentially heritable condition even in the absence of demonstrable cytogenetic or molecular genetic abnormalities. Indirect evidence suggests that a majority of the underlying genetic lesions evade detection with current clinical and laboratory protocols. The commonly familial nature of infertility could imply that among offspring of patients treated with ICSI, suboptimal fertility might recur with a considerable likelihood.
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Acknowledgments |
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Notes |
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References |
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Submitted on December 7, 1999; accepted on March 13, 2000.