The effect of structural aberrations of the chromosomes on reproductive fitness in man. I. Methodology

Inheritance of a Ring Chromosome 21 in a Couple Undergoing In Vitro Fertilization (IVF): A Case Report

An amniotic fluid sample from an in vitro fertilized pregnancy was referred for cytogenetic analysis based on a Down syndrome screening risk of 1 : 21. Routine cytogenetic analysis showed a nonmosaic karyotype of 46,XX,r(21)(p11.2q22.3), with partial monosomy for chromosome 21 due to a ring chromosome replacing one of the normal homologues. Detailed ultrasound scanning for the remainder of the pregnancy did not reveal any unusual findings. Parental bloods showed that the mother was mosaic for the ring 21 with a karyotype of 46,XX,r(21)(p11.2q22.3)/46,XX and the father had an unrelated Robertsonian translocation, with a karyotype of 45,XY,rob(13;14)(q10;q10). Microarray analysis of cultured amniocytes determined the extent of the deletion of chromosome 21 material in the ring. The parents were given genetic counselling, and a phenotypically normal female baby was delivered at term. This case highlights the importance of karyotyping as an initial step in the management of couples referred for in vitro fertilization.

FRAXA and FRAXE: evidence against segregation distortion and for an effect of intermediate alleles on learning disability

There have been several claims of segregation distortion (meiotic drive) for loci associated with diseases caused by trinucleotide repeats, leading us to test for this phenomenon in a large study of the X-linked loci FRAXA and FRAXE. We found no evidence of meiotic drive in females and no convincing evidence in males, where the limitation of risk to daughters creates a testing bias for alleles of interest. Alleles for pre- and full mutation, intermediate alleles, and common alleles were analyzed separately, with the same negative results that are extended in the discussion to claims of meiotic drive for other diseases. On the other hand, an excess risk of learning difficulties was confirmed for intermediate FRAXA alleles (relative risk, 2.58 +/- .74) and suggested for intermediate FRAXE alleles. The penetrance of learning difficulty is low, the risk being estimated as .039 for FRAXA common alleles and .101 for intermediate alleles. Because of their lower gene frequency, full mutations are a less frequent cause of learning difficulty than intermediate alleles, which contribute .0020 to total prevalence and .0012 to attributable prevalence of learning difficulty.

Familial pericentric and paracentric inversions of chromosome 1

We investigated 33 individuals (21 carriers) from one family with a pericentric inversion involving a large part of chromosome 1 (1p36.1----1q32). In addition, we investigated 15 individuals (10 carriers) from another family with a paracentric inversion of a small part of chromosome 1 (1p32----1p36.1). In each family, the index patient was ascertained because three miscarriages had occurred. Each carrier of these inversions was phenotypically normal. If the miscarriages of the index patients are excluded, the frequency of recognized miscarriages among the carriers of childbearing age was 9% (4 of 46) for the family with pericentric inversion and 17% (4 of 23) for the family with paracentric inversion. One of the pericentric inv(1) carriers had had a stillborn daughter. The carriers of the pericentric inversion who were of childbearing age had 41 children; carriers of the paracentric inversion who were of childbearing age had 19 children. No live-born children with birth defects were observed in either family. This evidence, together with the low frequency of miscarriages, suggests that crossover within the inversion loop occurs much less frequently than might be expected from the large size of this inversion. Our investigation suggests that the risk of recognized miscarriages, stilbirths, and live-born children with recombinant chromosomes who have birth defects may be much lower for inv(1) carriers than previously reported. The risk of having a malformed child because of a recombinant chromosome is probably less than 3% for carriers of the pericentric inversion and less than 6% for the carriers of the paracentric inversion.

Role of chromosome aberrations in recurrent abortion: a study of 269 balanced translocations

We have studied a sample of 5,445 couples in which the woman was ascertained to have had two or more spontaneous abortions: 396 from our Cytogenetics Unit (present series) and 5,049 from the literature (literature series). In approximately 5% of these couples one of the members was a carrier of a balanced translocation, either reciprocal (2/3 of cases) or Robertsonian (1/3). In 1% of the couples there were other chromosome anomalies, mostly gonosomal aneuploidies or mosaicisms. A pericentric inversion of the heterochromatic region of chromosome 9 was present in 3% of the couples of the present series and in 1% of the literature series. The number of female carriers exceeded significantly that of males. The probability for one member of the couple to be a carrier increased with the number of abortions at the time of ascertainment, but it does not seem modified by the concomitant presence of term pregnancies. The analysis of the cytogenetic findings in 80 cases of Robertsonian and 156 cases of reciprocal translocations suggests that some chromosomes are preferentially involved, and that in reciprocal translocations the breakpoints are not distributed at random on the chromosome arms. There is an excess of breakpoints on chromosomes 6, 7, and 22 and a dearth on chromosome 12. This distribution is significantly different from that of a sample of reciprocal translocations ascertained for a malformed child. In both samples the breakpoints seem associated with fragile sites more frequently than expected by chance. An analysis of the potential and effective chromosome imbalance suggests that in subjects with unbalanced chromosomes survival is correlated with a minimum imbalance.

Pericentric inversions. Problems and significance for clinical genetics

A review is given of the incidence, cytogenetics, and biologic relevance of pericentric inversions (pii). In 251 cases in the literature and our patients, 96 different inversion forms with different breakpoints are found. Eighteen of these cases have been observed several times in unrelated families; they are classified as types. The problem of pii in the heterochromatic regions of chromosomes 1 and 9 is especially emphasized and the investigations required are pointed out. The significance of the individual pii is checked with regard to their behavior in meiosis and their phenotypical relevance. An approximately 1:1 segregation is found. Fertility, stillbirth, and rates of abortion are not statistically altered. The gonadal findings available at present in man are reported and commented on. The occurrence of aneusomic recombinants among the live offspring of carriers shows a marked dependence on the length of the relative inversion segments. Since these are distinctly below average in inversion types, they only result in recombinants in exceptional cases. Certain pointers to an above-random common occurrence of other chromosomal aberrations are not found in families with pii. A correlation between pii and clinical symptoms likewise cannot be detected. However, in this connection it is pointed out that trisomic mosaics were observed jointly with pii(9) and pii(22). The review is completed by a brief examination of the literature concerning the significance of pii in evolution.

Reproductive risks for translocation carriers: cytogenetic study and analysis of pregnancy outcome in 58 families

Here we report on studies of the reproductive risks for heterozygous carriers of chromosome translocations. Pregnancy outcome, breakpoints, mode of segregation of the translocated chromosomes, and resulting chromosome imbalance were analyzed in 58 families (46 with reciprocal and 12 with Robertsonian translocations) ascertained for birth of a malformed child, recurrent spontaneous abortion, or hypogonadism. These families include a total of 122 informative sibships. The analysis of the data, after correction for ascertainment bias, showed that the incidence of spontaneous abortion is nearly 50% in reciprocal and between 20 and 25% in Robertsonian translocation families ascertained for malformed child or recurrent abortion. The risk of malformed infants with unbalanced genome is approximately 6% among the liveborn offspring of reciprocal translocation carriers and 23% among the liveborn offspring of carrier mothers of t(14q21q). The distribution of the breakpoints on the chromosomes involved in reciprocal translocations ascertained through a malformed child is nonrandom, with an excess on chromosomes 5, 9, 13, and 15. The study of chromosome imbalance, expressed as gain or loss of a portion of genetic information relative to the total haploid autosome length (percent HAL), shows that among the common types of disjunction-segregation leading to unbalanced gametes, adjacent 1 seems to be the one producing on the average the least level of genome imbalance. This explains why it is the most frequently observed type of segregation giving rise to gametes from which subjects with a chromosome imbalance compatible with life can be generated.

The effect of variant chromosomes on reproductive fitness in man

Reproductive fitness of carriers of heterochromatic variants of the human karyotype was found to be normal. The method was based on a comparison between known carriers and known non-carriers from the same pedigree in respect of live births, generation time and survival of offspring to reproductive age. A subset of the data had been included in an earlier study in which reproductive fitness of carriers was found to be significantly reduced. Our analysis suggests that the result may have been fortuitous, since it was not supported by the additional data. There was no evidence of heterogeneity between carriers of different types of variant or of different sex. There were indications of increased fetal losses to carriers, but the number of spontaneous abortions was insufficient to produce a detectable effect on gross reproductive fitness.

On the probability of demonstrating differential fertility in genetic studies

Computer simulation has been used to investigate the probability of detecting genetic differences in fertility in small cohorts. It was found that in cohorts of size 200 which have the demographic characteristics of females born in the United States in 1921--5, the probability is approximately 0.50 of detecting, by analysis of variance, either a 20% reduction in the monthly probability of conception or a 15--20% increase in the rate of foetal loss in the first month after conception. Consideration is given to the relative sensitivities of various observable measures of fertility to variation in underlying fertility components.

The effect of structural aberrations of the chromosomes on reproductive fitness in man. II. Results

Reproductive fitness was measured in the following seven classes of pedigrees: (1) D/D Robertsonian translocations ascertained through a euploid proband; (2) D/G Robertsonian translocations ascertained through a euploid proband: (3) reciprocal translocations ascertained through a euploid proband; (4) inversions ascertained through a euploid proband; (5) all translocations and inversions ascertained through an aneuploid proband; (6) those ascertained through a proband with a ring, marker, or supernumerary chromosome; (7) those ascertained through a proband with an extreme variant chromosome. For each group reproductive fitness was calculated in two ways. One method was based on live births, fetal and infant deaths and generation time of those individuals carrying a chromosome abnormality or variant by comparison with their first degree relatives who were known to have a normal chromosome constitution. The second method was based on the proportion of sporadic cases obtained from segregation analysis. The results obtained using both methods are presented and discussed.