Comments on estimations of risks to translocation carriers

Risk evaluation of carriers with chromosome reciprocal translocation t(7;13)(q34;q13) and concomitant meiotic segregation analyzed by FISH on ejaculated spermatozoa

We performed the segregation analysis of a relatively large pedigree of t(7;13)(q34;q13) carriers together with the sperm karyotype analysis of the one carrier using a tri-color fluorescence in situ hybridization (FISH) method. The risk assessments for unfavorable pregnancy outcomes in a series of 36 pregnancies in eight reciprocal chromosome translocation (RCT) couples of carriers were estimated directly from a pedigree after ascertainment correction. The individual probability rate for unbalanced child was predicted according to Stengel-Rutkowski and co-workers. The unbalanced karyotypes in the form of monosomy 7q34-->qter and trisomy 13q13-->qter were detected among stillborn/early death newborns with holoprosencephaly (HPE), cyclopia and other malformations. Based on clinical description of unkaryotyped stillbirth progeny, it can be assumed that the phenotype distinctions were connected with the unbalanced karyotype from 2:2 segregation (monosomy 7q with trisomy 13q) and 3:1 segregation as interchange trisomy 13 (Patau syndrome). Probability rates for miscarriages, stillbirth/early death were 12.9 +/- 6% (4/31) and 29 +/- 8.2% (9/31), respectively. The results of the meiotic segregation pattern indicated the rate of unbalanced spermatozoa for about 60%, with the unusual high rate (29.4%) of 3:1 segregant (i.e., 13.4% of the tertiary segregation and 16% of the interchange segregation). Adjacent-1 segregation followed with 23.5% and adjacent-2 followed with 7.2% of analyzed spermatozoa. The high rate of unbalanced gametes in comparison to the number of stillborn/early death and miscarriages detected in pedigree suggests a strong selection against unbalanced chromosomal constitutions during fetal development. It corresponds to a very small probability rate (about 0.3%) of viable unbalanced progeny from 3:1 meiotic segregation predicted for maternal carriers. This knowledge can be used in genetic counseling of families with similar RCT ascertained in a different way.

Genetic counselling in carriers of reciprocal chromosomal translocations involving short arm of chromosome X

A central concept in genetic counselling is the estimation of the probability of occurrence of unbalanced progeny at birth and other unfavourable outcomes of pregnancy (miscarriages, stillbirths and early death). The estimation of the occurrence probability for individual carriers of four different X-autosome translocations with breakpoints at Xp, namely t(X;5)(p22.2;q32), t(X;6)(p11.2;q21), t(X;7)(p22.2;p11.1), and t(X;22)(p22.1;p11.1), is presented. The breakpoint positions of chromosomal translocations were interpreted using GTG, RBG and FISH-wcp. Most of these translocations were detected in women with normal phenotype, karyotyped because of repeated miscarriages and/or malformed progeny. A girl with very rare pure trisomy Xp22.1-->pter and a functional Xp disomy was ascertained in one family and her clinical picture has been described in details. It has been suggested that not fully skewed X chromosome inactivation of X-autosome translocation with breakpoint positions at Xp22 (critical segment) could influence the phenotype and risk value. Therefore, the X inactivation status was additionally evaluated by analysis of replication banding patterns using RBG technique after incorporation of BrdU. In two carriers of translocations: t(X;5)(p22.2;q32) and t(X;7)(p22.2;p11.1), late replication state of der(X) was observed in 5/100 and 10/180 analysed cells, respectively. In these both cases the breakpoint positions were clustered at the critical segment Xp22.2. In two other cases, one with the breakpoint position within [t(X;22)(p22.1;p11.1)] and one outside the critical region [t(X;6)(p11.2;q21)], fully skewed inactivation was seen. Therefore, we suggest that neither the distribution of the breakpoint positions nor fully skewed inactivation influenced the phenotype of observed t(X;A) carriers. The occurrence probabilities of the unbalanced progeny were calculated according to Stene and Stengel-Rutkowski along with application of updated available empirical data. In the studied group the values of occurrence probability for unbalanced offspring at birth ranged from 2.1% to 17%. Information on the magnitude of the individual figures may be important for women carrying a reciprocal X;A translocation when deciding upon further family planning.

Cartographic study: breakpoints in 1574 families carrying human reciprocal translocations

Reciprocal translocations (rcp) are among the most common constitutional chromosomal aberrations in man. Using a European database of 1574 families carrying autosomal rcp, a cartographic study was done on the breakpoints involved. The breakpoints are non-randomly distributed along the different chromosomes, indicating "hot spots". Breakpoints of rcp that result in descendants that are unbalanced chromosomally at birth are more frequent in a distal position on chromosomal arms, and 65% of them are localised in R-bands. Among the R-bands, bands rich in GC islands and poor in Alu repetitive sequences are more frequently the site of breakpoints, as well as bands that include a fragile site. This result suggests that the variation in degree of methylation in GC islands could be involved in chromosomal breakage and hence in chromosomal rearrangements. The heterogeneity of the human chromosomal structure has been demonstrable by metaphase banding techniques since 1970. In contrast to G-bands, R-bands are sites of high gene concentration (Korenberg et al. 1978), are relatively rich in cytosine plus guanine (GC), and in Alu repetitive DNA sequences (Korenberg and Rykowski 1988). More recently Holmquist (1992) has proposed four types of R-bands, depending on their relative richness in GC and Alu DNA sequences. R-bands rich in GC correspond almost exactly to T-bands (Dutrillaux 1977). They contain 65% of all genes while they represent only 15% of the genome (Holmquist 1992). The aim of this study is to analyse the distribution of the breakpoints along chromosomes from a European database of autosomal rcp in order to relate it to the specificity of different chromosomal regions.

Risk estimates for balanced reciprocal translocation carriers--prenatal diagnosis experience

An analysis was performed on 40 families at risk for an unbalanced rearrangement in the fetus because one of the parents is a reciprocal translocation carrier. The overall risk at second trimester prenatal diagnosis was 14% (8/57). The individual risk for unbalanced offspring at second trimester prenatal diagnoses and at birth were estimated using empirical data by Stengel-Rutkowski et al. (1988). The risks at birth ranged from 0%-21.6%. Most reciprocal translocations (22 or 55%) were at low risk. Without risk (7 or 17.5%), medium risk (6 or 15%) and high risk (5 or 12.5%) translocations were about equally represented and relatively infrequent. The analysis shows that the mode of ascertainment as well as the measurement of lengths of observed or probable imbalances cannot serve as a reliable risk predictor in individual counselling. In the translocations ascertained through spontaneous abortions the risk is frequently small or nonexistent, but remarkable exceptions to this rule are observed. Translocations discovered through unbalanced offspring were found to belong to different risk groups with the exception of the no risk group. Individual risk estimates have to be performed as a basis of genetic counselling before or during pregnancy so that parents with reciprocal translocations can make their choices regarding the available options.

Logistic regression model to estimate the risk of unbalanced offspring in reciprocal translocations

The aim of this study was to estimate the risk of viable unbalanced offspring for a parental carrier of reciprocal translocation. On a large computerized database of reciprocal translocations we used logistic regression to model this risk. The status of the progeny is the outcome variable. Explanatory covariates are cytogenetic characteristics of the translocation, age and sex of the parental carrier, and potential viability of the gametes. The results obtained by the logistic model demonstrate the important role of certain variables such as the sex of the parental carrier and the R band length of the translocated segments. Within the group of lower risk (risk of viable unbalanced offspring less than 5%), 97% of the individuals are correctly classified with this model. For this group, the choice prenatal diagnosis can be best discussed by considering both the risk for viable unbalanced offspring and the risk of induced abortion following prenatal diagnosis.

Human reciprocal translocations: is the unbalanced mode at birth predictable?

Two methods of prediction for the risk of unbalance at birth were tested on a large data base of reciprocal translocation (1376 families): the pachytene diagram predictive method (PDP method) and the discriminant method (D method). These method succeeded in correctly predicting the segregation mode in 66% of the data for the PDP method and in 80% of the data for the D method. The quality of chromosome material (in particular R bands) must be taken into account for more accurate prediction. Some difficulties still exist in predicting the 3:1 tertiary segregation mode, which can frequently be incorrectly classified as the adjacent 1 mode.

Experiences with risk estimates for carriers of chromosomal reciprocal translocations

The risk estimates for individual carriers of ten different familial reciprocal translocations detected among 500 couples with reproductive failures are presented. These were established by application of the empirical data analysed by Stengel-Rutkowski et al. (1988) and the guidelines given in Stene & Stengel-Rutkowski (1988). Different risks were estimated for unbalanced offspring at birth or at second trimester prenatal diagnosis for abortions, or stillbirths/early deaths. These risk estimates varied considerably from translocation to translocation. Carriers of five translocations had risks for offspring with single-segment imbalances. The birth risk figures ranged from 0.1% to 13.8%. Carriers of five other translocations had risks for double-segment imbalances with birth risks ranging from 0% to 3.2%. The estimated risk figures were independent of the method of ascertainment. Among the parents of the index cases we found nine maternal carriers and only one paternal carrier. This presentation illustrates the need for individual risk counselling of each carrier with reciprocal translocation regarding further family planning.