INHERITANCE OF A CHROMOSOME 3 AND 21 TRANSLOCATION IN THE FETUSES, WITH ONE ALSO HAVING TRISOMY 21, IN THREE PREGNANCIES IN ONE FAMILY
Pazarbasi A1,*, Demirhan O1, Alptekin D1, Ozgunen FT2, Ozpak L1, Yilmaz MB1, Nazlican E3, Tanriverdi N1, Luleyap U1, Gümürdülü D4
*Corresponding Author: Associate Professor Ayfer Pazarbasi (Ph.D.), Çukurova University, Faculty of Medicine, Department of Medical Biology and Genetics, 01330 Adana, Turkey; Tel.: +90-322-338-6060/3498; Fax: +90-322-338-6572; E-mail: payfer@cu.edu.tr
page: 91

DISCUSSION

Autosomal reciprocal translocations have been proposed as the most common chromosomal changes in couples who have recurrent pregnancy loss (RPL). In parallel, reciprocal translocations were the most common abnormalities (2.9%) in our series as reported in the literature, and all the chromosomes that are involved in these reciprocal translocations were found in autosomes [10]. Although reciprocal translocations are balanced rearrangements, they are important for the offspring of carriers who have increased risk of a chromosomal imbalance during gameto-genesis due to unequal meiotic segregation. Especially when one of the parents is a carrier of a balanced reciprocal translocation, pregnancy may result in one of the three different types of offspring: a child with a normal karyotype, a child with a balanced reciprocal translocation, or a conceptus with an unbalanced karyotype that may lead to a spontaneous miscarriage or a live-born child with malformations and mental retardation. Cytogenetic findings do not only lead to RPL but also increase the frequency of bearing a malformed child, therefore, genetic counseling for subsequent pregnancies of couples who have balanced translocation is important [1,3]. It is generally accepted that balanced rearrangements lead to increased non disjunction of other chromosomes during meiosis. Carriers of balanced translocations may be apparent because of recurrent miscarriages with or without healthy and/or affected children [5]. Prenatal diagnosis is essential if there are recurrent miscarriages for couples with a known chromosome rearrangement and if there is advanced maternal age. Maternal age is the only well-established risk factor for Down Syndrome, and the associated risk increases exponentially at age 35 years and over [7,8,16]. As a result, amniocentesis is regularly recommended for women at age 35 and over. It is also recommended that case reports of clinically normal subjects with balanced karyotypes should be published so that an informed decision can be made prenatally when a similar rearrangement is identified. In the rare group of parents carrying a balanced chromosome rearrangement affecting whole or partial 21q, the risk of having a child with a complete or partial trisomy 21 is relatively high, with about 20.0% in females and 10.0% in males. If the translocation chromosomes and their normal homologous show pairing difficulties in meiosis I, the additional risk of a 3:1 segregation has to be taken into account, leading to a recurrence risk of up to 30.0% [6]. We concluded that carriers of reciprocal translocations including chromosome 21 are at increased risk of having offspring with trisomy 21. Thus, the t(3;21) could promote formation of trisomy 21 in the offspring. Recent studies quite clearly indicate that interchromosomal effects (ICEs) do exist [9,17-19]. It has been hypothesized that a familial translocation frequently induces errors of pairing of het-erologous chromosomes in the prophase of meiosis I leading to an aneuploid gamete (ICE). Recent investigations could not prove this hypothesis. Kovaleva [20] found that carriers of balanced reciprocal translocations or inversions but not a Robertsonian translocation, are at increased risk of bearing a trisomy 21 offspring. According to this researcher, these data do not support the existence if ICE in its common sense, i.e., as an effect of rearrangement on another chromosomes’ segregation at the carrier’s meiosis. Nowadays, it is assumed that the reduced fertility of the translocation carriers is the reason for pregnancies at increased maternal age that leads to an elevated risk for pregnancies with trisomy 21 [6,15,16]. Translocation of chromosome 21 (4.0% of Down Syndrome) recurrence risk varies between 10.0- 25.0%, if one parent is a carrier of a translocation comprising chromosome 21 [12]. The risk of unbalanced translocation in the offspring will depend on both the type of translocation in the parents, and which parent is affected and whether the translocation is between homologous or non homologous chromosomes. If the parents are carriers of balanced translocation, risk for unbalanced translocation in the fetus is high and all subsequent pregnancies require prenatal sampling. Once an unbalanced translocation in the fetus/ child has been identified, parental karyotype is essential. More than 50.0% of the translocations in a fetus are de novo. So if parents have a normal karyotype, no matter what type of translocation in the fetus, recurrence risk is minimal <1.0% [12]. We may then expect carriers of balanced parental structural rearrangements to be at a greater risk of having an offspring with a distinct aneuploidy.



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