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

INTRODUCTION

Balanced reciprocal translocations result from exchange of fragments between two chromosomes, without any gain or loss of genetic material, and are a common form of chromosomal abnormalities, occurring in about 1 in every 625 newborns [1-4]. Although, these translocation carriers usually do not exhibit any particular phenotypes, there is a balanced complement of genes. These are responsible for a high incidence of infertility, pregnancy loss, mental retardation, behavioral abnormalities, morbidity and mortality. Carriers of reciprocal translocations have reduced fertility and thus form an increased risk of having a spontaneous abortion or an unbalanced karyotype in their offspring [1,3,5-11]. Pure trisomy due to non disjunction of chromosome 21 is responsible for 96.0% of Down Syndrome with a recurrence risk of less than 1.0%. Parental karyotype is not required in non disjunction type of trisomies [12-15]. The population risk for trisomy 21 is 1 in 700 births but some couples are at a much higher risk owing to parental translocation or mosaicism [2]. Trisomy 21 due to reciprocal translocations are caused by exchange of euchromatic regions of chromosome 21 with the euchromatin regions of different autosomes or gonosomes. In addition to trisomic regions in various lengths and location of chromosome 21, unbalanced forms also show partial mono-somy for the exchanged regions of the other translocation chromosome. As a result, the phenotype in the carriers of an unbalanced translocation is not consistent [6]. Since these translocations rarely occur, there are no reliable data for their incidence, but their frequency is assumed to be less than 1:1000 in standard trisomy cases. According to reports in previous literature, the most common partners for a reciprocal translocation seem to be chromosomes 18 and 22 [11]. The true mechanisms responsible for structural rearrangement at segregation remain unknown. There is some evidence that considers chromosomal translocations as a risk factor for aneuploidy, therefore, translocations have to be considered in combination with aneuploidy analysis [10]. In one of our recent studies we assessed a family in which the translocation between chromosomes 12 and 16 segregates; one of the eight progenies carried the 47,XY, +21,t(12;16)(q24;q24) karyotype and presented with Down Syndrome [10]. His mother was phenotypically normal, one brother and one sister also carried the same translocation. Apparently, this rearrangement occurred due to the unbalanced chromosome segregation of the mother [t(12;16)(q24;q24)mat]. Here, we present a segregation of a balanced translocation between chromosomes 3 and 21 [t(3;21) (q21; q22] of a phenotypically normal mother that led to one offspring with a viable balanced translocation and Down Syndrome, one child with a normal karyotype and one phenotypically normal fetus with the translocation. This rearrangement apparently originated from the mother [t(3;21)(q21;q22)mat].



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