ANALYSIS OF MEIOTIC SEGREGATION PATTERNS AND INTERCHROMOSOMAL EFFECTS IN SPERM FROM 13 ROBERTSONIAN TRANSLOCATIONS
Wang B1,*, Nie B1,*, Tang D2,*, Li R3,*, Liu X1, Song J1, Wang W1, Liu Z1,** *These authors contributed equally to this study.
*Corresponding Author: Dr. Zhi Liu, Department of Clinical Laboratory, Hubei Maternal and Child Health Hospital, 745 WuLuo Road, Wuhan, Hubei, People’s Republic of China. Tel: +86-27-8716-9190. Fax: +86-27-8716-9097. E-mail: wangbo1005@163.com.
page: 43

INTRODUCTION

The frequency of the Robertsonian (ROB) translocation in newborn babies is approximately one in 1000 [1]. The ROB translocation is an unusual type of chromosome rearrangement caused by two particular chromosomes joining together. In humans, it occurs in the five acrocentric chromosomes, such as chromosomes 13, 14 and 15 (group D), and 21 and 22 (group G). During a ROB translocation, the participating chromosomes break at their centromeres and the long arms fuse to form a single chromosome with a single centromere. The short arms also join to form a reciprocal products that typically contain nonessential genes and are usually lost within a few cell divisions. The most common ROB translocation in humans is between chromosomes 13 and 14, constituting 75.0% of all ROB translocations [1]. Since ROB translocation carriers have a balanced chromosomal complement, they may never be aware of their unusual chromosome rearrangement. Therefore, ROB translocation can be passed down in the family for many generations without detection. In ROB translocations, at the end of meiosis I, segregation of the translocated and non translocated chromosomes from the two different implicated chromosome pairs, lead to the formation of either balanced (alternate segregation mode) or unbalanced (adjacent 1, adjacent 2, and 3:0 segregation modes) gametes that can segre gate in different ways at anaphase [2,3]. Only alternate segregation produces normal/balanced karyotypes. The other segregation modes (adjacent 1, adjacent 2, 3:0) produce unbalanced gametes with disomies and nullisomies of chromosomes involved in ROB translocations. It is well-known that meiotic tetravalent configuration tends to segregate in an alternate way [4], resulting in the production of normal/balanced spermatozoa. However, a certain percentage of unbalanced gametes deriving from adjacent segregation are also produced, and could be responsible for the miscarried or the severely affected aneuploidy offspring frequently born to these carriers. Analysis of the chromosomal constitution in sperm of ROB translocation carriers is of great interest to assess the risk of unbalanced offspring for genetic counseling. Sperm fluorescence in situ hybridization (FISH) using chromosome specific probes has proven to be a useful technique to determine the meiotic segregation, especially in translocation carriers [5-9]. During the last decade, meiotic segregation in spermatozoa has been repeatedly studied in male carriers of ROB translocations. Despite the large variations, the rate of unbalanced gametes is generally not conclusive enough for genetic counseling. Although most studies showed strong prevalence of alternate segregation, which lead to balanced spermatozoa, one recent study showed a high percentage of unbalanced spermatozoa in two ROB translocation carriers [10]. More recently, inter-chromosomal effects (ICE) have been reported in several chromosome pairs in ROB translocations. Inter-chromosomal effects refer to a disturbance of meiosis, where rearranged chromosomes disrupt dis-junction and distribution of chromosome pairs not involved in the rearrangement. The concept of ICE was first postulated by Lejeune [11], who noticed an excess of carriers of balanced reciprocal translocations among the fathers of children with Down syndrome. Since then, contradictory data have been reported in ROB translocation carriers. Several studies have found such an ICE in male carriers of ROB translocations [12,13], but others did not [14,15]. The aims of this study were to analyze the segregation pattern of ROB translocation in 13 male carriers, and evaluate the occurrence of ICE on chromosomes 18, X and Y in these carriers.



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