CYTOGENETIC FINDINGS IN MENTALLY RETARDED IRANIAN PATIENTS
Nasiri F1, Mahjoubi F1,2,*, Manouchehry F1, Razazian F1, Mortezapour F1, Rahnama M1
*Corresponding Author: Dr. Frouzandeh Mahjoubi, Genetics Department, Iran Blood Transfusion Organization Research Centre, High Institute for Research and Education in Transfusion Medicine, Hemmat Express Way, Next to the Milad Tower, Tehran, Iran; Tel.: +9821-44580389; Fax: +9821-44580399; E-mail: Frouz@ nigeb.ac.ir
page: 29

DISCUSSION

There is great variation in the frequency of the reported chromosomal abnormalities found in MR patients. A cytogenetic study of 419 MR school children in southern Taiwan, by Shiue et al [2], found chromosomal abnormalities in 22.43% of the cases, with trisomy 21 occurring in 77 cases (18.38%). Sex chromosome aneuploidies were found in three cases (0.72%). Structural abnormalities of autosomes were found in 13 cases (3.10%) (2). Another study of 341 MR children in Taiwan found chromosomal abnormalities in 89 cases (20.3%) including 63 of trisomy 21 (10.7%) and 13 of fragile X (3.8%) [4]. Coco and Penchaszadeh [5] reported on a cytogenetic study in 200 MR children in Argentina. They found chromosomal abnormalities in 42 (21%) with 26 cases having structural chromosome defects [5]. Two studies were performed in The Netherlands. One study done in Amsterdam (in the south of The Netherlands) indicated that a chromosomal base in 22.1% of the patients was responsible for their MR. Of these, 14.3% were Down’s syndrome patients, and 6.1% had other chromosomal abnormalities [6]. Another study done in Amsterdam indicated that 20 patients had chromosomal anomalies (7.5%) in 266 karyotyped MR children. Interestingly, these were mainly structural chromosome aberrations [7]. A study performed in Poland showed that the incidence of abnormal karyotypes in MR patients was 10.1% [8]. However, the percentage of chromosome aberrations found in patients with non specific mental retardation was 2.2% [8]. A study done by Butler and Singh [9] in America showed that 39 out of 201 (6.6%) institutionalized MR patients had abnormal chromosome with Down’s syndrome noted in 31 of the patients. While the overall frequency of chromosomal abnormalities in these reports was similar, there are reports of either low or high percentages of chromosomal aberrations in other studies. For example, Celep et al. [10] reported the percentages of chromosomal abnormalities in 457 Turkish MR Patients to be only 4.81%. Chromosomal abnormalities and polymorphisms were detected in 65 (14.21%) (structural and numerical chromosomal abnormalities in 22 patients and polymorphisms in 43) of 457 MR and/ or multiple congenital anomaly (MCA) patients. On the other hand, a study done in Slovakia revealed a very high percentage of chromosome abnormalities in MR patients. Of 324 MR patients, 104 (53.0%) had chromosomal aberrations [11]. The differences between the incidences of chromosomal abnormalities in the literature could be caused by the criteria for patient selection, and the techniques applied [cytogenetics only or in combination with molecular cytogenetics such as fluorescent in situ hybridization (FISH) and comparative genomic hybridization (CGH)]. In our study of 865 screened subjects, chromosomal anomalies were identified in 205 of the patients (23.6%). The majority were Down’s syndrome cases (n = 138, 15.9%). Interestingly, we found three cases with marker chromosomes (0.34%). Liehr and Weise [15] found that the incidence of marker chromosomes is about 0.288% in MR patients. In general, van Karnebeek et al. [3] showed that the mean yield of chromosome aberrations in classical cytogenetics is about 9.5% (variation: 5.4% in school populations to 13.3% in institute populations; 4.1% in borderline-mild MR to 13.3% in moderateprofound MR; more frequent structural anomalies in females). They also indicated that for fragile X anomalies, yields were 5.4% (cytogenetic studies) and 2.0% (molecular studies) [3]. The incidence of fragile X positive cases in our study is slightly higher than some other reports although we only employed cytogenetic tests for fragile X. For example, Butler and Singh [9] reported 2.0% fragile X positive in his cases, while in our study it was 3.8%. Nevertheless, our results indicate that the diagnostic contribution of the fragile X screening could be considered equally important as conventional chromosome banding techniques for the detection of structural chromosome abnormalities. Some of the chromosome aberrations were detected in more than one case. For example: in two cases, chromosome 2 was involved with a very close breakpoint of q22 and q23 (Table 1; patients #1 and #14); in two cases, chromosome 4 with breakpoints p16 and p15.3 (Table 1; patients #5 and #15); and in another two cases, chromosome 6 with breakpoints p25 and p23 (Table 1; patients #2 and #17). Even more interesting, chromosome 11 in two cases and chromosome 18 in another two cases had the same breakpoints (Table 1; patients #6 and #16, and #4 and #23, respectively). This could be very interesting because some of the genes responsible for MR may be located in these breakpoints. For example, the chromosomal breakpoints for the two MR patients (patients #1 and #14) were at 2q22 and 2q23, respectively. Heterozygous mutations or deletions of the ZEB2 gene, which is located near to 2q22, is known to be responsible for Mowat-Wilson syndrome, with MR being one of the main features of this syndrome (16). Furthermore, heterozygous mutations/ deletions of the DPAGT1 gene located at 11q 23 (the breakpoint for patients #6 and #16) may reduce up to 88.0% of the mature mRNA and cause clinical features including MR [17].



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