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Genetics and Molecular Biology

Print version ISSN 1415-4757On-line version ISSN 1678-4685

Genet. Mol. Biol. vol.25 no.1 São Paulo  2002

http://dx.doi.org/10.1590/S1415-47572002000100002 

Fragile X founder effect and distribution of CGG repeats among the mentally retarded population of Andalusia, South Spain

 

Yolanda de Diego, Abdelkrim Hmadcha, Francisco Moron, Miguel Lucas, Mercedes Carrasco and Elizabeth Pintado
Departamento de Bioquímica Médica y Biología Molecular. Facultad de Medicina y Hospital Universitario Virgen Macarena, Universidad de Sevilla, Spain.
Send correspondence to Elizabeth Pintado, Dpto. de Bioquímica Médica y Biología Molecular, Facultad de Medicina, Avda. Sánchez Pizjuán, 4, 41009 Sevilla, Spain. E-mail: elizabet@us.es.

 

 

ABSTRACT

Fragile X syndrome is the most common inherited form of mental retardation. We investigated the prevalence of the Fragile X syndrome in the population with mental retardation of unknown etiology in Andalusia, South Spain. We analyzed 322 unrelated patients (280 males and 42 females), and found a fragile X syndrome frequency of 6.5%. Among the non-fragile X chromosomes, the 29 CGG repeat was the most common allele. At the linked microsatellite DXS548 locus, we found a new allele which we called "allele 10" (17 CA). Similar to other south European populations, allele 2 (25 CA) at the DXS548 locus and the fragile X allele were in linkage disequilibrium supporting the idea of a common founder chromosome predisposing to the CGG expansion.

Key words: mental retardation, fragile X syndrome, CGG repeats, genetic screening

Received: March 5, 2002; accepted: March 25, 2002.

 

 

INTRODUCTION

Fragile X syndrome is the most common cause of hereditary mental retardation. It is characterized by mental handicap, facial dysmorphism and expression of a fragile site at Xq27.3 (Martin and Bell, 1943; Lubs, 1969; Escalante and Frota-Pessoa, 1969; Escalante et al., 1971; Sutherland, 1977; Sutherland and Ashford, 1979; Sherman et al., 1985; Chakrabarti and Davies, 1997; Kooy, 2000). In white populations of European origin its estimated prevalence is 1 in 4,000 males (Turner et al., 1996; Morton et al., 1997). The molecular basis of the fragile X syndrome is an expansion of the (CGG)n triplet repeats located within the 5’ UTR region of the FMR-1 gene, resulting in the absence of the encoded protein (FMRP), which is a ribosome-associated RNA-binding protein (Verkerk et al., 1991; Fu et al., 1991; Feng et al., 1997; Jin and Warren, 2000). The presence of large expansions (n > 200) is associated with abnormal methylation of the surrounding DNA and suppression of FMR-1 expression and translation (Piretti et al., 1991; de Vries et al., 1997; Willemsen et al., 1997).

The CGG repeats are polymorphic, their mode of distribution varying according to the population studied (Brown et al., 1996; Chiurazzi et al., 1996b; Tzeng et al., 1999; Chiang et al., 1999; Saha et al., 2001). Similar to several other diseases involving dynamic mutations, there is evidence of a founder effect based on the demonstration of linkage disequilibrium between the fragile X locus and its flanking polymorphic markers (Richards et al., 1992; Buyle et al., 1993; Oudet et al., 1993; Macpherson et al., 1994; Zhong et al., 1994a; Zhong et al., 1994b; Chiurazzi et al., 1996c; Eichler and Nelson, 1996; Syrrou et al., 1996; Jara et al., 1998). The two most frequent DXS548/ FRAXAC1 haplotypes in fragile X chromosomes (2-1 and 6-4) were found in non-fragile X chromosomes whose CGG repeat structure would predispose to expansions, leading to a founder effect (Eichler et al., 1996).

In this work, we investigated the prevalence of the Fragile X syndrome in subjects with mental retardation of unknown etiology, in Andalusia, South Spain. We also studied the allele frequencies at the linked DXS548 loci in normal and fragile X chromosomes.

 

MATERIAL AND METHODS

Subjects

This study included 322 unrelated patients (280 males and 42 females) with mental retardation of unknown etiology, referred to us by pediatricians, child neurologists, psychiatrists and clinical geneticists. In a subgroup of 142 male patients the DXS548 locus was genotyped and the FRAXA/DXS548 haplotypes, determined. The FRAXA locus was also analyzed in 30 X chromosomes from the normal population.

DNA analysis

DNA was isolated from peripheral blood samples by the salt precipitation method (Miller et al., 1988). Fragile X syndrome was diagnosed by Southern blotting as described previously (Pintado et al., 1995). PCR amplification of the CGG repeats at the FRAXA locus of non-fragile X chromosomes was achieved using the c and f primers described by Fu et al. (1991). The analysis of CA repeats at the flanking DXS548 locus was carried out as previously reported (Hallmayer et al., 1994). The aliquots of the PCR products were loaded on 6% denaturing acrylamide gels. Alleles were sized by running in parallel lambda gt11 a-35S-labelled sequencing plasmid or a-32P-labelled pBR322 MspI-digested fragments. To calculate the exact number of CGG repeats at the FRAXA locus, we used different sequenced alleles as reference, one of them with 29 triplets, kindly provided by Dr. B. Oostra (Erasmus University, Rotterdam).

Statistical methods

The significance of the differences between fragile X and control samples at the DXS548 locus was assessed by means of the chi-square test (SigmaStatTM 1.0. Statistical Software).

 

RESULTS AND DISCUSSION

Among the 322 individuals studied, we found 21 with a fragile X (20 males and one female), corresponding to a 6.5% frequency. In previous studies performed on unselected retarded males, the frequencies of affected subjects ranged from 2.9% to 15% (Turner et al., 1986; Mazurczak et al., 1996; Mornet and Simon-Bouy, 1996; Gonzalez-del Angel et al., 2000; Limprasert et al., 2001).

Various studies have revealed an allele with 30 CGG repeats as the most frequent one at the FRAXA locus in Caucasian populations, and the 29 CGG repeats initially reported has been considered a miscalculation due to differences in C+G content which affect the migration of the PCR products (Brown et al., 1996; Chiurazzi et al., 1996b). In order to avoid this artefact, we determined the exact number of CGG repeats in non-fragile X chromosomes by running different sequenced CGG repeat alleles in parallel. We identified 23 different normal alleles ranging in size from 10 to 43 CGG repeats (Figure 1). Six alleles (with 23, 28, 29, 30, 31 and 32 repeats) accounted for 75% of the total, the 29 triplet allele having been the most frequent one, which is in contrast with the aforementioned studies (Brown et al., 1996; Chiurazzi et al., 1996b). The allele with 29 CGG repeats is also the most frequent allele in the Asian populations (Cheng et al., 1997), although other studies suggest the allele with 28 CGG repeats to be the most common allele in China (Chiang et al., 1999). The 29 CGG allele is also the most frequently reported in India (Saha et al., 2001). Since our study was performed on a population with mental retardation of unknown etiology, it could be argued that this could lead to a bias in the ascertainment of X chromosomes. However, our study of 30 X chromosomes from non-retarded persons, showed similar allele frequencies. Based on these results, we considered that non-fragile-X individuals were representative of the normal population for the FRAXA locus, although a larger number of X chromosomes from our normal population should be studied. Considering the range between 5 and 52 triplets as normal, our sample contained 90% of small alleles (< 35 CGG) and 10% of large alleles (>35 CGG), in agreement with a previous report (Milá et al., 1994).

 

 

In order to verify the presence of a founder chromosome, we analyzed the distribution of alleles at DXS548 locus, a polymorphic marker located 150 Kb centromeric from the CGG repeats, that co-segregates, in the majority of the cases, without recombination with the fragile-X locus (Fu et al., 1991; Dreesen et al., 1994). In addition to all the previously described DXS548 alleles, we detected a new allele which we called "allele 10" (17 CA), following the terminology recommended by Macpherson et al. (1994) (Figure 2). The frequencies of DXS548 alleles in our sample show a slightly higher genetic diversity than in other populations, which probably reflects Spain’s more heterogeneous genetic background, as it has been previously reported for other loci (Bertrantpetit and Cavalli-Sforza, 1991; Cavalli-Sforza and Piazza, 1993; Chillon et al., 1994; Milá et al., 1994; Milá, 1997). In the non-fragile X chromosomes, we observed that the most frequent DXS548 allele was allele 7 (20 CA, 47%), followed by allele 6 (21 AC, 23%) (Table 1). Table 2 shows the DXS548/ (CGG)n-FRAXA haplotypes in the non-fragile X chromosomes, compared to fragile X. Similar to previous work in south European countries and black African populations, allele 2 at the DXS548 locus was present in 52% of non-related fragile-X-positive subjects, whereas it was very uncommon (9%) in non-related fragile-X-negative mentally retarded subjects. Therefore, a statistically significant linkage disequilibrium between the fragile X chromosomes and allele 2 at the DXS548 locus was demonstrated (X2 = 19.4; p = 0.002; df = 3). No disequilibrium with regard to the normal (CGG)n repeats was detected. These results are consistent with the idea of at least one founder chromosome for the fragile X syndrome in our population, corresponding to one of the original predisposing chromosome in Indo-European populations that could derive from an ancient African founder, as postulated by Chiurazzi et al. (1996c).

 

 

 

 

 

 

In summary, we showed that the frequency of fragile X syndrome in our population with mental retardation of unknown etiology was similar to the frequency described in other populations. We found the allele with 29 triplets at the FRAXA locus to be the most frequent one in our geographic area, at least in the mentally retarded non-fragile-X population. We also found a new allele of 17 CA at the DXS548 locus, and showed that the allele frequencies at this locus had a broader distribution than in other populations. As previously reported in south European and black African populations, allele 2 at the DXS548 locus showed linkage disequilibrium with the fragile-X alleles.

 

ACKNOWLEDGMENTS

We are grateful to Dr. J. López-Barneo for helpful discussion and criticism of the manuscript. We thank Dr. J.L. Mandel for the StB12.3 probe, and Dr. B. Oostra for the sequenced CGG samples used in this study. This work was supported by Grants Nº 84/99 and 21/00 from Servicio Andaluz de Salud.

 

REFERENCES

Bertrantpetit J and Cavalli-Sforza LL (1991) A genetic reconstruction of the history of the population of the Iberian Peninsula. Ann Hum Genet 55:51-67.        [ Links ]

Buyle S, Reyniers E, Vits L, De Boulle K, Handing I, Wuyts FE, Deelen W, Halley DJJ, Oostra BA and Willems PJ (1993) Founder effect in a Belgian-Dutch fragile X population. Hum Genet 92:269-272.        [ Links ]

Brown WT, Nolin S, Houck G Jr, Ding X, Glicksman A, Li SY, Stark-Houck S, Brophy P, Duncan C, Dobkin C and Jenkins E (1996) Prenatal diagnosis and carrier screening for fragile X by PCR. Am J Med Genet 64:191-195.        [ Links ]

Cavalli-Sforza LL and Piazza A (1993). Human genomic diversity in Europe: A summary of recent research and prospects for the future. Eur J Hum Genet 1:3-18.        [ Links ]

Chakrabarti L and Davies E (1997) Fragile X syndrome. Curr Opin Neurol 10:142-147.        [ Links ]

Cheng TA, Lu XF, Che PK and Ho WK (1997) Variation of the CGG repeat in FMR-1 gene in normal and fragile X Chinese subjects. Ann Clin Biochem 34:517-520.        [ Links ]

Chiang SC, Lee YM, Wang TR and Hwu WL (1999) Allele distribution at the FMR1 locus in the general Chinese population. Clin Genet 55:352-355.        [ Links ]

Chillon M, Casals T, Gimenez J, Ramos MD, Palacios A, Morral N, Estivill X and Nunes V (1994) Analysis of the CFTR gene confirms the high genetic heterogeneity of the Spanish population: 43 mutations account for only 78% of CF chromosomes. Hum Genet 93:447-451.        [ Links ]

Chiurazzi P, Genuardi M, Kozak L, Giovannucci-Uzielli ML, Bussani C, Dagna-Bricarelli F, Grasso M, Perroni L, Sebastio G, Sperandeo MP, Oostra BA and Neri G (1996a) Fragile X founder chromosomes in Italy. A few initial events and possible explanation for their heterogeneity. Am J Med Genet 64:209-215.        [ Links ]

Chiurazzi P, Destro-Bisol G, Genuardi M, Oostra BA, Spedini G and Neri G (1996b) Extended gene diversity at the FMR1 locus and neighboring CA repeats in a Sub-Saharan population. Am J Med Genet 64:216-219.        [ Links ]

De Vries BB, Van-Den-Ouweland AM, Mohkamsing S, Duivenvoorden HJ, Mol E, Gelsema K, Van-Rijn M, Halley DJ, Sandkuijl LA, Oostra BA, Tibben A and Niermeijer MF (1997) Screening and diagnosis for the fragile X syndrome among the mentally retarded: an epidemiological and psychological survey. Collaborative Fragile X Study Group. Am J Hum Genet 61:660-667.        [ Links ]

Dreesen C, Smits A and van Oost B (1994) Recombination of DXS548 (RS46) with FRAXA locus. Am J Med Genet 51:535-537.        [ Links ]

Eichler EE and Nelson DL (1996) Genetic variation and evolutionary stability of the FMR1 CGG repeat in six closed human populations. Am J Med Genet 64:220-225.        [ Links ]

Eichler EE, Macpherson JN, Murray A, Jacobs PA, Chakravarti A and Nelson DL (1996) Haplotype and interspersion analysis of the FMR1 CGG repeat identifies two different mutational pathways for the origin on the fragile X syndrome. Hum Mol Genet 5:319-330.        [ Links ]

Escalante JA, Grunspun H and Frota-Pessoa O (1971) Severe sex-linked mental retardation. J Genet Hum 19:137-140.        [ Links ]

Escalante JA and Frota-Pessoa O (1973) Retardamento mental. In: Beçak W and Frota-Pessoa O (eds) Genética Médica. Sarvier, São Paulo, pp 300-308.        [ Links ]

Feng Y, Gutekunst CA, Eberhart DE, Yi H, Warren ST and Hersch SM (1997) Fragile X mental retardation protein: nucleocytoplasmic shuttling and association with somatodendritic ribosomes. J Neurosci 17:1539-1547.        [ Links ]

Fu YH, Kuhl DP, Pizzuti A, Pieretti M, Sutcliffe JS, Richards S, Verkerk AJ, Holden JJ, Fenwick RG, Warren ST, Oostra BA, Nelson DL and Caskey CT (1991) Variation of the CGG repeat at the fragile X site results in genetic instability: Resolution of the Sherman paradox. Cell 67:1047-1058.        [ Links ]

Gonzalez-del Angel A, Vidal S, Saldana Y, del Castillo V, Alcantara M, Macias M, Luna PJ and Orozco L (2000) Molecular diagnosis of the fragile X and FRAXE syndromes in patients with mental retardation of unknown cause in Mexico. Ann Genet 43:29-34.        [ Links ]

Hallmayer J, Pintado E, Lotspeich L, Spiker D, Mcmahon W, Peterson P, Nicholas P, Pingree C, Kraemer H, Lee Wong D, Ritvo E, Lin A, Hebert J, Cavalli-Sforza L and Ciaranello R (1994) Molecular analysis and test of linkage between the FMR-1 gene and infantile autism in multiplex families. Am J Hum Genet 55:951-959.        [ Links ]

Jara L, Aspillaga M, Avendano I, Obreque V, Blanco R and Valenzuela CY (1998) Distribution of (CGG)n and FMR-1 associated microsatellite alleles in a normal Chilean population. Am J Med Genet 75:277-282.        [ Links ]

Jin P and Warren ST (2000) Understanding the molecular basis of fragile X syndrome. Hum Mol Genet 9: 901-908.        [ Links ]

Kooy RF, Willemsen R and Oostra BA (2000) Fragile X syndrome at the turn of the century. Mol Med Today 6: 193-198.        [ Links ]

Limprasert P, Saechan V, Ruangdaraganon N, Sura T, Vasiknanote P, Jaruratanasirikul S and Brown WT (2001) Haplotype analysis at the FRAXA locus in Thai subjects. Am J Med Genet 98:224-229.        [ Links ]

Lubs HA (1969) A marker X chromosome. Am J Hum Genet 21:231-244.        [ Links ]

Macpherson JN, Bullman H, Youings SA and Jacobs PA (1994) Insert size and flanking haplotype in fragile X and normal populations: Possible multiple origins for the fragile X mutation. Hum Mol Genet 3:399-405.        [ Links ]

Martin JP and Bell J (1943) A pedigree of mental defect showing sex-linkage. J Neurol Psychiat. 6:154-157.        [ Links ]

Mazurczak T, Bocian E, Milewski M, Obersztyn E, Stanczak H, Bal J, Szamotulska K and Karwacki W (1996) Frequency of Fra X syndrome among institutionalized mentally retarded males in Poland. Am J Med Genet 64:184-186.        [ Links ]

Milá M, Kruyer H, Glover G, Sánchez A, Carbonell P, Castellví-Bel S, Volpini V, Rosell J, Gabarrón J, López I, Villa M, Ballesta F and Estivill X (1994) Molecular analysis of the (CGG)n expansion in the FMR-1 gene in 59 Spanish fragile X syndrome families. Hum Genet 94:395-400.        [ Links ]

Milá M (1997) Screening for FMR1 and FMR2 mutations in 222 individuals from Spanish special schools, identification of a case of FRAXE-associated mental retardation. Hum Genet 100:503-507.        [ Links ]

Miller SA, Dykes DD and Polesky HF (1988) A simple salting out procedure of extracting DNA from human nucleated cells. Nucleic Acids Res 16:1215-1216.        [ Links ]

Mornet E and Simon-Bouy B (1996) Molecular biology of fragile X syndrome: recent data and diagnostic applications. Arch Pediatr 3:814-821.        [ Links ]

Morton JE, Bundey S, Webb TP, MacDonald F, Rindl PM and Bullock S (1997) Fragile X syndrome is less common than previously estimated. J Med Genet 34:1-5.        [ Links ]

Oudet C, Mornet E, Serre JL, Thomas F, Lentes-Zengerling S, Kretz C, Deluchat C, Tejada I, Boué J, Boué A and Mandel JL (1993) Linkage disequilibrium between the fragile X mutation and two closely linked CA repeats suggests that fragile X chromosomes are derived from a small number of founder chromosomes. Am J Hum Genet 52:297-304.        [ Links ]

Pieretti M, Zhang F, Fu Y H, Warren ST, Oostra B A, Caskey CT and Nelson DL (1991) Absence of expression of the FMR-1 gene in Fragile X syndrome. Cell 66:817-822.        [ Links ]

Pintado E, De Diego Y, Hmadcha A, Carrasco M, Sierra and Lucas M (1995) Instability of the CGG repeat at the FRAXA locus and variable phenotypic expression in a large fragile X pedigree. J Med Genet 32:907-908.        [ Links ]

Richards RI, Holman K, Friend K, Kremer E, Hillen D, Staples A, Brown WT, Goonewardena P, Tarleton J, Schwartz C and Sutherland GR (1992) Evidence of founder chromosomes in fragile X syndrome of mental retardation. Nature Genet 1:257-260.        [ Links ]

Saha S, Karmakar P, Chatterjee C, Banerjee D, Das S and Dasgupta UB (2001) Fragile X syndrome in Calcutta, India. Ann Clin Biochem 38:264-271.        [ Links ]

Sherman SL, Jacobs PA, Morton NE, Froster-Iskenius U, Howard-Peebles PN, Nielsen KB, Partington MW, Sutherland GR, Turner G and Watson M (1985) Further segregation analysis of the fragile X syndrome with special reference to transmitting males. Hum Genet 69:289-299.        [ Links ]

Sutherland GR (1977) Fragile sites on human chromosomes: demonstration of their dependence on the type of tissue culture medium. Science 197:265-266.        [ Links ]

Sutherland GR and Ashford PLC (1979) X-linked mental retardation with macro-orchidism and the fragile site at Xq27 or 28. Hum Genet 48:117-120.        [ Links ]

Syrrou M, Patsalis PC, Georgiou I, Hadjimarcou MI, Constantinou-Deltas CD and Pagoulatos G (1996) Evidence for high-risk haplotypes and (CGG)n expansion in fragile X syndrome in the Hellenic population of Greece and Cyprus. Am J Med Genet 64:234-238.        [ Links ]

Turner G, Robinson H, Laing S and Purvis-Smith S (1986) Preventive screening for the fragile X syndrome. N Engl J Med 315:607-609.        [ Links ]

Turner G, Webb T, Wake S and Robinson H (1996) Prevalence of fragile X syndrome. Am J Med Genet 64:196-197.        [ Links ]

Tzeng CC, Cho WC, Kuo PL and Chen RM (1999) Pilot fragile X screening in normal population of Taiwan. Diagn Mol Pathol 8:152-156.        [ Links ]

Verkerk AJ, Pieretti M, Sutcliffe JS, Fu YH, Kuhl DP, Pizzuti A, Reiner O, Richards S, Victoria MF, Zhang F, Eussen BE, van Ommen GJ, Blonden LA, Riggins GJ, Chastain L, Kunst CB, Galjaard H, Caskey CT, Nelson BA and Warren ST (1991) Identification of a gene (FMR-1) containing a CGG repeat coincident with a breakpoint cluster region exhibiting length variation in fragile X syndrome. Cell 65:905-914.        [ Links ]

Willemsen R, Smiths A, Mohkamsing S, van Beerendonk, de Haan A, de Vries B, van den Ouweland A, Sistermans E, Galjaards H and Oostra BA (1997) Rapid antibody test for diagnosing fragile X syndrome: a validation of the technique. Human Genet 99:308-311.        [ Links ]

Zhong N, Ye L, Dobkin C. and Brown WT (1994a) Fragile X founder chromosome effects: linkage disequilibrium or microsatellite heterogeneity? Am J Med Genet 51:405-411.        [ Links ]

Zhong N, Liu X, Gou S, Houck Jr GE, Li S, Dobkin C and Brown WT (1994b) Distribution of FMR-1 and associated microsatellite alleles in a normal Chinese population. Am J Med Genet 51:417-422.        [ Links ]

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