Characterization of the full fragile X syndrome mutation in fetal gametes

Malter, Henry; Iber, Jane; Willemsen, Rob; Graaff, Esther de; Tarleton, Jack; Leisti, J; Warren, Stephen T.; Oostra, Ben A.

doi:10.1038/ng0297-165

Cited by 176 publications

(156 citation statements)

References 34 publications

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“…However, one of the poorly understood features of expansion is the gender bias that is associated with transmission. Large changes in repeat number are known to be transmitted through the paternal line in HD (1, 2), SCA1 (3), dentatorubral-pallidoluysian atrophy (DRPLA) (4), MachadoJoseph disease (5), and spinal and bulbar muscular atrophy (SBMA) (6) and through the maternal line in Fragile X (7,8). The gender bias transmission has also been documented in transgenic mice (9 -12).…”

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confidence: 99%

CAG Repeat Lengths in X- and Y-bearing Sperm Indicate That Gender Bias during Transmission of Huntington's Disease Gene Is Determined in the Embryo

Kovtun

Welch

Guthrie

et al. 2004

Journal of Biological Chemistry

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The size of the CAG tract at the Huntington's disease (HD) locus upon transmission depends on the gender of the parent. However, the basis for the parent-of-origin effect is unknown. To test whether expansion and contraction in HD are "imprinted" in the germ cells, we isolated the X-and Y-bearing sperm of HD transgenic mice. Here we show that CAG repeat distributions in the X-and Y-bearing spermatozoa of founding fathers do not differ. These data show that gender-dependent changes in CAG repeat length arise in the embryo.The mechanism for expansion in HD 1 and other trinucleotide disorders is not known. However, one of the poorly understood features of expansion is the gender bias that is associated with transmission. Large changes in repeat number are known to be transmitted through the paternal line in HD (1, 2), SCA1 (3), dentatorubral-pallidoluysian atrophy (DRPLA) (4), MachadoJoseph disease (5), and spinal and bulbar muscular atrophy (SBMA) (6) and through the maternal line in Fragile X (7, 8). The gender bias transmission has also been documented in transgenic mice (9 -12). Similar to humans, HD transgenic mice transmit expansions predominantly through the male germ line (9,10,12). Moreover, we found that expansion and contraction of CAG repeat size in the hHD transgene depended on the gender of the progeny (9).Expansions are primarily seen in males and contractions dominate in females. The molecular basis for the parent-oforigin effect is unknown. The gender dependence of expansion can be due to differences in CAG repeat distribution in X-and Y-bearing sperm. Alternatively, the change in repeat number, contraction, or expansion can take place after fertilization in early embryogenesis. In order to distinguish between these two mechanisms, the isolation and purification of the X-and Ybearing sperm were required. This, however, was not possible using conventional technology.In present work, we have developed techniques and used advanced technology to separate pure populations of X-and Y-bearing germ cells from HD transgenic mice. We show here that CAG repeat distributions in X-and Y-bearing parental sperm are not different. Repeat expansion occurs equally well within both X and Y chromosomes. This indicates that the gender-dependent processing of CAG repeats must take place post-zygotically. Thus, these data provide evidence for a new kind of imprinting that may be important in the interpretation of genetic data in other systems. EXPERIMENTAL PROCEDURESAnimals-HD transgenic male mice (line B6CBA-TgN R6/1) were originally purchased from The Jackson Laboratory. The colony was maintained at the animal core facility, Mayo Clinic/Foundation. Animals were routinely screened for the presence of HD transgene by PCR (10, 18).Preparation of Single Cell Suspension and Flow Sorting-Mouse epididymis was dissected from the animal, placed in phosphate-buffered saline, and chopped with a razor. Resulting suspension was filtered through cheesecloth. The supernatant was centrifuged at 1500 rpm, and the pellet was resuspended...

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confidence: 99%

CAG Repeat Lengths in X- and Y-bearing Sperm Indicate That Gender Bias during Transmission of Huntington's Disease Gene Is Determined in the Embryo

Kovtun

Welch

Guthrie

et al. 2004

Journal of Biological Chemistry

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“…Little is known about meiotic instability of the repeat tract. In humans, trinucleotide repeats show extreme meiotic instability, and expansion of the repeat tract has been suggested to occur in the germ-line mitotic divisions or postmeiotically during early divisions of the embryo (34,35). Recently, Cohen et al (36) have shown that instability of CAG repeats in yeast is severalfold higher during meiosis compared with mitosis.…”

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confidence: 99%

Meiotic instability of CAG repeat tracts occurs by double-strand break repair in yeast

Jankowski

Nasar

Nag

2000

Proc. Natl. Acad. Sci. U.S.A.

104

100

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Expansion of trinucleotide repeats is associated with a growing number of human diseases. The mechanism and timing of expansion of the repeat tract are poorly understood. In humans, trinucleotide repeats show extreme meiotic instability, and expansion of the repeat tract has been suggested to occur in the germ-line mitotic divisions or postmeiotically during early divisions of the embryo. Studies in model organisms have indicated that polymerase slippage plays a major role in the repeat tract instability and meiotic instability is severalfold higher than the mitotic instability. We show here that meiotic instability of the CAG͞CTG repeat tract in yeast is associated with double-strand break (DSB) formation within the repeated sequences, and that the DSB formation is dependent on the meiotic recombination machinery. The DSB repair results in both expansions and contractions of the CAG repeat tract. E xpansion of trinucleotide repeats has been shown to be associated with a growing number of human diseases (1). Several of these genetic diseases are associated with alterations of the CAG͞CTG (hereafter CAG) repeat tract length. The expansion mutation is highly dependent on the repeat tract length: the longer the repeat tract, the greater the possibility for expansion (2-4). Such mutational changes are dynamic, since the mutated region can undergo further changes in subsequent generations and during the lifespan of an individual.Two mechanisms have been used to explain the expansion of trinucleotide repeats: unequal genetic recombination and error in DNA replication caused by polymerase slippage (5, 6). In the recombination model, unequal crossing-over or gene conversion between triplet repeats either on sister chromatids or on homologs results in an altered tract length. In the slippage model, the replicating strand becomes dissociated, and then it misaligns during reassociation. This misalignment causes the formation of an unpaired sequence either on the template strand or on the nascent strand. A failure to repair the unpaired sequence will result in a DNA molecule containing a larger or smaller number of repeats after the next round of DNA replication. Several studies using model organisms indicate that replication slippage plays a major role in the repeat tract instability (7-14). The latter model is also supported by the in vitro observations that CAG repeats can form hairpin structures, with the CTG hairpin being more stable than the CAG hairpin (15-18). Recent studies with diploid yeast strains containing heterozygous trinucleotide repeat-insertion mutations suggest that trinucleotide repeats in single-stranded DNA are likely to form hairpin structures in vivo (19). During DNA replication, the hairpin formation on the template strand or on the newly synthesized strand would produce a deletion or an expansion of the repeat tract, respectively. This hypothesis is consistent with the observation that CAG-repeat instability depends on the direction in which the replication fork proceeds through the repeat tract (...

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“…The utility of the model system is immense in resolving certain unique attributes of the fragile X syndrome. For instance the developmental timing of repeat expansion is debated due to the occurrence of monozygotic twin pairs showing discordant repeat numbers at FMR1 locus (41,45) on one hand and the results with abortuses from fragile X patients which showed full mutation alleles suggesting that expansions are perhaps prezygotic event and that the variability in repeat number occurs during early embryonic divisions (46).…”

Section: Animal Modelsmentioning

confidence: 99%

Chromosomal fragility and human genetic disorders

Baskaran

Brahmachari

2000

Indian J Clin Biochem

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The first report of X-linked mental retardation correlated with the presence of marker chromosome came in 1940. It was in 1990 that the molecular basis of fragile X syndrome was deciphered. This elucidation marked the discovery of a novel process of mutation designated as dynamic mutations, resulting in the expansion of a triplet repeat sequence within the human genome. Subsequently several human genetic disorders involving triplet repeat expansion have beer= discovered. Almost all the disorders are known to affect the nervous system and/or the brain. This review presents an overview of fragile sites in the genome and the molecular genetics of fragile X syndrome.

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Characterization of the full fragile X syndrome mutation in fetal gametes

Cited by 176 publications

References 34 publications

CAG Repeat Lengths in X- and Y-bearing Sperm Indicate That Gender Bias during Transmission of Huntington's Disease Gene Is Determined in the Embryo

CAG Repeat Lengths in X- and Y-bearing Sperm Indicate That Gender Bias during Transmission of Huntington's Disease Gene Is Determined in the Embryo

Meiotic instability of CAG repeat tracts occurs by double-strand break repair in yeast

Chromosomal fragility and human genetic disorders

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