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Heterozygosity for a hypomorphic Polβ mutation reduces the expansion frequency in a mouse model of the Fragile X-related disorders.

Lokanga RA, Senejani AG, Sweasy JB, Usdin K - PLoS Genet. (2015)

Bottom Line: The FXDs result from expansion of a CGG/CCG repeat tract in the 5' UTR of the FMR1 gene.Somewhat surprisingly, while the number of expansions is smaller, the average size of the residual expansions is larger than that seen in WT animals.This may have interesting implications for the mechanism by which BER generates expansions.

View Article: PubMed Central - PubMed

Affiliation: Section on Gene Structure and Disease, Laboratory of Cell and molecular Biology, National Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland, United States of America; Department of Biochemistry, University of Cape Town Medical School, Cape Town, South Africa.

ABSTRACT
The Fragile X-related disorders (FXDs) are members of the Repeat Expansion Diseases, a group of human genetic conditions resulting from expansion of a specific tandem repeat. The FXDs result from expansion of a CGG/CCG repeat tract in the 5' UTR of the FMR1 gene. While expansion in a FXD mouse model is known to require some mismatch repair (MMR) proteins, our previous work and work in mouse models of another Repeat Expansion Disease show that early events in the base excision repair (BER) pathway play a role in the expansion process. One model for repeat expansion proposes that a non-canonical MMR process makes use of the nicks generated early in BER to load the MMR machinery that then generates expansions. However, we show here that heterozygosity for a Y265C mutation in Polβ, a key polymerase in the BER pathway, is enough to significantly reduce both the number of expansions seen in paternal gametes and the extent of somatic expansion in some tissues of the FXD mouse. These data suggest that events in the BER pathway downstream of the generation of nicks are also important for repeat expansion. Somewhat surprisingly, while the number of expansions is smaller, the average size of the residual expansions is larger than that seen in WT animals. This may have interesting implications for the mechanism by which BER generates expansions.

No MeSH data available.


Related in: MedlinePlus

The effect of heterozygosity for the PolBC mutation on the distribution of repeat number changes seen in the gametes of 3-month-old male mice.The percentage of alleles with the indicated gains or losses in repeat number for PolB+/+ and PolB+/C mice was plotted. The mean gain of repeats was 3.17 (SD = 2.52) for PolB+/+ and 5.43 (SD = 4.48) for PolB+/C. This resulted in a distribution of expanded alleles that was significantly different in the two genotypes (p = 0.0001; t test). The mean loss of repeats was 10.83 (SD = 11.05) for PolB+/+ and 19.68 (SD = 31.58) for PolB+/C. The very high standard deviations due to the presence of some very large contractions particularly in the PolB+/C mice resulted in a distribution of contracted alleles that was not significantly different in the two genotypes. Inset: PolB+/C mice have fewer small expansions and more large expansions than PolB+/+ mice. The error bars represent the 95% confidence interval. Repeat size classes that are significantly different in PolB+/C mice are marked with an asterisk. The decrease in the number of alleles with 1–5 repeats was significant at p = 0.0001, and the increase in the number of alleles with >10 repeats was significant at p = 0.005.
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pgen.1005181.g002: The effect of heterozygosity for the PolBC mutation on the distribution of repeat number changes seen in the gametes of 3-month-old male mice.The percentage of alleles with the indicated gains or losses in repeat number for PolB+/+ and PolB+/C mice was plotted. The mean gain of repeats was 3.17 (SD = 2.52) for PolB+/+ and 5.43 (SD = 4.48) for PolB+/C. This resulted in a distribution of expanded alleles that was significantly different in the two genotypes (p = 0.0001; t test). The mean loss of repeats was 10.83 (SD = 11.05) for PolB+/+ and 19.68 (SD = 31.58) for PolB+/C. The very high standard deviations due to the presence of some very large contractions particularly in the PolB+/C mice resulted in a distribution of contracted alleles that was not significantly different in the two genotypes. Inset: PolB+/C mice have fewer small expansions and more large expansions than PolB+/+ mice. The error bars represent the 95% confidence interval. Repeat size classes that are significantly different in PolB+/C mice are marked with an asterisk. The decrease in the number of alleles with 1–5 repeats was significant at p = 0.0001, and the increase in the number of alleles with >10 repeats was significant at p = 0.005.

Mentions: As can be seen in Fig 1, the sperm of 3-month-old PolB+/C mice had a significantly lower number of expansions than the sperm of PolB+/+ animals (Fig 1; 42% vs 74%; p = 0.0001). However, of the expansions that did occur, only 63% involved the addition of 1–5 repeats compared to 88% of the expansions seen in the sperm of WT mice (see Fig 2 inset). This relative deficit of small expansions is associated with a higher frequency of gametes that have gained >10 repeats: Only 3% of the expanded gametes of WT animals had gained this number of repeats while 22% of the expanded gametes of PolB+/C mice had done so. In addition, PolB+/C mice also had a 4.9-fold more gametes with alleles smaller than the parental allele (36.6% vs 7.4%, p = 0.0001). An increase in both large and small contractions was seen (Fig 2).


Heterozygosity for a hypomorphic Polβ mutation reduces the expansion frequency in a mouse model of the Fragile X-related disorders.

Lokanga RA, Senejani AG, Sweasy JB, Usdin K - PLoS Genet. (2015)

The effect of heterozygosity for the PolBC mutation on the distribution of repeat number changes seen in the gametes of 3-month-old male mice.The percentage of alleles with the indicated gains or losses in repeat number for PolB+/+ and PolB+/C mice was plotted. The mean gain of repeats was 3.17 (SD = 2.52) for PolB+/+ and 5.43 (SD = 4.48) for PolB+/C. This resulted in a distribution of expanded alleles that was significantly different in the two genotypes (p = 0.0001; t test). The mean loss of repeats was 10.83 (SD = 11.05) for PolB+/+ and 19.68 (SD = 31.58) for PolB+/C. The very high standard deviations due to the presence of some very large contractions particularly in the PolB+/C mice resulted in a distribution of contracted alleles that was not significantly different in the two genotypes. Inset: PolB+/C mice have fewer small expansions and more large expansions than PolB+/+ mice. The error bars represent the 95% confidence interval. Repeat size classes that are significantly different in PolB+/C mice are marked with an asterisk. The decrease in the number of alleles with 1–5 repeats was significant at p = 0.0001, and the increase in the number of alleles with >10 repeats was significant at p = 0.005.
© Copyright Policy
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4401650&req=5

pgen.1005181.g002: The effect of heterozygosity for the PolBC mutation on the distribution of repeat number changes seen in the gametes of 3-month-old male mice.The percentage of alleles with the indicated gains or losses in repeat number for PolB+/+ and PolB+/C mice was plotted. The mean gain of repeats was 3.17 (SD = 2.52) for PolB+/+ and 5.43 (SD = 4.48) for PolB+/C. This resulted in a distribution of expanded alleles that was significantly different in the two genotypes (p = 0.0001; t test). The mean loss of repeats was 10.83 (SD = 11.05) for PolB+/+ and 19.68 (SD = 31.58) for PolB+/C. The very high standard deviations due to the presence of some very large contractions particularly in the PolB+/C mice resulted in a distribution of contracted alleles that was not significantly different in the two genotypes. Inset: PolB+/C mice have fewer small expansions and more large expansions than PolB+/+ mice. The error bars represent the 95% confidence interval. Repeat size classes that are significantly different in PolB+/C mice are marked with an asterisk. The decrease in the number of alleles with 1–5 repeats was significant at p = 0.0001, and the increase in the number of alleles with >10 repeats was significant at p = 0.005.
Mentions: As can be seen in Fig 1, the sperm of 3-month-old PolB+/C mice had a significantly lower number of expansions than the sperm of PolB+/+ animals (Fig 1; 42% vs 74%; p = 0.0001). However, of the expansions that did occur, only 63% involved the addition of 1–5 repeats compared to 88% of the expansions seen in the sperm of WT mice (see Fig 2 inset). This relative deficit of small expansions is associated with a higher frequency of gametes that have gained >10 repeats: Only 3% of the expanded gametes of WT animals had gained this number of repeats while 22% of the expanded gametes of PolB+/C mice had done so. In addition, PolB+/C mice also had a 4.9-fold more gametes with alleles smaller than the parental allele (36.6% vs 7.4%, p = 0.0001). An increase in both large and small contractions was seen (Fig 2).

Bottom Line: The FXDs result from expansion of a CGG/CCG repeat tract in the 5' UTR of the FMR1 gene.Somewhat surprisingly, while the number of expansions is smaller, the average size of the residual expansions is larger than that seen in WT animals.This may have interesting implications for the mechanism by which BER generates expansions.

View Article: PubMed Central - PubMed

Affiliation: Section on Gene Structure and Disease, Laboratory of Cell and molecular Biology, National Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland, United States of America; Department of Biochemistry, University of Cape Town Medical School, Cape Town, South Africa.

ABSTRACT
The Fragile X-related disorders (FXDs) are members of the Repeat Expansion Diseases, a group of human genetic conditions resulting from expansion of a specific tandem repeat. The FXDs result from expansion of a CGG/CCG repeat tract in the 5' UTR of the FMR1 gene. While expansion in a FXD mouse model is known to require some mismatch repair (MMR) proteins, our previous work and work in mouse models of another Repeat Expansion Disease show that early events in the base excision repair (BER) pathway play a role in the expansion process. One model for repeat expansion proposes that a non-canonical MMR process makes use of the nicks generated early in BER to load the MMR machinery that then generates expansions. However, we show here that heterozygosity for a Y265C mutation in Polβ, a key polymerase in the BER pathway, is enough to significantly reduce both the number of expansions seen in paternal gametes and the extent of somatic expansion in some tissues of the FXD mouse. These data suggest that events in the BER pathway downstream of the generation of nicks are also important for repeat expansion. Somewhat surprisingly, while the number of expansions is smaller, the average size of the residual expansions is larger than that seen in WT animals. This may have interesting implications for the mechanism by which BER generates expansions.

No MeSH data available.


Related in: MedlinePlus