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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

PolB+/C mice show a reduced somatic instability index in testis and tail.The somatic instability index of different organs of three 16 month old PolB+/+ and three 16 month old PolB+/C mice with ~140 repeats was determined as previously described [46]. Tail 1 and tail 2 refer to tail samples taken at 3 weeks of age and tail samples taken at 16 months respectively. The error bars represent the standard deviations. The significance of the differences in the SII for different genotypes was determined using Student’s t-test. The tissues in which the SII was significantly lower in PolB+/C mice are indicated by asterisks. The SII for PolB+/C testis was significantly lower at p = 0.001 and the SII for the PolB+/C tail 2 sample was significantly lower at p = 0.013.
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pgen.1005181.g005: PolB+/C mice show a reduced somatic instability index in testis and tail.The somatic instability index of different organs of three 16 month old PolB+/+ and three 16 month old PolB+/C mice with ~140 repeats was determined as previously described [46]. Tail 1 and tail 2 refer to tail samples taken at 3 weeks of age and tail samples taken at 16 months respectively. The error bars represent the standard deviations. The significance of the differences in the SII for different genotypes was determined using Student’s t-test. The tissues in which the SII was significantly lower in PolB+/C mice are indicated by asterisks. The SII for PolB+/C testis was significantly lower at p = 0.001 and the SII for the PolB+/C tail 2 sample was significantly lower at p = 0.013.

Mentions: As can be seen from Fig 5, some tissues of PolB+/C mice have an average SII that is lower than their WT counterparts. The difference in SII was significant for testis, and the tail sample taken at euthanasia (Tail 2). While the expansion frequency seen in sperm of 11-month-old animals is not significantly different from WT animals, the lower SII seen in the testis as a whole may reflect the contribution of other cells of the testis. The limited number of tissues affected by the presence of the PolBC mutation is consistent with the limited effect of PolB mutations on the mutation rates of different tissue [32], an observation that has been interpreted to mean that DNA repair in most tissues is not sensitive to PolBC heterozygosity. Since we know that contractions do not occur post-natally in somatic cells [38], the decrease in the SII would be consistent with a role for Polβ in generating somatic expansions. While it is possible that some expansions seen in testis are derived from developing gametes, the fact that a decreased SII is also seen in tail indicates that the effect of the Polβ mutation is not confined to germ cells. While in sperm a lower expansion frequency is offset by a larger average expansion size, this effect is not apparent in either total testis DNA or tail. It may be that the decrease in the expansion frequency caused by the Y265C mutation is more marked in these cells than it is in sperm, such that the larger jumps produced by those few alleles that do expand is not apparent. Alternatively, the pathway that generates these larger jumps may not occur at high enough frequency for the effect to be seen in somatic cells.


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)

PolB+/C mice show a reduced somatic instability index in testis and tail.The somatic instability index of different organs of three 16 month old PolB+/+ and three 16 month old PolB+/C mice with ~140 repeats was determined as previously described [46]. Tail 1 and tail 2 refer to tail samples taken at 3 weeks of age and tail samples taken at 16 months respectively. The error bars represent the standard deviations. The significance of the differences in the SII for different genotypes was determined using Student’s t-test. The tissues in which the SII was significantly lower in PolB+/C mice are indicated by asterisks. The SII for PolB+/C testis was significantly lower at p = 0.001 and the SII for the PolB+/C tail 2 sample was significantly lower at p = 0.013.
© Copyright Policy
Related In: Results  -  Collection

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

pgen.1005181.g005: PolB+/C mice show a reduced somatic instability index in testis and tail.The somatic instability index of different organs of three 16 month old PolB+/+ and three 16 month old PolB+/C mice with ~140 repeats was determined as previously described [46]. Tail 1 and tail 2 refer to tail samples taken at 3 weeks of age and tail samples taken at 16 months respectively. The error bars represent the standard deviations. The significance of the differences in the SII for different genotypes was determined using Student’s t-test. The tissues in which the SII was significantly lower in PolB+/C mice are indicated by asterisks. The SII for PolB+/C testis was significantly lower at p = 0.001 and the SII for the PolB+/C tail 2 sample was significantly lower at p = 0.013.
Mentions: As can be seen from Fig 5, some tissues of PolB+/C mice have an average SII that is lower than their WT counterparts. The difference in SII was significant for testis, and the tail sample taken at euthanasia (Tail 2). While the expansion frequency seen in sperm of 11-month-old animals is not significantly different from WT animals, the lower SII seen in the testis as a whole may reflect the contribution of other cells of the testis. The limited number of tissues affected by the presence of the PolBC mutation is consistent with the limited effect of PolB mutations on the mutation rates of different tissue [32], an observation that has been interpreted to mean that DNA repair in most tissues is not sensitive to PolBC heterozygosity. Since we know that contractions do not occur post-natally in somatic cells [38], the decrease in the SII would be consistent with a role for Polβ in generating somatic expansions. While it is possible that some expansions seen in testis are derived from developing gametes, the fact that a decreased SII is also seen in tail indicates that the effect of the Polβ mutation is not confined to germ cells. While in sperm a lower expansion frequency is offset by a larger average expansion size, this effect is not apparent in either total testis DNA or tail. It may be that the decrease in the expansion frequency caused by the Y265C mutation is more marked in these cells than it is in sperm, such that the larger jumps produced by those few alleles that do expand is not apparent. Alternatively, the pathway that generates these larger jumps may not occur at high enough frequency for the effect to be seen in somatic cells.

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