DYNAMIC MUTATIONS

Sandep Yadav

…now the defect found in Arabidopsis thaliana

DYNAMIC MUTATION ??

Unstable heritable element where probability of mutation is function of no. of copies of mutation.

Unique Properties:

Expanding, Unstable DNA repeats. Repeats are usually polymorphic in copy number, with mitotic and meiotic

instability. Copy number related to the severity and/or age of onset Imprinting effects Reverse mutation - The mutation can revert to normal or to a premutation

carrier state. Founder effects are observed in association with dynamic mutation as a

higher frequency of particular allele in the affected population.

TYPES OF DYNAMIC MUTATION

Expanded repeats that cause disease are found in many different genic locations

Genic location of disease-associated repeats

How do expanded repeats cause disease?

Gain of function:• Repeats in protein coding sequences - toxic proteins e.g. polyglutamine disease.• Repeats in RNA coding regions - altered RNA function e.g. splicing in Myotonic dystrophy.

Loss of function:• Repeats in non-coding regions - reduced transcription

or translation e.g. fragile X syn. & IIL1 in A. thaliana

Processes involved in repeat expansion

Repeat expansion can take place in the germ line or in the somatic cell

Pf-0 at 27 C Bur-0 at 27 C Bur-0 at 23 C

Genetic Variation in A. thaliana’s Bur-0 strain

Strains were grown at 27 C and short day condition All except Bur-0 strain showed normal growth. Bur-0 strain showed abnormal growth and no flowering. Phenotype was named as “irregularly impaired leaves”(iil). At 23 C this defect disappeared but worsen at 30 C.

RNA level of At4g13430Location of expanded repeats

Genetic basis of iil defect

Interval mapping by PCR revealed expansion of GAA repeats in Chrm. 4 At 23 C RNA level didn’t differ between genotypes while RNA level decreased by 50% in Bur-0 strain at 27 C

Overexpression of mature Bur-0 transcript

IIL expression w.r.t. Overexpression

Bur-0 plant at 27 C (5 week)

Bur-0 plant harboring 35S:IIL1-Bur-0 transgene

DNA blot analysis of IIL1 triplet repeat containing region

PCR analysis of IIL1 repeat.

GAA repeat and somatic variability Phenotypic revertant showed the variability in the repeat no. EMS promotes the contraction of triplet repeats. After treatment 156 phenotypic revertants, 27 had reduced triplet copy no.

CONCLUSION:

1. Bur-0 strain provided a genetically tractable model for TNR studies.

2. Phenotypic revertants highlight the potential of IIL1 repeats for further study.

3. The molecular mechanisms underlying the repeat disorders involve either loss of protein function or gain of function at the protein or RNA level.

4. Repeat instability involves the formation of unusual DNA structures during DNA replication, repair and recombination.

5. Simple repeats can be associated with phenotypic variability of evolutionary significance.

Suggested reading:1.Kumar, S., Todesco, M., Schneeberger, K., Harilal, R., Balasubramanian, S., Weigel, D. (2009). A Genetic Defect Caused by a Triplet Repeat Expansion in Arabidopsis thaliana. Sciencexpress 1126: 1-10.2.Al-Mahdawi, S. et al. (2008). The Friedreich ataxia GAA repeat expansion mutation induces comparable epigenetic changes in human and transgenic mouse brain and heart tissues. Hum. Mol. Genet. 17, 735-746.3.Richards, R.I. (2001). Dynamic mutations: a decade of unstable expanded repeats in human genetic disease. Human Molecular Genetics 10: 2187-2194.4.Pearson, C. E., Edamura, K. N., Cleary, J. D. (2005). Repeat instability: mechanisms of dynamic mutations. Nat. Rev. Gen. 6: 729-742.

Thank You

for Your Attention!

Expansion mechanisms

Replication associated instability

Repair associated instability

Recombination associated