Sita Reddy

PIBBS MENTOR

Associate Professor

Biochemistry & Molecular Biology
Keck School of Medicine
Institute for Genetic Medicine

Research Topics

  • Developmental Biology
  • Cardiovascular & Skeletal Muscle Diseases
  • DNA Replication, Repair, Modification, Neurogenetics

Research Overview

My laboratory is interested in the molecular defects that contribute to the development of myotonic dystrophy. The myotonic dystrophies, DM1 and DM2, are multi-symptom disorders characterized by a wide range of muscle and neurological defects. We are using both mouse genetics and biochemical approaches to understand the molecular basis of these diseases.

The genetic defects in DM1 and DM2 are expansions of CTG and CCTG repeat tracts located in untranslated regions of two genes, DMPK and ZNF9, located on chromosome 19q and 3q, respectively. DM1 is the more serious disorder, exhibiting both unique features and demonstrating increased incidence and severity of several symptoms shared between the two disorders. We are currently testing the following hypotheses:

(i) Unique features of DM1 arise from locus specific cis effects of CTG expansion

(ii) A dominant RNA mechanism underlies shared features of both diseases

(iii) Disruption of aberrant interactions between the muscleblind proteins and the mutant RNAs is sufficient to rescue pathological features that are common to both diseases.

Cis effects of CTG expansion in DM1: CTG expansions in DM1 patients have thus far been shown to cause stochastic decreases in the steady-state levels of two genes, DMPK and SIX5, which are located in the vicinity of the CTG tract. We are testing the hypothesis that locus specific cis effects of CTG expansion contribute to the increased severity and complexity of the symptoms exhibited by DM1 patients. This hypothesis predicts that inactivation of DMPK and SIX5 should result in partial DM1 phenotypes in model animals. To test this model, we have developed mice in which Dmpk and Six5 have been functionally inactivated. Analyses of these mouse strains demonstrate that decreased levels of Dmpk and Six5 result in a unique set of pathophysiological features that are observed in DM1 patients. Specifically, reduced Dmpk levels results in skeletal muscle weakness, ion channel defects and cardiac conduction disease while Six5 loss increases the incidence of congenital cataracts, cardiac hypertrophy and gonadal dysfunction. Defining the number of genes affected at the DM1 locus and understanding their contribution to the DM1 phenotype are current interests of the lab. These analyses will be carried out using a mouse model of DM1, in which expanded CTG tracts have been introduced into the corresponding mouse Dm1 locus.

RNA dominant mechanism: An RNA dominant mechanism has been shown to underlie the development of several pathological features that are common to both DM1 and DM2. Specifically, mutant RNAs encoding expanded CUG and CCUG repeat sequences are known to sequester the muscleblind family of proteins to form aberrant nuclear foci. We have shown that the muscleblind proteins are RNA splice regulators and that dysregulation of their activity results in RNA splice defects in DM patient cells. In ongoing experiments we are assessing the role of the muscleblind family of genes in DM by developing single and double mutants of the muscleblind family of genes in mice. In a parallel series of experiments we are attempting to understand the molecular basis of the toxicity associated with expanded CUG/CCUG tracts by functional characterization of the protein profile of DM nuclear foci using molecular and biochemical approaches.

Screening chemical libraries for small molecules that rescue DM pathology: We are currently developing molecular screens to identify small molecules that disaggregate DM foci in patient cells and allow a rescue of the DM associated splice defects. The effectiveness of such molecules in rescuing features of DM pathology will be further assessed in vivo using mouse models for DM that are under construction in my lab.