Therapeutic targeting of mismatch repair proteins in triplet repeat expansion diseases

Triplet repeat expansion diseases are a class of ∼20 inherited neurological disorders. Many of these diseases are debilitating, sometimes fatally so, and they have unfortunately proved difficult to treat. New compelling evidence shows that somatic repeat expansions in some diseases are essential to the pathogenic process, accelerating the age of onset and the rate of disease progression. Inhibiting somatic repeat expansions, therefore, provides a therapeutic opportunity to delay or block disease onset and/or slow progression. Several key aspects enhance the appeal of this therapeutic approach. First, the proteins responsible for promoting expansions are known from human genetics and model systems, obviating the need for lengthy target searches. They include the mismatch repair proteins MSH3, PMS1 and MLH3. Second, inhibiting or downregulating any of these three proteins is attractive due to their good safety profiles. Third, having three potential targets helps mitigate risk. Fourth, another protein, the nuclease FAN1, protects against expansions; in principle, increasing FAN1 activity could be therapeutic. Fifth, therapies aimed at inhibiting somatic repeat expansions could be used against several diseases that display this shared mechanistic feature, offering the opportunity for one treatment against multiple diseases. This review will address the underlying findings and the recent therapeutic advances in targeting MSH3, PMS1, MLH3 and FAN1 in triplet repeat expansion diseases.