FAN1 protects against repeat expansions in a Fragile X mouse model

•FAN1 protects against repeat expansions in a mouse model of the FXDs.•Protection is seen in somatic tissues, including brain, but is not seen for intergenerational expansions.•As GWAS suggest, FAN1 mutations may affect the age at onset of some Repeat Expansion Diseases. The Fragile X-related disorders (FXDs) are members of a large group of human neurological or neurodevelopmental conditions known as the Repeat Expansion Diseases. The mutation responsible for all of these diseases is an expansion in the size of a disease-specific tandem repeat tract. However, the underlying cause of this unusual mutation is unknown. Genome-wide association studies have identified single nucleotide polymorphisms (SNPs) in the vicinity of the FAN1 (MIM* 613534) gene that are associated with variations in the age at onset of a number of Repeat Expansion Diseases. FAN1 is a nuclease that has both 5′-3′ exonuclease and 5′ flap endonuclease activities. Here we show in a model for the FXDs that Fan1−/− mice have expansions that, in some tissues including brain, are 2–3 times as extensive as they are in Fan1+/+ mice. However, no effect of the loss of FAN1 was apparent for germ line expansions. Thus, FAN1 plays an important role in protecting against somatic expansions but is either not involved in protecting against intergenerational repeat expansions or is redundant with other related enzymes. However, since loss of FAN1 results in increased expansions in brain and other somatic tissue, FAN1 polymorphisms may be important disease modifiers in those Repeat Expansion Diseases in which somatic expansion contributes to age at onset or disease severity.