|Number of repeats||Counts (alleles)||Percentage|
Overview of repeats
Point mutations in the exon 1 of NIPA1 gene, such as the c.134C>G has been shown to cause spastic paraplegia type 6 (SPG6) (Svenstrup et al., 2011). SPG6 disease-causing mutations in the GCG repeat region upstream from the mutation, at the N-terminus, has not yet been characterised. However, repeat lengths over eight GCG repeats were associated with increased susceptibility to amyotrophic lateral sclerosis (ALS) and the repeat size seemed to be limited to the addition of one or two GCG repeats (Tazelaar et al., 2019). In a healthy population the repeat region comprises of 8 repeats for 72% of individuals and 7 repeats for 25% of them (Tazelaar et al., 2019). Shorter and longer stretches (6, 9, 10) have also been reported in healthy individuals with a very low frequency (Chai et al., 2003).