STRs database » CNBP locus
Overview of repeats
Myotonic dystrophy type 2 (DM2) is a disease similar to DM1 but it is caused by tetranucleotide CCTG expansions in intron 1 of the CNBP gene (also known as ZNF9) which can reach more than 11000 copies of the repeat (Liquori et al. (2001)). In most individuals the repeat tract contains (CCTG)n repeats which are interrupted by TCTG, GCTG and ACTG motifs. Uninterrupted alleles have been shown to be significantly more unstable than interrupted alleles (Bachinski et al. (2009)) and instability has been correlated with the length of uninterrupted tracts. The cut-off between stable and unstable uninterrupted alleles is approximately 30 CCTG repeats (Radvanszky et al. (2013)). Normal alleles generally contain up to 26 CCTG repeats, while disease-associated expansions generally contain approximately 75 or more. Alleles between these ranges are classified here as intermediate or uncertain, including premutation alleles (Kamsteeg et al. (2012)). Although alleles as short as 55 repeats have been reported (Bachinski et al. (2009)), their pathogenicity is not firmly established (Radvanszky et al. (2021)). Repeat interruptions also affect stability, so length alone does not fully determine interpretation.
The structure of expanded alleles has also been described in more detail. Alfano et al. (2022) identified downstream TCTG repeat blocks in seven of nine affected individuals studied. Wendlandt et al. (2025) subsequently reported an additional TCTG component in at least 84% of a cohort of 168 patients with confirmed DM2.