Molecular Genetic Pathogenesis
The mechanism by which WRN pathogenic variants cause the Werner syndrome phenotype is not clear. WRN encodes a multifunctional nuclear protein of 1,432 amino acids [Yu et al 1996] that is a member of the RecQ family of DNA helicases. The N-terminal region of the protein encoded by WRN has exonuclease activity as well.
DNA-type helicases are ATP-dependent 3'→ 5' helicases that are necessary to maintain genomic integrity in cells. Other human RecQ helicases are encoded by RECQL, BLM (responsible for the Bloom syndrome), RECQL4 (responsible for the Rothmund-Thomson syndrome), and RecQL5 (reviewed by Croteau et al [2014]).
The WRN helicase preferentially unwinds DNA structures, such as tetraplex DNA, double-strand DNA with mismatch “bubbles,” and Holliday junctions. It unwinds DNA-DNA double strands as well as DNA-RNA double strands. WRN exonuclease activity also preferentially digests single strands in complex DNA structures, such as double-stranded DNA with mismatched ends or bubbles. WRN helicase and exonuclease activities are modified by binding to interacting proteins (e.g., Ku complex, p53, replication protein A) and by phosphorylation [Croteau et al 2014].
Biochemical and cell biologic studies suggest that WRN protein is involved in DNA repair, recombination, replication, and transcription as well as combined functions such as DNA repair during replication. WRN protein can potentially unwind or digest aberrant DNA structures accidentally generated during various DNA metabolisms and can also regulate DNA recombination and repair processes by unwinding or digesting intermediate DNA structures. WRN protein is also involved in the maintenance of telomeres. These findings are consistent with the notion that WRN plays a role in maintenance of genomic stability [Croteau et al 2014].
Gene structure.
WRN has a transcript of 5765 bp and consists of 35 exons. For a detailed summary of gene and protein information, see Table A, Gene.
Variants of uncertain significance. The allelic variant of uncertain significance was found in Latino populations with a heterozygote frequency of 0.02. This change greatly reduces helicase and exonuclease activities in vitro. Homozygous individuals could exhibit some of the phenotypes of Werner syndrome, but this has not been demonstrated [Kamath-Loeb et al 2004].
Pathogenic allelic variants. More than 70 different WRN pathogenic variants have been identified. The majority of pathogenic variants are stop codons, insertions, or deletions that result in a frame shift or splice donor or acceptor site variant that result in exon skipping. Several missense variants that abolish helicase activity or confer protein instability have been reported.
Pathogenic variants that occur in an intron and result in creation of a new exon as well as multiexonic deletions and duplications have also been reported [Huang et al 2006, Uhrhammer et al 2006, Friedrich et al 2010].
The most common pathogenic variant is , which accounts for 20%-25% of pathogenic variants in the European and Japanese populations [Matsumoto et al 1997, Friedrich et al 2010].
Founder variants have been reported in some populations (see ).
While protein analysis does not distinguish different types of pathogenic variants, it provides important supportive evidence. For example, the Sardinian founder variant, , is an intronic variant that creates a new exon resulting in a protein of altered length.
Table 2.
View in own window
Variant Classification | DNA Nucleotide Change | Predicted Protein Change | Reference Sequences |
|---|
Uncertain significance | c.2500C>T 1 | p.Arg834Cys | NM_000553.4 NP_000544.2 |
| Pathogenic | c.1105C>T | p.Arg369Ter |
| c.2089-3024A>G 2 | See footnote 3 |
| c.2179dupT 4 | p.Cys727LeufsTer5 |
| c.3139-1G>C 5 | See footnote 6 |
| c.3460-2A>C 7 | See footnote 8 |
| c.3590delA 9 | p.Asn1197ThrfsTer2 |
Normal gene product. The normal gene product has 1,432 amino acids. The central region of the WRN ATP-dependent helicase contains the consensus domains of RecQ type helicases [Gray et al 1997] and the N-terminal region contains exonuclease domains [Huang et al 1998]. A nuclear localization signal is at the C-terminal end of the protein [Suzuki et al 2001]. A highly acidic transactivation sequence is present between exonuclease and helicase domains [Balajee et al 1999]. There are two consensus domains in the C-terminal region whose functions have not been completely elucidated: a RecQ C-terminal conserved (RCQ) region and a helicase RNaseD C-terminal (HRDC) conserved region. The RCQ region, which contains a zinc finger motif and a winged helix motif (WH) may be involved in the regulation of helicase enzymatic activity by modulating DNA binding as well as protein folding of WRN helicase [Kitano et al 2010]. The HDRC region is speculated to mediate protein-protein interactions.
Abnormal gene product. A majority of pathogenic variants result in the truncation of the protein. In addition to the loss of the nuclear localization signal in WRN mutated proteins [Huang et al 2006], the mutated mRNAs and the resulting mutated proteins exhibit shorter half-lives than do the wild-type mRNA and WRN protein [Yamabe et al 1997].