Sequence alterations in DC associated genes are summarized in the Telomerase Database (http://telomerase.asu.edu/diseases.html
) along with references to the original work. In X-linked DC, with the exception of a small terminal deletion removing the last 20 amino acids of dyskerin and an in-frame deletion of a single amino acid (L37) mutations in dyskerin are mainly point mutations causing a single amino acid change, suggesting that these are all hypomorphic mutations with some residual dyskerin function (see also above). The promoter and splice site mutations described in rare patients with DC are consistent with this interpretation. The solving of the crystal structure of dyskerin revealed that pathogenic DKC1
mutations cluster in the RNA binding domain of dyskerin [63
], which is consistent with the decreased TERC
RNA levels in dyskerin mutant cells. Large and small TERC
RNA deletions have been described to be responsible for the development of autosomal dominant DC. Point mutations alter the tertiary structure of the RNA affecting activity or processivity of the telomerase enzyme, or the stability of the RNA. Mutations in TERT spread throughout the coding sequence and may affect any of its three domains, the N-terminal region, reverse-transcriptase motifs, or the C-terminal region. The study of 45 individuals with a de novo
chromosomal deletion spanning the TERT
gene showed that telomere shortening, although marginally accelerated, was not sufficient to cause a DC-related disease in the absence of anticipation [66
]. Alternatively, it is possible that in patients with a gene deletion compensatory mechanisms exist that might not operate in patients who express a mutant protein. Interestingly in patients with DC due to TINF2
mutations the mutations cluster in exon 6 [36
]. The mutations spare the TRF1 binding site, with TIN2 mutations after the binding site giving rise to a more severe phenotype (see also below).
The majority of mutations in patients with DC are unique or private mutation occurring in an individual family with DC. Exceptions are the A353V mutation in DKC1
and R282H and R282C TINF2
that are recurrent mutations. Occasionally DKC1 or TERT
gene mutations have been described independently in more than one family. Interestingly the recurrent A353V mutation in DKC1
and the R282H and R282C in TINF2
most frequently occur sporadically (see also above) suggesting that these occur within mutational hotspots and indeed these three mutations all arise from the relatively frequent CpG to TpG transitions that can occur by deamination of the cytosine residue [37
]. This cannot wholly explain the high frequencies of these mutations, and it is expected the role of these frequently mutated residues will hold important clues about the pathogenetic mechanism.
The fact that many of the gene alterations are only described once in a patient or family makes it difficult to determine whether the identified sequence alteration is indeed responsible for disease (pathogenic). This is more difficult when the same gene alteration has also been identified in unaffected family members or even in unrelated individuals, as can happen because of the variable penetrance of TERC and TERT mutations. Functional in vitro assays of telomerase activity or quantitative measurements of the level of telomerase RNA may support the diagnosis but often lead to variable results depending on the investigator. Moreover in vitro telomerase activity may not necessarily reflect the telomerase activity at the telomere end in vivo. Family studies and segregation with disease or short telomeres may lend support, but cannot on their own necessarily consolidate or exclude the pathogenicity of a novel identified gene alteration.
Several polymorphism have been described in DKC1, TERC, TERT and TINF2 genes, some of which have been shown to affect the expression levels of the respective cDNA or decrease the in vitro telomerase activity (functional polymorphism). However, to what extent these functional polymorphisms contribute or predispose to the development of disease is unclear and remains to be determined. In the context of genetic counseling or clinical decision-making such genetic variations should be carefully interpreted to avoid unnecessary anxiety in a patient who may remain healthy or to exclude a transplant from a sibling donor who otherwise has no clinical or laboratory signs of disease.