These are the first reports of homozygous amino acid substitutions in TERT. They result in a clinical phenotype of either classical DC or HH, which is a severe variant of DC. They also represent the first genetic characterization of any case of AR-DC/HH. Individuals with heterozygous TERT
mutations published to date33,35-39
do not have the features of nail dystrophy and skin pigmentation abnormalities that have been observed in the index cases described here. This study therefore shows that a clinical phenotype close to classical DC can be produced in patients with homozygous mutations in TERT
and can be associated with a “pure telomerase defect.” This defect is highlighted by the presence of very short telomeres observed in the index case of family B. The reduced telomerase activity observed in the TRAP assays for both novel mutations suggests that the index cases in both of these 2 families would have less than 50% of normal telomerase activity in vivo.
TERT is a member of a large family of nucleic acid–dependent polymerases with conserved primary sequence motifs. While the tertiary structure of the TERT RT domain is unknown, telomerase-specific areas have been located in both the N- and C- termini peptide regions.51-53
Key residues in the TERT peptide (), termed the RT motifs, were found to alter RT catalytic function and reduce telomerase activity.54,55
The R811C amino acid substitution is positioned within the RT domain but in a section of low conservation (). The amino acid at position 811 in human TERT has no specific feature linking it to the equivalent amino acid in other species (ie, charge, aromatic ring, etc). The R901W amino acid substitution is also located in the RT domain but it is within the highly conserved D motif (). It lies beside the perfectly conserved K902 amino acid that was found to be mutated in another DC family,36
although the R901 amino acid itself is not highly conserved between the 5 species investigated. Of the 2 mutations that we have identified here, the R901W substitution resulted in a lower telomerase activity and is associated with a more severe clinical phenotype (HH). It is possible that homozygous mutations in highly conserved residues would be lethal as these would be predicted to result in negligible telomerase activity in vivo, and therefore would be incapable of maintaining normal growth and development.
DC is a genetically heterogeneous genetic disorder.56
But regardless of the inheritance pattern, progressive loss of cellular renewal associated with short telomeres has been observed. In cells from patients with X-linked recessive DC, lower TERC levels have been observed along with short telomeres.25,57
Haploinsufficiency of TERC leads to the AD inheritance of DC.26,33
This also appears to be the case with heterozygous inheritance of mutated TERT
In this paper, we demonstrate that homozygous TERT
mutations lead to reduced telomerase activity, shorter-than-expected telomeres, and a suggested increase of TERC levels. It has been documented that both X-linked DC and AD-DC may arise due to limitation of TERC levels.57,58
Our study now shows that classical DC can also arise due to severe deficiency of TERT, but that low TERC levels do not appear to be a uniform feature of DC. Indeed the index cases of both families with homozygous TERT
mutations were found to have raised TERC levels reflecting possible feedback mechanisms to compensate for the severe TERT deficiency. Therefore it seems that impaired telomerase function through dyskerin, TERC,
defects is sufficient to induce a DC clinical phenotype.
It is noteworthy that some of the heterozygous individuals in these families had subtle features of DC (). While the heterozygous R901W parents in family B had relatively short telomeres but were asymptomatic, the parents in family A had mild nail or skin pigmentation abnormalities. As there is a 20-year age difference between these 2 sets of parents, it is possible that the clinical features in family B simply require more time to develop, in a similar manner to the disease anticipation observed in AD-DC TERC
We also note that while the presence of the homozygous R901W TERT
mutation led to a significant reduction in telomere length, this shortening appeared to be reversed when the R901W alleles were absent, as the unaffected healthy sibling has relatively long telomeres.
The clinical observations seen in heterozygous individuals of both families are consistent with previous reports of a variable range of presentation in individuals carrying heterozygous TERT
The mild but variable phenotypic expression in part probably reflects the position of the different mutations within the TERT molecule as well as the effect of other environmental and/or genetic factors and therefore the various functional consequences that they might have on TERT activity, stability, or accumulation. The fact that as a group these heterozygotes are usually phenotypically mild compared with the classical presentation of DC, which we see in the homozygous index cases described here, highlights the dosage effect of having 1 versus 2 mutant TERT
It is unusual, but not unprecedented, for mutations in the same gene to give rise to both dominant and recessive forms of the same disease. The situation we describe here in TERT
is partly analogous to β-thalassemia where heterozygous mutations usually produce only mild hematologic abnormalities, but may on occasion give rise to a dominant β-thalassemia.59
In contrast, those individuals with biallelic mutations in the β-globin
gene usually present with a severe hematologic phenotype necessitating regular blood transfusion therapy. The situation with mutations to the telomerase complex is complicated, however, by the phenomenon of disease anticipation in which the dominant disease appears to worsen through successive generations. However, it is clear that the heterozygous TERC
families published to date never have a clinical phenotype as severe as seen in X-linked DC and AR-DC, particularly with respect to the cutaneous features. This is in contrast to the cases with the homozygous TERT
mutations described here where the phenotype closely mimics that observed in X-linked DC. This therefore highlights that a severe telomerase deficiency alone is capable of producing a classical and severe DC phenotype.
In conclusion, we describe the first clinical and functional characterization of families with homozygous TERT mutations. The findings of this study elucidate the first genetic characterization of AR-DC/HH syndrome and show that a “pure” but severe telomerase deficiency due to constitutional homozygous TERT mutations can produce a classical phenotype of DC.