Three human diseases, dyskeratosis congenita (DKC), aplastic anemia (AA), and idiopathic pulmonary fibrosis (IPF), have been linked to mutations within the genes that encode for the two telomerase essential core components, telomerase RNA (TR) and telomerase reverse transcriptase (TERT); telomerase-associated proteins, DKC1 encodes for dyskerin and Nola3 encodes for Nop10; and recently to one of the six proteins that for the shelterin complex, TINF2 encodes for TIN2. Heterogeneous mutations show that even half the dose of telomerase is insufficient to maintain telomere length, resulting in erosion and loss of function, senescence, and apoptosis. The maintenance of telomere length in highly prolific cells, germline and stem cells, is crucial for the preservation of high populations and human health. In general point mutations, which leads to a single amino acid substitution, are better tolerated than frame shift and splicing junction mutations, limiting but not abolishing telomerase activity. The toleration of reduction and loss of telomerase function decreases with each subsequent generation. This anticipation, or progression of symptoms within the next generation, is characteristic of telomerase-deficiency diseases. The telomeres of the parental generation erode and when passed to their offspring begin this generation with shorter telomeres. The increase in severity of symptoms is linked with the progressive decrease of telomere length.
Dyskeratosis congenita (DKC) is an inherited disorder with clinical manifestations of skin hyperpigmentation (dark patches of skin), oral leukoplakia (white spots inside the mouth), and nail dystrophy (lack of nails). The majority of deaths occur from bone marrow failure, immunodeficiency, pulmonary complications, and malignancies. The X-linked recessive form has severe clinical presentations and caused by mutations found within the DKC1 gene that encodes for the dyskerin protein. The autosomal dominant form of the disease has been shown to be caused by mutations within the genes that encode TR as well as TERT. The autosomal recessive form of the disease has been shown to be caused by a mutation within Nola3, the gene that codes for Nop10. These mutations cause a reduction of telomerase activity leading to a limitation in stem cell capacity for proliferation. This reduction in proliferative capacity for high turnover cells leads in low counts for blood and immune cells, resulting in aplastic anemia. Bone marrow failure, brought on by the aplastic anemia, is the most common cause of death for patients with DKC. Families with this disease show anticipation, or the worsening of symptoms in subsequent generations because each generation begins with shorter telomeres than the previous Vulliamy et al, 2006.
Aplastic anemia (AA) is characteristically an acquired disease, however, there are rare constitutional forms of the disease that are found in patients with a strong familial history of various blood diseases. The disease has is connected with mutations with the genes that encode TR and TERT. This constitutional form of bone marrow failure is defined by low peripheral blood cell counts, hypocellular bone marrow, does not respond to immunosuppressive therapy, and included typical physical anomalies. The constitutional has associated with patients with DKC, however, there are cases lacking symptoms of DKC. The most common cause of death is due to bone marrow failure Fogarty et al, 2003.
Idiopathic pulmonary fibrosis (IPF) is a specific form of plumonary fibrosis with unknown cause, Plumonary fibrosis involves fibrotic lesion and scarring of the lung. The build up of excess scar tissue in the lungs results in reduced lung volume. The symptoms that typify the disease are chronic cough and shortness of breath. Some familiar types of IPF are caused by mutations in the genes that encode TR and TERT Armanios et al, 2007.
A standard nomenclature for telomerase RNA and protein components is used throughout the online database for consistency and clarity. Mutations designated according to the guidelines and recommendations for mutation nomenclature from the Human Genome Variation Society (HGVS). The nucleotides are numbered according to the coding DNA reference sequence, intronic positions are numbered as an addition to the last nucleotide from the preceding exon or as a difference from the first nucleotide from the proceeding exon. The (?) indicates uncertainty in the description of the mutation while frame shifts are described by the first affected amino acid and from this position the number of amino acids to the stop codon.