Diminished Telomeric 3′ Overhangs Are Associated with Telomere Dysfunction in Hoyeraal-Hreidarsson Syndrome

BACKGROUND:Eukaryotic chromosomes end with telomeres, which in most organisms are composed of tandem DNA repeats associated with telomeric proteins. These DNA repeats are synthesized by the enzyme telomerase, whose activity in most human tissues is tightly regulated, leading to gradual telomere shortening with cell divisions. Shortening beyond a critical length causes telomere uncapping, manifested by the activation of a DNA damage response (DDR) and consequently cell cycle arrest. Thus, telomere length limits the number of cell divisions and provides a tumor-suppressing mechanism. However, not only telomere shortening, but also damaged telomere structure, can cause telomere uncapping. Dyskeratosis Congenita (DC) and its severe form Hoyeraal-Hreidarsson Syndrome (HHS) are genetic disorders mainly characterized by telomerase deficiency, accelerated telomere shortening, impaired cell proliferation, bone marrow failure, and immunodeficiency. METHODOLOGY/PRINCIPAL FINDINGS:We studied the telomere phenotypes in a family affected with HHS, in which the genes implicated in other cases of DC and HHS have been excluded, and telomerase expression and activity appears to be normal. Telomeres in blood leukocytes derived from the patients were severely short, but in primary fibroblasts they were normal in length. Nevertheless, a significant fraction of telomeres in these fibroblasts activated DDR, an indication of their uncapped state. In addition, the telomeric 3' overhangs are diminished in blood cells and fibroblasts derived from the patients, consistent with a defect in telomere structure common to both cell types. CONCLUSIONS/SIGNIFICANCE:Altogether, these results suggest that the primary defect in these patients lies in the telomere structure, rather than length. We postulate that this defect hinders the access of telomerase to telomeres, thus causing accelerated telomere shortening in blood cells that rely on telomerase to replenish their telomeres. In addition, it activates the DDR and impairs cell proliferation, even in cells with normal telomere length such as fibroblasts. This work demonstrates a telomere length-independent pathway that contributes to a telomere dysfunction disease

Reduced telomeric overhang signal in the HHS-affected cells.

<p>(A) Genomic DNA samples prepared from S2 and control LCLs (PDL of 44 and 50, respectively; 2 and 4 µg) or fibroblasts (PDL of 14 and 16 for S2 and C, respectively; 2 µg) were digested with MboI and AluI and electrophoresed in a 0.7% agarose gel. The average length of the 3′ overhang was estimated by in-gel hybridization of native DNA to a C-rich telomeric probe (native panels). The DNA was subsequently denatured <i>in situ</i> and re-hybridized to the same probe to measure the total TTAGGG repeat signal (denatured panels). (B) The histograms below the images represent the quantified native (overhang) signals normalized to the denatured (total) signals and presented as percentage of the normalized overhang signals of the controls.</p

Genealogical tree of the HHS-affected family.

<p>Open circles and squares represent unaffected females and males, respectively. Black circles and squares represent affected females and males. A gray square indicates a family member who died from pulmonary fibrosis. Tilted lines indicate mortality, and the ages of mortality are indicated underneath.</p

Severely short telomeres in the HHS patients.

<p>Genomic DNA was prepared from blood leukocytes (A) and EBV-infected lymphoblastoid cells (B), digested with the restriction endonuclease HinfI, and analyzed by Southern with a telomeric probe. The identity of the HHS-affected and unaffected individuals is indicated above the lanes by the same labels used in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0005666#pone-0005666-g001" target="_blank">Figure 1</a>. C1 and C2 indicate control samples taken from healthy unrelated individuals. The ages at which the blood samples were taken for telomere length analysis were (years): C1, 0.5; S5, 4; C2, 6; G2, 66; S1, 20; G1, 68; S4, 7; S2, 17; P2, 46; and P1, 46. The mean telomere length, calculated by the computer program <i>MATELO </i><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0005666#pone.0005666-Yehezkel1" target="_blank">[27]</a>, is indicated below the lanes. (C) S2 and C3 (control) LCLs were analyzed by FISH with a telomeric PNA probe. Both cultures grew for about the same population doublings and were processed side by side under the same conditions.</p

The HHS-affected cells express normal hTR levels and assemble active telomerase.

<p>Total RNA was prepared from blood leukocytes (A) and LCLs (B) derived from the affected siblings (red), unaffected parents (dark gray), or unaffected individuals (C – control; light gray). (A,B) hTR levels were measured by SYBR-green real-time RT-PCR, normalized to the levels of the U93 small nucleolar RNA, and presented as values relative to the controls. S2a indicates a later passage of the S2 LCL. Normalization to the levels of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA did not significantly change the results (data not shown). (C) Whole-cell extracts were prepared from LCLs and assayed for telomerase activity by TRAP assay, as described in the <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0005666#s4" target="_blank">Materials and Methods</a>. The amounts of the extracts used in total protein were 10, 30, and 100 ng for the left panel, and 100 ng for the right panel. The PDL of the LCLs at which the samples were taken are: S2a, 38; C, 40; P1, 20; P2, 28; and S2, 25.</p

TIF formation in HHS-affected fibroblasts with normal telomere length.

<p>(A) Control (C) and HHS-affected (S2) primary fibroblast cultures (established at the ages of 30 and 17 years, and grown to PDL of 14 and 10, respectively) were immunostained for TRF1 (green) and γ-H2AX (red), and with DAPI for the nuclei (blue), as indicated above the images. The bottom panels show enlarged images that include several telomeres. The images of the affected and control cells were obtained and processed in the same way, side by side. (B) The number of TIFs (defined as colocalized TRF1 and γ-H2AX foci) was counted in randomly-chosen 67 affected and 58 control cells. The graph shows the percentage of cells with at least five such foci. (C) Genomic DNA was prepared from these cultures and the average length of telomeres estimated by Southern analysis.</p

Impaired proliferation of cell cultures prepared from an HHS-affected patient.

<p>EBV-infected lymphoblastoid (A) and primary fibroblast (B) cultures were grown as described in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0005666#s4" target="_blank">Materials and Methods</a>. The cumulative population doubling level (PDL) of the HHS-affected (S2) and control (C) cultures is drawn as a function of days in culture.</p