Very Short Telomeres Are Characteristic of Dyskeratosis Congenita and Not Other Inherited Bone Marrow Failure Syndromes
Authors: Alter Blanche P. , Baerlocher Gabriela , Giri Neelam , Lansdorp Peter M. , Savage Sharon A 
Published: April 2020

Abstract
Telomeres protect the ends of chromosomes, shorten with age, and are very short in dyskeratosis congenita (DC), an inherited bone marrow failure syndrome (IBMFS) associated with mutations in telomere biology genes. “Short telomeres” were reported in Fanconi Anemia (FA), Diamond-Blackfan Anemia (DBA) and Shwachman-Diamond Syndrome (SDS) using telomere restriction fragment length or Q-FISH assays of total leukocyte or mononuclear cell DNA. These reports focused on group averages, not results from individual patients. Our objective was to determine which categories of IBMFS patients have very short telomeres, and in which leukocyte subsets, using a more sensitive and specific assay. Telomere length was measured in granulocytes, lymphocytes, naïve T-cells, memory T-cells, B-cells, and NK cells using automated multicolor flow fluorescence in situ hybridization (FISH). We previously showed that very short telomeres (&lt;1st percentile for age) in lymphocytes, naïve T-cells, and B-cells were sensitive and specific for the diagnosis of DC (Alter et al, Blood110:1439, 2007). Granulocytes were not specific, since in that study neutropenic patients without DC often had very short granulocyte telomeres. The current study included 53 DC patients and 87 relatives, 19 non-transplanted FA patients and 12 relatives, 21 DBA patients and 11 relatives, and 7 SDS patients and 7 relatives. There were 400 normal controls, ages 0 to 100; very short telomeres were defined as &lt;1st percentile of normal for age. Z-scores were calculated to adjust for age; 0 is normal and -2 or below is significantly short (i.e. &lt;2 standard deviations below the mean). Lymphocyte telomeres were &lt;1st percentile (very short) in 96% of DC patients, 7% of DC relatives, 16% of FA patients, 5% of DBA patients, 14% of SDS patients, and none of the non-DC relatives. Granulocyte telomeres were very short in 94% of DC patients, 16% of DC relatives, 44% of FA patients, 19% of DBA patients, 14% of SDS patients, 8% of FA relatives, and none of the non-DC relatives. All three hallmark leukocyte subsets (lymphocytes, naïve T-cells, and B-cells) were very short in 81% of DC patients, 5% of DC relatives, 5% of DBA patients, and none of the FA or SDS patients or their relatives. Although 4 out of 47 (9%) IBMFS other than DC had very short telomeres in lymphocytes, only one, a patient with DBA (without mutations in known DC genes) had very short telomeres in all 3 of the hallmark lineages. Four of 87 DC relatives also had very short telomeres in all 3 lineages; they may be silent carriers in families in which DC genes have not yet been identified. The mean Z-score for lymphocyte telomeres in DC was -4.7; it was above -2 in all other categories (-0.3 in FA, -0.9 in DBA, -1 in SDS). The mean Z-score for granulocytes was -4.2 in DC, -1.9 in FA, -1.2 in DBA, and -1.5 in SDS. Thus, based on the Z-scores, the average telomere length was very short in DC lymphocytes and granulocytes; normal in FA lymphocytes and borderline in FA granulocytes; and normal in DBA and SDS lymphocytes and granulocytes. Flow-FISH provides analysis of multiple cell types in individual patients, and identification of specific individuals with very short telomeres, as well as the age-adjusted mean telomere length in a diagnostic subset. No FA or SDS patients, and only one with DBA, met the DC diagnostic criteria of very short telomeres in three or more lymphocyte subsets. The telomere length deficit in DC appears to be more severe and more frequent than in the other inherited bone marrow failure syndromes.