Telomere Length Plays No Role in Clinical Unit Dominance After Double-Unit Cord Blood Transplantation
Authors: Ashbridge Beth , Zehir Ahmet , Shen Lingbo , Barker Juliet N , Moore Malcolm A.S. AS 
Published: October 2019

Abstract
Abstract 4661

Background:
Umbilical cord blood (CB) is an alternative source of allogeneic hematopoietic stem cells for transplantation and has the advantage of a reduced stringency of the required human leukocyte antigen-match. It is limited, however, by low total nucleated cell (TNC) dose and a low progenitor number in single CB units, thus restricting the use of CB transplantation (CBT) in larger children and adults. One strategy to augment engraftment is to combine 2 units from 2 different donors in a double-unit graft. We have previously shown that in most cases, one unit emerges as the sole source of hematopoiesis long-term, but as yet the mechanism of unit dominance remains unknown (Blood, 2010, 116(19):3999–4006). CB units are known to have an inherent biological variation in telomere length, the repeat sequence capping the ends of chromosomes. Telomere length variation and progressive shortening of telomere could later hematopoietic potential.

Methods:
We evaluated if telomere length has a role in unit dominance and how telomere length progresses over time post-transplant. We purified mononuclear cells from small aliquots of each unit of double-unit grafts and post-transplant peripheral blood (days +28, 100, 180 and 1 year) in 12 adult double-unit CBT recipients transplanted for hematologic malignancies at MSKCC. Average telomere length was measured using the TeloTTAGGG Telomere Length Assay (Roche) which utilizes Southern analysis of terminal restriction fragments (TRF) that are obtained by the digestion of isolated genomic DNA.

Results:
All 12 patients engrafted with one unit showing predominance. While there was a range of telomere length (5.57–12.09 kb) in the 24 units evaluated on the day of transplant, when comparing the telomere length of engrafting to non-engrafting units, there was no association between telomere length at day 0 and subsequent unit dominance. In 5 patients the engrafting unit had longer telomeres and in 7 patients the non-engrafting unit had longer telomeres. In serial assays, 7 of 12 patients demonstrated telomere length stabilization post-transplant in the dominant unit with a mean percentage loss of telomere length of 7.15% ± 2.33%. A second group (n = 5) demonstrated a decrease in telomere length with a mean percentage loss of 29.48% ± 5.12%. While this difference is significant (p = 0.0015) the clinical significance of this finding is uncertain. We are currently following these patients in order to correlate telomere length (stabilization versus decrease) with clinical outcomes.

Conclusions:
This data suggests that unit dominance is not influenced by telomere length. It is likely, based on emerging data from our laboratory as well as others, that unit dominance is immune mediated. However, the influence of telomere length on the quality of engraftment is of interest and analysis of this is ongoing with larger numbers of patients required to also consider the infused cell dose. It is notable that 7/12 patients had telomere length stabilization which correlates with the high levels of telomerase activity previously reported in the in vitro expansion of CB (Blood, 2004, 103(12):4440-8).

Disclosures:
No relevant conflicts of interest to declare.