Question

Cells lacking telomerase can grow for several cell divisions without obvious defects. However, after more cell divisions, such cells tend to show chromosomes that have fused together. Propose an explanation for the formation of the chromosomes.

Answer 1

The ends of human chromosomes are bound by telomere repeats which have the sequence TTAGGG, followed by a terminal 3’G-rich single stranded overhang. These overhands invade the DNA by forming a D-loop. Telomere repeats cannot be formed by DNA polymerase during the lagging strand synthesis.

During DNA replication, there is leading and lagging strand synthesis. The DNA polymerase is able to polymerize and extend the 3’ GC rich overhang. The 5’ to 3’ sequence which is complementary to the 3’ overhang is cleaved by a G-rich sequence specific nuclease. Processing of the 5’ strand leads to formation of the G-tail (G-overhang). The 3’ overhang is the formed by 5’ resection.   

Such replication and formation of 3’ overhang is not possible on the lagging strand. The RNA primers are removed from Okazaki fragments to join these fragments after DNA replication. However, no complementary region for the 3’ overhang can be created. Telomerase, an enzyme consisting of reverse transcriptase (TERT), and a telomerase RNA component (TERC) is therefore involved in generation of the telomere repeats. Telomerase is able to extend the repeats at the G rich region. However, after telomerase action, the last primer is removed. This creates a gap on this strand. Hence, telomere repeats are incompletely added in lagging strand, thereby decreasing telomere length every replication cycle.

If cells lack telomerase, lagging strands cannot form telomere repeats, despite telomere repeats being present on the leading strand. Hence, first few generations, the cells will not show major effect. However, repeated defective lagging strand telomere addition cause chromosomal instability. The chromosomal ends will start fusing to each other. The chromosomal ends lacking the telomere repeats are recognized as double stranded breaks. This will result in occurrence of interchromosomal fusion.

When there is a loss of telomere repeats, the cell recognizes it as damage via binding of MRN an ATM. This will resection or remove the 3’ overhang. The chromatin structure is altered resulting in recruitment of 53BP1. 53BP1 increases mobility of the chromosomes towards each other. The chromosomes bind to similar uncapped chromosomal ends. Fusion of chromosomal ends is via NHEK covalent fusion. These fused chromosomes can continue to cycle if the p53 and pRb dependent tumor suppressor pathways are absent (in tumor cells). When they enter mitosis, due to inefficient separation of sister chromatids, the genetic material is unequally divided in daughter cells. This cycle during mitosis is called fusion-breakage-bridge cycle. Multiple such cycles will result in genomic instability.   As a result, chromosomes fusion results in degradation of the chromosomes. Thus, there is loss of chromosomes and death.