PREAMBLE: Before there is debate, let me preface this question by stating that I am indeed aware of the question Why do replicants have a short lifespan? However, none of its answers adequately answer my question below, and I believe the spirit of the question is entirely different. (That question is asking for the motivation behind giving the replicants a short lifespan, not about how the short lifespan is achieved.)
In Blade Runner, Roy, Pris, Leon, and Zhora — replicants manufactured by the Tyrell Corporation — suffer from "accelerated decrepitude", to quote Pris. These particular replicants have a four-year lifespan.
In the Blade Runner universe, replicants are synthetic lifeforms that include genetically-engineered organic components, such as skin, hair, and eyes. These are grown according to custom genetic patterns supplied by the Tyrell Corporation. (In the film, we see that eye production is outsourced, for instance. The character Hannibal Chew grows eyes for replicants in development.)
The conversation between Tyrell and Roy suggests the lifespan problem is biological in nature:
TYRELL: The facts of life. To make an alteration in the evolvment of an organic life system is fatal. A coding sequence cannot be revised once it's been established.
When pressed for more information, Tyrell explains:
Because by the second day of incubation, any cells that have undergone reversion mutations give rise to revertant colonies like rats leaving a sinking ship. Then the ship sinks.
After Batty offers a different possible solution, Tyrell counters with:
...but it does give rise to an error in replication so that the newly-formed DNA strand carries the mutation....
Assuming Tyrell is telling the truth, then there is a natural question:
Is Tyrell's explanation indicative of exceptionally short telomeres in the cells of replicants?
The types of problems regarding life extension put forward by Tyrell would support this. One way to combat the telomere length problem would be to use telomerase to protect nucleotides on chromatids. However, this would theoretically leave the subject open to opportunistic malignancies resulting from mutated cells that form stable colonies due to reduced cell apoptosis.
Is this what Tyrell is describing?