Nathan Klein, an early PD gene therapy trialist, had a good recovery then his "new" cells started to die and he was back to square one.

However he got back a good few years and remember DBS doesn't halt the progression of PD and Prosavin is competing with DBS.

In terms of losing the functions of the converted brain cells, well I am no expert but :

It is estimated, the human brain consists of 100 billion brain cells (neurons) and around 1 trillion supporting cells (glia), which helps neurons. The brain weighs about 1.4 kg (3 pounds). But after the age of 20, naturally people will lose about 1 gram of brain mass per year.

If the human brain has a weight of 1.4 kg (1,400 grams) and there are about 100 billion neurons, then it can be calculated that there are about 70 million neurons per gram.

If people lose 1 gram of brain mass per year, this means that the brain loses 70 million neurons per year with about 190 thousand per day slowing down or dyeing.

Even if it is said that about 5 percent from 1 gram of neurons really die, then the human brain loses about 9,000 neurons per day. These numbers may seem high, but when compared with the remaining neurons, the number is small.

So there is still a lot of capacity to go round even allowing for the "loss" of the converted cells.

Take care,
Neil.