A race of giants may have sprung from a mutant gene that first emerged around 1,500 years ago and causes uncontrolled body growth, scientists believe.
The “gigantism gene” was identified in the DNA of an 18th century man known as the “Irish Giant” who stood 2.3m (7ft 7in) tall.
Copies of the same mutation have been found in living patients suffering from gigantism and other symptoms of over-growth.
Scientists writing in the New England Journal of Medicine said they suspect all inherited the gene from the same common ancestor who lived up to 66 generations ago. Around 200 to 300 people may be carrying the same mutation today.
The Irish Giant was Charles Byrne - also known by his stage name O’Brien - born in Littlebridge, Co Derry in 1761.
In the 1780s he found fame exhibiting himself as a curiosity or “freak” in London. Despite claims that he towered more than eight feet tall, skeletal evidence shows his height to have been just over seven feet seven inches.
Celebrity life eventually got the better of Byrne, who took to drink and died at his home in Charing Cross aged just 22.
After his death Byrne’s body was acquired by the 18th century surgeon John Hunter, and his skeleton remains at the Hunterian Museum at the London headquarters of the Royal College of Surgeons.
British and German scientists conducting the new research extracted DNA from two of the Bryne’s teeth.
They discovered a mutant version of the aryl hydrocarbon-interacting protein gene (AIP) which matched those found in living patients from four Northern Irish families.
The gene variant triggers tumour growth in the pituitary gland at the base of the brain. Among its many functions, the gland releases hormones that regulate growth.
Pituitary tumours can cause tissue to grow abnormally, which may lead to gigantism - as suffered by Byrne - or acromegaly. Symptoms of acromegaly include thickened skin, enlarged hands and feet, distorted facial features, and overgrown organs.
By analysing the DNA, the scientists calculated that all the carriers of the mutant AIP gene had inherited the genetic fault from a common ancestor.
Calculations suggest that the mutation is about 1,500 years old, and between 200 and 300 people may carry it today.
Professor Marta Korbonits, from Barts and the London School of Medicine and Dentistry, who led the research, said: “The idea initially looked quite bizarre, but as we gained more data and identified new families affected by the mutation, our findings confirmed our theory.
“The most important clinical aspect of our study is that it is now possible to trace down carriers of this gene in time and treat patients before they grow to be a giant.”
Co-author Professor Mark Thomas, from University College London, said: “The striking thing about this research is that with modern genetic techniques we can say so much about somebody who died so long ago and use that information to identify and treat people at risk today.”
The discovery means anyone thought to be at risk can now be screened for the AIP mutation with a blood test, said the scientists.