We tend to think of diseases being exogenous to our world – something that the gods send to punish us. In the real world, diseases arise from our genes. This story about a small town in Brazil where 30% of the people are related to each other due to marriage highlights how intermarriage amplifies the risk of genetic mutation and that in turn leads to rare diseases. Giulia Granchi & Vitor Tavares write:
“Before Silvana Santos arrived in the little town of Serrinha dos Pintos more than 20 years ago, residents had no idea why so many local children had lost the ability to walk.
The remote town in north-eastern Brazil is home to fewer than 5,000 people, and is where biologist and geneticist Santos identified and named a previously unknown condition: Spoan syndrome.
Caused by a genetic mutation, the syndrome affects the nervous system, gradually weakening the body. It only appears when the altered gene is inherited from both parents.
Santos’s research marked the first time the disease had been described anywhere in the world…
Years of investigation would lead Santos and a research team to identify these as symptoms of Spoan syndrome.
They would go on to find 82 other cases worldwide.”
The BBC article then explains how intermarriage, genetic mutation and disease are related to each other:
“Serrinha’s geographical isolation and little inward migration mean that many of the population are related, making marriage between cousins far more likely and more socially acceptable.
Worldwide, marriages between relatives were estimated at around 10% in the early 2010s. More recent data shows the rate varies widely, from over 50% in countries like Pakistan, to 1-4% in Brazil and less than 1% in the US and Russia. Most children born to pairs of cousins are healthy, experts say.
But these marriages do face a higher risk of a harmful genetic mutation being passed down through the family.
“If a couple is unrelated, the chance of having a child with a rare genetic disorder or disability is about 2–3%. For cousins, the risk rises to 5–6% per pregnancy,” explains geneticist Luzivan Costa Reis from Brazil’s Federal University of Rio Grande do Sul.
A 2010 study led by Santos showed that more than 30% of couples in Serrinha were related, and a third of them had at least one child with a disability…
Santos’s team found that the mutation involves the loss of a small fragment of a chromosome, which causes a gene to overproduce a key protein in brain cells…”
In a mindblowing bit of research, the Brazilian scientists use gene sequencing to trace the underlying genetic mutation back into the mists of time:
“…the genetic mutation behind Spoan is far older than the legend of Old Maximiano: it likely arrived more than 500 years ago with early European settlers in the north-east of Brazil.
“Sequencing studies show strong European ancestry in patients, supporting records of Portuguese, Dutch, and Sephardic Jewish presence in the region,” says Santos.
The theory gained strength after two Spoan cases were found in Egypt, and further studies showed that the Egyptian cases also shared European ancestry, pointing to a common origin in the Iberian Peninsula.
“It likely came with related Sephardic Jews or Moors fleeing the Inquisition,” says Santos. She believes more cases may exist globally, especially in Portugal.”
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