by Cristina Brown
The scientific enigma still unsolved. The two 12-year-old Spanish sisters share the same genetic mutation, but only one has a rare disease that turns her muscles into bones, forcing her to have her legs amputated
Cayetana and Celia are two 12-year-old Spanish girls, identical twins, they share the same DNA and also share the same genetic mutation. Healthy Celia leads the normal twelve-year-old life, while Cayetana’s muscles are rapidly turning into bones: the girl suffers from an ultra-rare disease that spared her sister instead.
The incredible story was told by The country who also interviewed the pediatrician Federico Martinon Torres, head of the medical team of the University Hospital of Santiago de Compostela who is trying to understand why such a terrible disease hit a girl and did not manifest itself in her twin sister. A year and a half ago, Cayetana requested and obtained that both her legs, already petrified, be amputated, to make her difficult life easier for her. The doctors are convinced that Celia holds the secret to curing her sister’s condition and are doing everything they can to find a clue and help Cayetana.
Progressive bone heteroplasia
The two girls, due to a genetic mutation, have theprogressive bone heteroplasia, an ultra rare disease which, according to the calculations of the American doctor Fred Kaplan , discoverer of the disorder thirty years ago, affects only a hundred people in the world. The disease resembles another unusual condition, fibrodysplasia ossificans progressive, or stone person syndrome. the most serious case I know of, he told El Pais. For Cayetana the disease progressed explosively. A small callus under the foot exposed on Friday could turn into a bone plaque by Sunday. There is no curative or preventative treatment, other than having surgery all the time to remove new bones. The mutation that affected the sisters involves the loss of only four letters of the three billion that make up the DNA: G, A, C and T, the initials of guanine (C₅H₅N₅O), adenine (C₅H₅N₅), cytosine (C₄H₅N₃O) and thymine ( C₅H₆N₂O₂). Those four missing letters cause the gene to be inactivated GNAS, in charge of preventing bone formation in the wrong place. But in Celia the terrible mutation did not express itself.
A life with pain
The bone plates sometimes stick out like needles, explains the girls’ mother. “Caye told me she doesn’t know what it’s like to have a day without pain,” her mother said Lol Surez. Cayetana, intelligent and cheerful, accepted the amputation of her legs with some humor. The other day his mother said at home: It smells like feet in here! And the girl replied with a laugh: They won’t be mine, mom!.
The genetic hypotheses
There are only hypotheses about why the terrible disease affected only one of the two sisters. Geneticist Antonio Salas, co-director of the research group that treats Cayetana, suspects that the mutation appeared after fertilization of the egg, before the embryo split in two to give rise to two identical sisters. In fact, the parents do not have the mutation. Epigenetic changes would have come into play (caused by environmental factors, chemical or physical agents) which can modify the expression of genes but not the DNA sequence. These changes can cause a gene to turn on or off, a complex phenomenon called imprinting. Researchers believe that the two sisters share the same mutation but Cayetana also has an imprint that causes the GNAS gene to be turned on or off in parts of her body. Scientists call this phenomenon mosaicism: the girl would have two types of cells, each with a different epigenetic composition. Instead, according to Fred Kaplan she has discovered the disease in 1994, the twins share the same mutation, but the one affected by the disease probably suffered a second mutation in the GNAS gene or in another gene that interacts with it, very early in the formation of the embryo. Which of the two hypotheses is correct is not yet clear.
July 18, 2023 (change July 18, 2023 | 08:13)
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