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Cystinosis

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Content Reviewers:

Yifan Xiao, MD, Viviana Popa, MD

With cystinosis, “cystin-” refers to cystine, an amino acid, and “-osis” implies disease.

So, cystinosis is a rare condition caused by mutations of the CTNS gene that leads to a cystine buildup in the body.

This can cause tissue damage, especially in the kidneys and eyes.

Cystine is an amino acid that comes from our diet.

When food travels through the stomach and intestines, the proteins within, are broken down into tiny fragments, called oligopeptides, or small strings of amino acids.

Turnover of muscle, bone and other parts of the body provide another source of protein that can be broken down into oligopeptides.

Many of these oligopeptides end up in specialized vesicles, called lysosomes, found inside all of our cells.

Here, they are further broken up into amino acids like cystine.

Now, cystine, like any other amino acid has to leave the lysosome, and it does this with the help of a specific protein.

Generally, genes tell our bodies how to make proteins.

So, the CTNS gene encodes for the protein cystinosin, a transporter that is found embedded in the lysosomal membrane.

It’s function is to export cystine out of the lysosome.

Now, in cystinosis, any one of over 100 mutations can affect the CTNS gene, leading to a defective cystine transporter.

Without a working transporter, cystine has no way of leaving the lysosome, so it accumulates, turning into cystine crystals in the process.

Crystals that slowly damage organs like the kidneys and eyes.

In general, humans have two copies of their genes, so both must be damaged for there to be so little cystine transport that cystinosis occurs.

That means that a person with cystinosis must receive a mutated CTNS gene from both the mother and father.

For each such mating, there is a 25% chance that both parents will pass down their own CTNS mutation to the offspring.

Now, depending on the CTNS gene mutation, three types of cystinosis can develop that differ in the age of onset and severity of symptoms.

They are nephropathic, or infantile, late onset, and ocular.

Summary
Cystinosis is a lysosomal storage disease characterized by the abnormal accumulation of the amino acid cystine. It is a genetic disorder that typically follows an autosomal recessive inheritance pattern. Cystinosis is the most common cause of Fanconi syndrome in the pediatric age group. Fanconi syndrome occurs when the function of cells in renal tubules are impaired, leading to abnormal amounts of carbohydrates and amino acids in the urine, excessive urination, and low blood levels of potassium and phosphates.
Sources
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