A genetic glitch can mimic a severe vitamin deficiency

Science · Published 28 October 2026 Swipe more like this →

In short: Imagine developing a skin rash and balance issues that look like a severe nutritional deficiency, even if you are eating perfectly. This is the reality of Hartnup disease, a rare genetic disorder where the body's transport system fails to move essential amino acids into the bloodstream.

First described in 1956 by Baron et al., Hartnup disorder is an autosomal recessive condition. It was named after the English family, specifically an individual named Eddie Hartnup, who presented with a variety of symptoms including a photosensitive skin rash, cerebellar ataxia (loss of muscle coordination), and psychosis. The condition occurs at a frequency of approximately 1 in 30,000 in European populations.

The underlying mechanism involves a mutation in the SLC6A19 gene, which encodes the B0AT1 protein. This protein acts as a neutral amino acid transporter located in the epithelial cells of the renal proximal tubules and the intestinal mucosa. Because the transporter is impaired, the body cannot efficiently move neutral amino acids from the gut and kidneys into the rest of the body. This leads to a constant renal aminoaciduria, where these amino acids are lost in the urine instead of being absorbed.

While the 'classical' symptoms of the 1956 case are striking, modern clinical presentations are often much milder. Many individuals today are asymptomatic or only experience minor issues like diarrhea or a skin rash during infancy. However, in some cases—particularly those reported in China and Japan—patients have exhibited severe symptoms, including numbness, coldness, and pain in the limbs, alongside significant mobility issues. The disorder is also linked to complex physiological functions, including blood-pressure control and glomerular structure, as the SLC6A19 transporter requires specific proteins like collectrin and angiotensin-converting enzyme 2 for proper expression.

Biochemically, the disorder is significant because it affects the availability of amino acids like tryptophan. This is crucial because tryptophan is a precursor for niacin (vitamin B3), as well as serotonin and melatonin. When transport is blocked, the body may struggle to produce these vital compounds, leading to the 'pellagra-like' dermatosis that gives the disease its name.

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