What is De Sanctis-Cacchione Syndrome?

De Sanctis-Cacchione Syndrome (DSCS) is an extremely rare and severe form of xeroderma pigmentosum (XP). XP is a group of rare inherited conditions where the body is unable to repair damaged DNA. DNA is crucial for the creation of proteins, which are essential for everyday function. Think of DNA as the instruction manual or blueprint for building and running your body. Every functioning cell has a copy of this manual. In DSCS, there is a problem in a fundamental part of the manual: the section responsible for repairing damaged DNA. We can think of DNA repair as the maintenance system. In DSCS, this maintenance system is faulty, causing damaged cells to accumulate, which leads to serious health consequences. DSCS is tied to mutations in genes called XPA and XPD, which are typically involved in repairing DNA damage. These genes produce proteins that are crucial for a process called nucleotide excision repair (NER), which acts as the DNA repair system that fixes bulk damage. When XPA or XPD are mutated, the repair system does not work properly. Without functioning XPA and XPD, damaged cells accumulate in neurons (cells that communicate and send messages throughout the nervous system), causing neurological deficits and impacting daily function.

This article examines how faults in two DNA repair genes, XPA and XPD, not only make the skin highly sensitive to sunlight but also lead to damage in the brain, helping to explain the neurological difficulties seen in some patients.

What are XPA and XPD Genes?

The XPA protein acts like the coordinator or team leader during DNA repair. It helps identify and locate the damaged DNA and brings together the appropriate tools to conduct the repair.¹ Essentially, XPA ensures that damaged sections are correctly identified and removed. XPD functions as the ‘unzipping tool’ in the DNA repair team. When DNA is damaged, the repair crew needs to open up the double helix around the broken spot to identify exactly what went wrong. XPD’s role is to unwind or ‘unzip’ the DNA to create a small open space where the damage can be identified by XPA and cut out.² Without XPD performing this unzipping, the rest of the repair process does not run as it should. When one or both of these genes are mutated, the repair system does not work properly, which leads to a build-up of damaged cells and causes many of the problems seen in DSCS.

De Sanctis-Cacchione Syndrome: Beyond Skin Symptoms to Neurological Decline

As previously mentioned, xeroderma pigmentosum (XP) is characterised by photosensitivity, pigmentary changes, premature skin ageing, and malignant tumour development due to heightened cellular sensitivity to ultraviolet rays, resulting from a defect in DNA repair. DSCS is considered one of the rarest and most severe forms of XP, characterised by neurological deficits alongside significant skin problems.³^,⁴ It is considered the most severe form of XP because it combines all of the hallmark features with additional widespread complications that other XP subtypes do not usually present, as their features are mainly confined to the skin and eyes, though still serious.

In contrast, DSCS goes beyond these features, with individuals developing progressive neurological decline. The most consistently reported features include learning and developmental difficulties, a smaller-than-average head size (microcephaly), problems with balance and coordination that worsen over time (progressive ataxia), and hearing loss due to inner ear damage (sensorineural hearing loss).³^,⁵ Other reported issues include involuntary movements (chorea), nerve damage in the arms and legs (peripheral neuropathy), and weakened or absent reflexes.⁵ Because it affects multiple organ systems in addition to the usual skin cancers of XP, DSCS is regarded as the most severe and debilitating end of the XP spectrum.

How XPA and XPD Mutations Lead to Neurological Degeneration

The DNA repair system is crucial for neurons (brain cells) because they are constantly active and communicating, which requires a great deal of energy. This constant activity creates toxic by-products (oxidative stress). We can think of brain cells like engines that are constantly running: they burn a lot of fuel, giving off exhaust emissions. The exhaust can be damaging if not properly managed.

These by-products can harm DNA inside the cells, which in normal functioning can be repaired by the NER pathway with the help of XPA and XPD. This clears the damage and keeps the DNA healthy. However, when XPA or XPD are not working properly, the DNA damage is not repaired and instead accumulates over time, causing the cells to malfunction. Over time, this accumulation disrupts how neurons produce proteins and communicate with each other. This gradual decline leads to progressive neurological problems such as developmental delay, loss of coordination, hearing loss, and cognitive impairment.

Unlike other cells in the body, neurons do not divide or renew themselves with ease. Neurons are like hard drives that cannot readily be replaced; if their DNA becomes damaged and the repair system proteins (XPA and XPD) are absent, the damage accumulates over time and cannot be managed. If this occurs consistently, the cells will progressively deteriorate. Research has shown that different gene mutations in XP affect the brain in different ways.

Mutations in the XPA gene have been shown to cause some of the most severe changes. They can lead to widespread loss of neurons throughout the central nervous system, along with damage to the long nerve fibres that carry messages from the brain to the rest of the body (axonal neuropathy).⁶^,⁷ In simple terms, this means that both the brain and the body gradually lose function, resulting in neurological decline.

Mutations in the XPD gene also produce profound neurological problems, but in a slightly different way. They can cause significant brain shrinkage (atrophy), and the supportive mesh of the brain tissue develops empty spaces. There is also loss of Purkinje cells, which are specialised nerve cells in the cerebellum that are critical for movement coordination. In addition, patients with XPD mutations often show a thickened skull bone. Interestingly, unlike XPA mutations, XPD mutations do not usually cause damage to the nerves outside the brain and spinal cord.⁷

More recent studies have implicated XPC mutations in DSCS. These mutations usually spare the nervous system altogether, meaning the brain and nerves remain relatively unaffected compared to those seen with XPA and XPD mutations. However, a rare case demonstrated that even with XPC mutations, severe neurological symptoms can sometimes appear. In this reported patient, symptoms included intellectual disability, problems with movement and balance, and a smaller-than-normal head size.⁸ The role of XPC mutations in neurological impairments therefore remains uncertain.

Current Understanding and Challenges

There is currently no cure for the neurological aspects of DSCS. Management focuses on appropriate protection from UV light and regular check-ups for early detection of skin cancers. Although neurological care is mostly supportive, some medications may be administered to help with muscle spasticity. A multidisciplinary approach is often implemented to manage the varying symptoms, and new treatments are being investigated to alleviate skin symptoms. Research is actively exploring gene-based strategies for related DNA repair disorders, including other forms of XP. These experimental approaches aim to correct or replace faulty DNA repair genes, or to deliver healthy copies of the missing proteins into cells. While these treatments are still in the early stages of research, they represent an important step towards tackling the root cause of the disease. Progress is being made, and further research is needed to understand XP-related mutations and develop future treatments.

Summary

De Sanctis-Cacchione Syndrome (DSCS) is the rarest and most severe form of xeroderma pigmentosum (XP), caused by mutations in DNA repair genes, most notably XPA and XPD. While all forms of XP are associated with extreme sun sensitivity, premature skin ageing, and an elevated risk of skin cancer, DSCS is uniquely distinguished by progressive neurological decline, including developmental delay, ataxia, microcephaly, and hearing loss. XPA and XPD mutations disrupt the NER pathway in neurons, leading to the irreversible accumulation of DNA damage and gradual loss of neurological function. There is currently no cure; treatment focuses on UV protection, early cancer surveillance, and supportive neurological care. Ongoing research into gene-based therapies offers hope for more targeted interventions in the future.

• Skin symptoms: Extreme sensitivity to sunlight, freckles, pigment changes, premature skin ageing, and early skin cancers make dermatological care essential
• Neurological distinction: Neurological problems are what make DSCS distinct from other XP subtypes
• XPA mutations: Lead to widespread neuronal loss and peripheral neuropathy
• XPD mutations: Cause brain shrinkage, Purkinje cell loss, and a thickened skull, but have little effect on peripheral nerves
• Management: There is no cure; management focuses on UV protection, early detection of skin cancers, and supportive neurological care, with gene-based therapies under active investigation