Trichothiodystrophy
Trichothiodystrophy (TTD) is a group of rare autosomal recessive disorders. Its defining feature is a marked reduction in sulfur and sulfur-containing amino acids (especially cysteine) in the hair shaft cortex, to less than 50% of normal levels, which leaves the hair inherently brittle and severely damaged.
At the molecular level, TTD stems from defects in the cell's transcription and DNA repair machinery:
- Mutations in the TFIIH transcription complex and NER: More than half of TTD cases result from mutations in genes encoding subunits of the transcription factor complex TFIIH, which also functions in nucleotide excision repair (NER). These include ERCC2/XPD, ERCC3/XPB and GTF2H5 (TTDA).
- Disrupted transcription of ultra-high sulfur proteins (UHSP): TFIIH is central to regulating transcription of the structural genes for ultra-high sulfur proteins in the matrix of the hair cortex. When TFIIH is impaired, production of the keratin matrix stalls.
- Loss of disulfide bonds and structural collapse: Without UHSP, many of the disulfide bridges (R-S-S-R) that cross-link keratin fibers are lost. The hair shaft becomes porous and hollow and cannot withstand mechanical stretching, so it snaps with a clean transverse fracture (trichoschisis) or breaks under everyday mechanical stress.
At Haute-Derma, we see TTD not simply as a cosmetic hair problem but as a multisystem syndrome. Recognizing it early with trichoscopy allows us to refer the child promptly to pediatrics, neurology and genetics, and to start strong sun-protection measures straight away.
Clinical Classification and the PIBIDS Acronym
Because TFIIH affects transcription throughout the body, TTD varies widely, from mild forms limited to the hair to severe, life-threatening ones. The clinical features are summarized by an acronym:
- P (Photosensitivity): Marked sensitivity to sunlight (about 50% of cases, due to mutations affecting NER).
- I (Ichthyosis): Congenital scaly skin (babies are often born encased in a shiny membrane as a collodion baby).
- B (Brittle hair): Fragile hair that breaks easily (a required diagnostic criterion for every form of TTD).
- I (Intellectual disability): Varying degrees of intellectual, cognitive and psychomotor delay.
- D (Decreased fertility / Dysmorphism): Reduced fertility and hypogonadism, with facial dysmorphism (bird-like face, low-set ears, small chin).
- S (Short stature): Short stature with marked growth delay in height and weight.
Main clinical forms:
- PIBIDS: The complete and most severe form (with photosensitivity).
- IBIDS / BIDS: Forms without sun sensitivity.
- SIBIDS / ONIDS: Involvement fixed to the hair, nails (onychodystrophy), eyes and skeleton.
Clinical Features, Microscopy and Trichoscopy
Children with TTD are usually brought in during the first months of life because their hair will not grow, looks dry and frizzy, and the skin and nails look abnormal.
- Clinical Signs:
- Hair: Short, uneven and sparse hair that is very brittle and breaks close to the scalp with washing or brushing. Eyebrows, eyelashes and body hair are affected in the same way.
- Nails: Fingernails and toenails split, flatten or develop longitudinal ridges (onychodystrophy).
- Skin: Dry, rough skin all over with fish-like scaling, which burns and reddens easily in the sun.
- Microscopy and Trichoscopy Findings:
- Trichoschisis: Numerous clean, flat transverse fractures running at 90 degrees to the long axis of the hair shaft.
- "Tiger-tail" pattern: The gold-standard diagnostic sign of TTD. Under polarized light microscopy, the hair shows regular alternating light and dark bands resembling a tiger's tail. The dark bands correspond to porous, severely sulfur-deficient stretches of cortex that have lost their birefringence.
Multidisciplinary Diagnosis and Management at Haute-Derma
Because TTD is a complex recessive genetic disorder, care requires close teamwork between dermatology, pediatrics, neurology and genetics:
Diagnosis and Genetic Testing
- Polarized light microscopy: The "tiger-tail" banding is looked for at the first visit in any child with brittle, breaking hair.
- Next-generation sequencing (NGS): Screening for mutations in ERCC2, ERCC3 and GTF2H5 to confirm the subtype and guide genetic counseling for the family.
Protecting the Hair Shaft From Mechanical Damage
- Gentle care: Use a pH-balanced shampoo (pH 5.5) and generous amounts of conditioner containing high-molecular-weight silicone (amodimethicone) to coat the hair and reduce friction.
- Silk pillowcases: These reduce friction during sleep. Avoid heat styling, chemical treatments and any brushing that pulls or stretches the hair.
Preventing Serious Sun Damage (Photosensitive Forms / PIBIDS)
- Comprehensive broad-spectrum protection: Mineral sunscreen (zinc oxide, titanium dioxide) with SPF 50+ and PA++++ is essential, together with UV-protective clothing and a wide-brimmed hat whenever the child goes outdoors.
- Skin surveillance: Although skin cancer is less common in TTD than in xeroderma pigmentosum (XP), regular checks for precancerous lesions are still essential.
Supporting Overall Development
- Early intervention: Monitoring of psychomotor milestones, with early rehabilitation and speech therapy.
- Genetic counseling: Carrier testing for the parents and advice on prenatal diagnosis in future pregnancies.
