To the Editor:
Richner-Hanhart syndrome, also known as tyrosinemia type II or oculocutaneous tyrosinemia, is a rare autosomal-recessive, childhood-onset, metabolic hereditary disease. 1 A deficiency of tyrosine aminotransferase leads to an accumulation of tyrosine amino acid. It is characterized by the association of palmoplantar hyperkeratosis, bilateral keratitis, and neurological disorders.
An 18-month-old girl with recurrent warts of 6 months' duration was admitted to the dermatology department. She had been treated repeatedly with acyclovir for recurrent bilateral herpetic keratitis with major photophobia since 9 months of age with no response. Clinical presentation included punctate hyperkeratosis of the fingers and toes (Figure, A), severe photophobia with decreased visual acuity, and speech delay.
Her medical record showed a break of the growth curve with a weight of 9.25 kg (3rd percentile), a height of 80 cm (50th percentile), and a head circumference of 45 cm (50th percentile). Her parents were nonconsanguineous. The association of bilateral dendritic keratitis with punctate palmoplantar keratosis suggested a diagnosis of Richner-Hanhart syndrome. Diagnosis was confirmed by an elevated plasma level of tyrosine (1580 µmol/L; reference range, 40-80 µmol/L).
A low tyrosine and low phenylalanine diet (no animal proteins) was immediately introduced, with supplementation of amino acids, vitamins, and trace elements. After 8 days, the plasma level of tyrosinemia decreased by a factor of 4 (392 µmol/L). After 1 month, the cutaneous and ocular lesions completely resolved (Figure, B). Discrete psychomotor slowing still persisted for 1 year and then reached complete normalization. Genetic analysis showed a composite heterozygous mutation of the tyrosine aminotransferase gene, TAT, on chromosome 16. The mutation detected in the patient's mother was an A to V substitution at codon 147 (A147V). The second mutation was detected in the father; it was an 8 nucleotides duplication and then a substitution leading to a premature stop codon at codon 37 (R37X).
Richner-Hanhart syndrome is a rare autosomal-recessive disorder that is more common in Italy and in areas where inbreeding is prevalent 1,2; however, no data are available on disease prevalence. It is caused by a homozygous mutation in the TAT gene located on chromosome 16q22. 3 Tyrosine aminotransferase is an important enzyme involved in the tyrosine and phenylalanine metabolic degradation pathway located in the hepatic cytosol. Symptoms are due to the accumulation of tyrosine and its metabolite. Diagnosis is confirmed by an elevated plasma level of tyrosine (>500 µmol/L). This oculocutaneous syndrome is characterized by bilateral pseudodendritic keratitis, palmoplantar hyperkeratosis, and a variable degree of mental retardation.1 In contrast to tyrosinemia type II, types I and III do not affect the skin.
Intrafamilial and interfamilial phenotypic variability is reported. A large spectrum of mutations within the TAT gene have been reported. 4-7 These mutations lead to a reduction or an absence in the activity of hepatic tyrosine aminotransferase. The degradation pathway of tyrosine involving TAT occurs mainly in the liver. This process also is present in the mitochondria where the enzyme is called aspartate aminotransferase. 1,2 The mechanism by which Richner-Hanhart syndrome causes painful palmoplantar keratosis and keratitis remains unknown. It has been suggested that intracellular L-tyrosine crystals initiate an inflammation process resulting in the typical skin lesions and keratitis. 8 There is some evidence that patients with higher values of tyrosine in early life are more likely to develop neurological problems. 1 In addition, phenotype variability has been observed, even among individuals sharing the same pathogenic mutation. 4