Tyrosinemia type III is an extremely rare autosomal recessive metabolic disorder resulting from biallelic pathogenic mutations in the HPD gene. To date, only a limited number of cases have been reported worldwide, and the full spectrum of clinical manifestations remains incompletely understood. While neurodevelopmental abnormalities are the most commonly described features, ocular involvement has rarely been documented. Here, we present a 9-month-old girl who exhibited strikingly atypical ocular symptoms characterized by persistent severe photophobia and allergic conjunctivitis at initial presentation. This combination of findings has been reported only sporadically in the literature. Biochemical and genetic investigations confirmed the diagnosis by identifying two novel heterozygous variants in the HPD gene. Implementation of a phenylalanine- and tyrosine-restricted diet led to a marked reduction in plasma tyrosine concentrations and improvement in clinical symptoms. This case underscores the diagnostic importance of considering inherited metabolic disorders in infants presenting with severe photosensitivity and conjunctival irritation, even in the absence of other systemic features. Furthermore, it highlights the need for long-term follow-up to monitor potential neurological and ocular complications associated with persistently elevated tyrosine levels.

Phenylketonuria (PKU) and tyrosinemia type 3 (HT3) are both rare autosomal recessive disorders of phenylalanine-tyrosine metabolism. PKU is caused by a deficiency in phenylalanine hydroxylase (PAH), leading to elevated phenylalanine (Phe) and reduced tyrosine (Tyr) levels. HT3, the rarest form of tyrosinemia, is due to a deficiency in 4-hydroxyphenylpyruvate dioxygenase (HPD). We report a 5-year-old girl diagnosed with both PKU and HT3. She presented with elevated Phe levels in neonatal screening, and subsequent biochemical tests revealed both hyperphenylalaninemia and elevated Tyr levels. Genetic analysis confirmed the diagnoses, identifying homozygous mutations in both the PAH and HPD genes. Dietary management to maintain optimal Phe and Tyr levels proved to be challenging due to the presence of these two coexisting pathologies especially during infections and due to dietary non-compliance, necessitating frequent adjustments in the treatment strategy. This case highlights the importance of considering multiple metabolic disorders in patients with unexplained clinical and biochemical findings. Early diagnosis and stringent dietary management are crucial for preventing neurological damage and ensuring favorable outcomes in patients with concurrent metabolic disorders.

The accumulation of amyloid β peptide (Aβ) in the brain of Alzheimer's disease (AD) patients begins many years before clinical onset. Such process has been proposed to be pathogenic through the toxicity of Aβ soluble oligomers leading to synaptic dysfunction, phospho-tau aggregation and neuronal loss. Yet, a massive accumulation of Aβ can be found in approximately 30% of aged individuals with preserved cognitive function. Therefore, within the frame of the "amyloid hypothesis", compensatory mechanisms and/or additional neurotoxic or protective factors need to be considered and investigated. Here we describe a modifier genetic screen in Drosophila designed to identify genes that modulate toxicity of Aβ42 in the CNS. The expression of Aβ42 led to its accumulation in the brain and a moderate impairment of negative geotaxis at 18 days post-eclosion (d.p.e) as compared with genetic or parental controls. These flies were mated with a collection of lines carrying chromosomal deletions and negative geotaxis was assessed at 5 and 18 d.p.e. Our screen is the first to take into account all of the following features, relevant to sporadic AD: (1) pan-neuronal expression of wild-type Aβ42; (2) a quantifiable complex behavior; (3) Aβ neurotoxicity associated with progressive accumulation of the peptide; and (4) improvement or worsening of climbing ability only evident in aged animals. One hundred and ninety-nine deficiency (Df) lines accounting for ~6300 genes were analyzed. Six lines, including the deletion of 52 Drosophila genes with human orthologs, significantly modified Aβ42 neurotoxicity in 18-day-old flies. So far, we have validated CG11796 and identified CG17249 as a strong candidate (whose human orthologs are HPD and PRCC, respectively) by using RNAi or mutant hemizygous lines. PRCC encodes proline-rich protein PRCC (ppPRCC) of unknown function associated with papillary renal cell carcinoma. HPD encodes 4-hydroxyphenylpyruvate dioxygenase (HPPD), a key enzyme in tyrosine degradation whose Df causes autosomal recessive Tyrosinemia type 3, characterized by mental retardation. Interestingly, lines with a partial Df of HPD ortholog showed increased intraneuronal accumulation of Aβ42 that coincided with geotaxis impairment. These previously undetected modifiers of Aβ42 neurotoxicity in Drosophila warrant further study to validate their possible role and significance in the pathogenesis of sporadic AD.

Tyrosinemia type 3 (HT3) is a rare inborn error of tyrosine metabolism caused by mutations in the HPD gene encoding 4-hydroxyphenyl-pyruvate dioxygenase, which is transmitted in an autosomal recessive trait. The disorder is characterized by tyrosine accumulation in body fluids and massive excretion of tyrosine derivatives into urine (www.orpha.net). Since it is the least frequent form of tyrosinemia, only few cases with the variable but rather mild clinical features have been described so far. We report an 11 year old girl presenting with no clinical symptoms and with normal mental development who has been diagnosed with HT3 through metabolic screening on the basis of elevated serum level of tyrosine ranging from 425 to 535 μmol/L (normal values: 29-86 μmol/L), and elevated urinary excretion of p-hydroxyphenyl derivatives confirmed genetically with the homozygous c.479A > G (p.Tyr160Cys) missense change in the HPD gene. The girl has been only presenting with recurrent proteinuria of unknown etiology. A phenylalanine- and tyrosine-restricted diet has never been administered. Presented case may suggest that high tyrosine concentration itself does not participate directly in neuronal damage described in patients with tyrosinemia type 3.