Clinical and genetic characteristics of two patients with tyrosinemia type 1 in Slovenia - A novel fumarylacetoacetate hydrolase (FAH) intronic disease-causing variant

Abstract

Tyrosinemia type 1 (HT1) is an inborn error of tyrosine catabolism that leads to severe liver, kidney, and neurological dysfunction. Newborn screening (NBS) can enable a timely diagnosis and early initiation of treatment. We presented the follow up of the only two Slovenian patients diagnosed with HT1. Metabolic control was monitored by measuring tyrosine, phenylalanine and succinylacetone from dried blood spots (DBSs). Retrograde screening of HT1 was performed from DBSs taken at birth using tandem mass spectrometry. First patient was diagnosed at the age of 6 months in the asymptomatic phase due to an abnormal liver echogenicity, the other presented at 2.5 months with an acute liver failure and needed a liver transplantation. The first was a compound heterozygote for a novel FAH intronic variant c.607-21A>G and c.192G>T whereas the second was homozygous for c.192G>T. At the non-transplanted patient, 66% of tyrosine and 79% of phenylalanine measurements were in strict reference ranges of 200-400 μmol/L and >30 μmol/L, respectively, which resulted in a favorable cognitive outcome at 3.6 years. On retrograde screening, both patients had elevated SA levels; on the other hand, tyrosine was elevated only at one. We showed that non-coding regions should be analyzed when clinical and biochemical markers are characteristic of HT1. DBSs represent a convenient sample type for frequent amino acid monitoring. Retrograde diagnosis of HT1 was possible after more than three years of birth with SA as a primary marker, complemented by tyrosine.

Keywords: AFP, alpha-fetoprotein; ALP, alkaline phosphatase; ALT, alanine transaminase; AST, aspartate transaminase; DBS, dried blood spot; Dried blood spot; FAH, fumarylacetoacetate hydrolase; Fumarylacetoacetate hydrolase; GGT, gamma glutamyl transferase; HT1, tyrosinemia type 1; INR, international normalized ratio; Intronic variant; MS/MS, tandem mass spectrometry; NBS, newborn screening; NTBC, nitisinone; Nitisinone; PTT, partial thromboplastin time; RF, reference range; SA, succinylacetone; Succinylacetone; Tyrosinemia.

Fig. 1

Tyrosine and phenylalanine concentrations in dried blood spots and plasma during follow up in patient 1. Start of treatment is illustrated with a vertical line at 6 months of age. Target range 200–400 μmol/L for tyrosine, >30 μmol/L for phenylalanine and <1 μmol/L for succinylacetone are presented in gray. Blue dot – tandem mass spectrometry analysis of dried blood spots, red dot – plasma analysis by amino acid analyzer.

Fig. 2

Average total daily protein intake of natural protein, protein equivalent from protein substitutes free from phenylalanine and tyrosine and phenylalanine substitution derived from dietary intake analysis in patient 1. Note: Values on the horizontal axis are not proportionately dispersed.

Fig. 3

Comparison of tyrosine and succinylacetone concentrations between healthy newborns and patients with HT1. Values from healthy controls are presented with a box and whiskers plot. The box represents the 25th and 75th percentiles, the line in the box represents the median, and the whiskers are the 1st and 99th percentiles. Succinylacetone and tyrosine concentrations were determined from dried blood spots using tandem mass spectrometry. Patient samples were analyses retrograde. Cut-off values for a positive screening result are presented with dotted lines.