Feline Congenital Erythropoietic Porphyria: Two Homozygous UROS Missense Mutations Cause the Enzyme Deficiency and Porphyrin Accumulation

Abstract

The first feline model of human congenital erythropoietic porphyria (CEP) due to deficient uroporphyrinogen III synthase (UROsynthase) activity was identified by its characteristic clinical phenotype, and confirmed by biochemical and molecular genetic studies. The proband, an adult domestic shorthair cat, had dark-red urine and brownish discolored teeth with red fluorescence under ultraviolet light. Biochemical studies demonstrated markedly increased uroporphyrinogen I in urine and plasma (2,650-and 10,700-fold greater than wild type, respectively), whereas urinary 5-aminolevulinic acid and porphobilinogen were lower than normal. Erythrocytic URO-synthase activity was <1% of mean wild-type activity, confirming the diagnosis and distinguishing it from feline phenocopies having acute intermittent porphyria. Sequencing of the affected cat's UROS gene revealed two missense mutations, c.140C>T (p.S47F) in exon 3 and c.331G>A (p.G111S) in exon 6, both of which were homozygous, presumably owing to parental consanguinity. Neither was present in 100 normal cat alleles. Prokaryotic expression and thermostability studies of the purified monomeric wild-type, p.S47F, p.G111S, and p.S47F/G111S enzymes showed that the p.S47F enzyme had 100% of wild-type specific activity but ~50% decreased thermostability, whereas the p.G111S and p.S47F/G111S enzymes had about 60% and 20% of wild-type specific activity, respectively, and both were markedly thermolabile. Molecular modeling results indicated that the less active/less stable p.G111S enzyme was further functionally impaired by a structural interaction induced by the presence of the S47F substitution. Thus, the synergistic interaction of two rare amino acid substitutions in the URO-synthase polypeptide caused the feline model of human CEP.

Pathology in Practice

“…9 It was initially believed that porphyria was inherited as an autosomal dominant trait, 4,26 but porphyria has since been shown to be inherited as an autosomal recessive trait. 6 Clinically, porphyria in cats correlates most closely with congenital erythropoietic porphyria in humans, with the exception of photosensitivity, which has not been reported for cats. 2 It is now known that cats with erythrodontia may have either acute intermittent porphyria or congenital erythropoietic porphyria.…”

“…5 However, the accumulation of hemosiderin in hemosiderophages is consistent with the disease, which results in rupture of erythrocytes. 6 Porphyrias are diseases caused by marked deficiencies of enzymes of the heme biosynthesis pathway. 7 Heme is an essential component of hemoglobin, myoglobin, and the cytochromes of the mitochondrial respiratory chain.…”

“…9 The congenital form of the disease is caused by an inherited defect of uroporphyrinogen III cosynthase. 6 Congenital and acquired forms of porphyria in cattle, [10][11][12] swine, 13,14 sheep, 15 dogs, 5,9 cats, 4,6,16-18 fox squirrels, 19 and mice 20 have been described. Although most forms of porphyria are inherited, acquired forms (ie, experimentally induced with drugs or chemicals) of porphyria have been established in dogs, 9 calves, 21 Figure 2-Photographs of a femur from a healthy 2-month-old kitten that died following blunt force trauma (control specimen; A) and a femur from the kitten in Figure 1 (B and C).…”

“…2 It is now known that cats with erythrodontia may have either acute intermittent porphyria or congenital erythropoietic porphyria. 6 Deposition of porphyrins in teeth and bones is believed to be a result of their affinity for mineral components 4 during active mineralization. 16,27 This suggests that the case described in the present report, with the presence of stained teeth and bones, was a form of inherited porphyria.…”

“…However, on the basis of the veterinary medical literature regarding porphyria in cats, it may be inferred that although more severe forms may be detectable as systemic disease within the first 1 to 2 years after birth, 4 other variants of porphyria in cats may be subclinical, permitting good quality of life for > 10 years. 6,16 …”