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CASE REPORTS
A mutation in the iron-responsive element of ALAS2 is a modifier of disease severity in a patient suffering from CLPX associated erythro- poietic protoporphyria
Porphyrias are a group of eight genetically distinct dis- orders, each resulting from a partial deficiency or gain-of- function of a specific enzyme in the heme biosynthetic pathway.1 Porphyrias are inherited as autosomal domi- nant, autosomal recessive or X-linked traits.2 Erythropoietic protoporphyria (EPP) is a constitutive hematological disorder characterized by protoporphyrin IX (PPIX) accumulation in erythrocytes and other tissues resulting in acute skin photosensitivity, mild microcytic anemia, and rarely, severe liver disease. The majority of the patients with EPP present the autosomal EPP form (OMIM #177000) due to a partial deficiency of fer- rochelatase (FECH), the last enzyme of the heme biosyn- thetic pathway.1,2 In most EPP patients, the clinical expression requires the coinheritance of a FECH muta- tion, that abolishes or markedly reduces FECH activity, in trans to an hypomorphic FECH allele (rs2272783, NM_000140.3; c.[315-48T>C]) carried by about 11% of Caucasians.3 In Europe and the USA, 4-10% of EPP patients have been reported to harbor a gain-of-function mutation in the 11th exon of the erythroid d-aminole- vulinic synthase gene (ALAS2).4 Rare cases of EPP have also been reported in few reference centers without any mutations in the FECH or ALAS2 genes (personal com- munication from JC Deybach). EPP due to gain-of-func- tion ALAS2 mutations are inherited as an X-linked trait and result in a distinct biochemical feature not only with overproduction and accumulation of free PPIX but also zinc protoporphyrin (ZnPP) (XLPP) (OMIM #300751). In these patients, the FECH enzyme is functional and uti- lizes all available iron for heme production. The excess PPIX is used to make ZnPP in a reaction catalyzed by FECH.3,5 Moreover, gain-of-function missense mutations altering the C-terminal part of ALAS2 exacerbate congen- ital erythropoietic porphyria, suggesting that ALAS2 is a gatekeeper of erythroid heme biosynthesis and may func- tion as a modifier gene.6
The 5’ untranslated region (UTR) of ALAS2 mRNA contains a cis-regulatory iron-responsive element (IRE) that confers iron-dependent posttranscriptional regula- tion by the iron regulatory proteins (IRP).7 IRE muta- tions are known to cause human diseases. IRE muta- tions in ferritin L mRNA cause hereditary hyperferritine- mia with cataract syndrome (HHCS) (OMIM #600886),8-9 and a single point mutation in the IRE of ferritin H is responsible for an autosomal dominant iron overload phenotype (OMIM #615517).10 These cases suggest a possible role for IRE mutations in the ALAS2 gene in modifying the severity of hematologic diseases.
Here, we describe an 18 year-old Caucasian female proband (III:2, Figure 1A) referred to the French Center of Porphyria because of early onset (9 months) acute photosensitivity characterized by painfully phototoxic reactions suggesting EPP. An incomplete genetic charac- terization of this case was previously reported to harbor a gain of function in the mitochondrial unfoldase gene, CLPX.11 Written informed consent was obtained for all participants. This study was approved through the local ethical committees in accordance with the World Medical Association Declaration of Helsinki ethical principles for medical research involving human sub- jects and its subsequent amendments (R162-16-7 and 145-15-4 French ethical agreement). In the proband, a high level of free erythrocyte PPIX and ZnPP confirmed the diagnosis of EPP (Table 1). At the age of diagnosis, the proband also presented with a microcytic iron defi- ciency anaemia (Table 1). FECH enzyme activity was normal (Table 1). No point mutation or large FECH gene deletion were identified, and chromosome 18, where FECH gene is located, was excluded by linkage and comparative genomic hybridization (CGH) array analy- ses. Moreover, the proband did not harbor the FECH low-expressed allele rs2272783, NM_000140.3; c.[315- 48T>C] (IVS3-48C). The father (II.4) and one uncle (II.2) of the proband presented with zinc- and free PP accu- mulation in erythrocytes that were associated with a mild photosensitivity, but without symptoms of EPP (Figure 1 and Table 1). Whole-exome sequencing (WES) analysis showed that proband (III.2), the father (II.4) and the uncle (II.2) carried a heterozygous single
Table 1. Clinical and biochemical data in affected subjects of the erythropoietic protoporphyria proband’s family. Pedigree II:2 II:4 III:2 II:5 I.3 II:6
Sex MMFFFF
Normal values
<20
<1.9
<28
>72
>3.5 11.5 – 16.0 12.0 - 26.0
Photosensitivity IREALAS2mutation
Age at the visit (years)
Total porphyrins in plasma (nmol/L) Erythroid PPIX (mmol/L RBC):
FreePPIX(%)
ZnPP(%)
FECH activity (nmol/mg prot. h) Hb (g/dL)
Serum iron (mmol/L)
Serum ferritin (mg/L)
Transferrin saturation (%)
Soluble Transferrin receptor (mg/L)
mild mild Severe no - - + +
no no + + 78 51 19 20 1.9 2.0
56 46
54 52
30.4 26.7
182043 936 780 22 140.9 89.5 2.1
33 38 71 65 20 17 22
67 62 29 35 80 83 78
4.4 3.9 15.5 15.9 25.0 20.0 196 171
31 32 1.24 1.41
3.6 3.9 10.9 12.1 4.0 11.0 5 11
5 15 3.95 1.75
4.3 4.2 4.2 12.2 13.3 14.0 21.0 14.0 20.0
12 137 85 15–250♂ 8–150♀
31 24 36 20-45 1.15 1.26 1.19 0.76-1.76
2030
The proband (III:2) was examined twice: first at the initial visit associated with iron deficiency and one year later after oral iron supplementation.PPIX: protoporphyrin IX; ZnPP: Zinc protoporphyrin; Hb: hemoglobin; FECH: ferrochelatase; RBC: red blood cells; m: male; f: female.
haematologica | 2021; 106(7)