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Molecular Expression and Characterization of Erythroid-Specific 5-Aminolevulinate Synthase Gain-of-Function Mutations Causing X-Linked Protoporphyria

Authors :
David F. Bishop
Vassili Tchaikovskii
Irina Nazarenko
Robert J. Desnick
Source :
Molecular Medicine, Vol 19, Iss 1, Pp 18-25 (2013)
Publication Year :
2013
Publisher :
BMC, 2013.

Abstract

Abstract X-linked protoporphyria (XLP) (MIM 300752) is a recently recognized erythropoietic porphyria due to gain-of-function mutations in the erythroid-specific aminolevulinate synthase gene (ALAS2). Previously, two exon 11 small deletions, c.1699_1670ΔAT (ΔAT) and c.1706_1709ΔAGTG (ΔAGTG), that prematurely truncated or elongated the ALAS2 polypeptide, were reported to increase enzymatic activity 20- to 40-fold, causing the erythroid accumulation of protoporphyrins, cutaneous photosensitivity and liver disease. The mutant ΔAT and ΔAGTG ALAS2 enzymes, two novel mutations, c.1734ΔG (ΔG) and c.1642C>T (p.Q548X), and an engineered deletion c.1670–1671TC>GA p.F557X were expressed, and their purified enzymes were characterized. Wild-type and ΔAGTG enzymes exhibited similar amounts of 54- and 52-kDa polypeptides on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), whereas the ΔAT and p.F557X had only 52-kDa polypeptides. Compared to the purified wild-type enzyme, ΔAT, ΔAGTG and Q548X enzymes had increased specific activities that were only 1.8-, 3.1- and 1.6-fold, respectively. Interestingly, binding studies demonstrated that the increased activity Q548X enzyme did not bind to succinyl-CoA synthetase. The elongated ΔG enzyme had wild-type specific activity, kinetics and thermostability; twice the wild-type purification yield (56 versus 25%); and was primarily a 54-kDa form, suggesting greater stability in vivo. On the basis of studies of mutant enzymes, the maximal gain-of function region spanned 57 amino acids between 533 and 580. Thus, these ALAS2 gain-of-function mutations increased the specific activity (ΔAT, ΔAGTG and p.Q548X) or stability (ΔG) of the enzyme, thereby leading to the increased erythroid protoporphyrin accumulation causing XLP.

Details

Language :
English
ISSN :
10761551 and 15283658
Volume :
19
Issue :
1
Database :
Directory of Open Access Journals
Journal :
Molecular Medicine
Publication Type :
Academic Journal
Accession number :
edsdoj.fa79fb16070242f1a71b134205799c13
Document Type :
article
Full Text :
https://doi.org/10.2119/molmed.2013.00003