title: A homozygous intronic branch-point deletion in the ALPL gene causes infantile hypophosphatasia.
abstract: Hypophosphatasia (HPP) is a multi-systemic inborn disease with an extraordinary spectrum of severity, ranging from the absence of mineralization to high lethality and it involves different organs including bone, muscle, kidney, lung, gastrointestinal tract and the nervous system. The disease is characterized by low levels of serum alkaline phosphatase, caused by loss-of-function mutations within the ALPL gene that encodes the tissue-nonspecific alkaline phosphatase TNAP. Here we present the functional characterization of a gene mutation, detected in intron 7 of the ALPL gene of a boy with infantile HPP in whom routine sequencing of the coding region failed to detect any mutation. The homozygous c.793del-14_33 mutation results in the loss of the branch-point motif, relevant for correct ALPL pre-mRNA splicing. The main transcript skips exon 8 and codes for a C-terminally truncated TNAP protein of 275 amino acids, which was detected in peripheral blood mononuclear cells and serum from the patient. The functional characterization of recombinant TNAP275revealed no enzymatic activity nor any dominant-negative effect, relevant for the heterozygous parents. Nevertheless correct pre-mRNA splicing can take place without the branch-point sequence to a limited extend, as concluded from the ALPL cDNA, obtained from patient's PBMC, and from the low serum AP activity. These data reaffirm that in clear cut clinical cases, where conventional sequencing including the coding sequence and direct exon-intron-boundaries fails to detect mutations, deeper analyses of regulatory important motifs like branch-point sequences are required to establish a genetic diagnosis.
authors: Orthopaedic Center for Musculoskeletal Research, Orthopaedic Department, University of Würzburg, Würzburg, Germany. Electronic address: b-mentrup.klh@uni-wuerzburg.de.
authors: Birgit
authors: B
authors: Mentrup
authors: Children's Hospital, Vivantes Hospital im Friedrichshain, Berlin, Germany.
authors: Hermann
authors: H
authors: Girschick
authors: Orthopaedic Center for Musculoskeletal Research, Orthopaedic Department, University of Würzburg, Würzburg, Germany.
authors: Franz
authors: F
authors: Jakob
authors: Children's Hospital, Pediatric Rheumatology and Osteology, University of Würzburg, Würzburg, Germany.
authors: Christine
authors: C
authors: Hofmann
mesh_terms: D000328:Adult; D000469:Alkaline Phosphatase / Q000235:genetics*; D017398:Alternative Splicing / Q000235:genetics; D001483:Base Sequence; D002462:Cell Membrane / Q000378:metabolism; D002675:Child, Preschool; D004247:DNA / Q000235:genetics; D005786:Gene Expression Regulation; D006720:Homozygote; D006801:Humans; D007014:Hypophosphatasia / Q000201:enzymology* / Q000235:genetics*; D007223:Infant; D007231:Infant, Newborn; D007438:Introns / Q000235:genetics*; D008297:Male; D009711:Nucleotides / Q000235:genetics; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D017384:Sequence Deletion / Q000235:genetics*
chemical_list: D009711:Nucleotides; D012333:RNA, Messenger; D004247:DNA; C548267:ALPL protein, human; D000469:Alkaline Phosphatase
keywords: Alkaline phosphatase; Branch-point; Hypophosphatasia; Intron deletion; Splicing; Transcript variant