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 TNAP275 revealed 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.