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substrates and so in addition to type I procollagen, they
are involved in processing type II and III procollagens
and in the activation of the lyxyl oxidase zymogen (LOX)
by removing its N-terminal domain. LOX is essential to
generate collagen crosslinking, which is necessary for the
general tensile strength of the connective tissue. Finally,
BMP1/tolloid proteins are involved in processing secreted
factors including the dorsalizing factor Chordin and
BMP2/4.
31
The
BMP1
gene (MIM 112264) maps to chromosome
8p21.3 (Chr 8 22022653…22069839) and comprises 22
exons. The major
BMP1/TLD
transcription start site was
found only 706 bp downstream of the polyadenylation
site of the
SFTP2
surfactant gene.
reduced. Similarly, overexpression of an F249L mutant
form of mTLD in murine NIH3T3 cells or normal human
primary cells showed no effect on procollagen α1(I) pro-
cessing, unlike the overexpression of wild-type mTLD.
Subsequently, using a combination of whole-exome
sequencing and filtering for homozygous stretches of
identified variants, another mutation was reported
in two affected sibs of a consanguineous family from
Turkey with increased bone mineral density and mul-
tiple recurrent fractures.
28
The authors identified homo-
zygosity for a missense mutation (c.34G>C transversion
in exon 1) in
BMP1
resulting in a Gly12-to-Arg (G12R)
substitution within the signal peptide: this signal peptide
is essential for the protein's localization to the endoplas-
mic reticulum, correct post-translational glycosylation,
and secretion. It was also confirmed in
in vitro
assays that
the mutation leads to severely reduced post-translational
N-glycosylation of the protein and impairs protein secre-
tion. They also showed that the mutation leads to both
reduced secretion and subsequent reduced processing of
the substrates chordin and collagen I.
28
PATIENTS REPORTED WITH
MUTATIONS IN
BMP1
In early 2012, we described two Egyptian sibs born
from a consanguineous family with a severe form of auto-
somal recessive OI (
Figures 19.3-19.5
). We used genome-
wide homozygosity mapping and direct sequencing of
candidate genes to find the cause of the disease in these
individuals.
29
We identified a region of homozygosity on
8p containing the
BMP1
gene and subsequently identified
homozygosity for a missense mutation (F249L) (
Figure
19.6
). The same mutation was found in homozygous
state in the proband's affected brother and in heterozy-
gous state in their parents and unaffected sibs. The muta-
tion was not found in 100 chromosomes from ethnically
matched controls or in 884 chromosomes from European
controls. Demonstration of the pathogenicity of the F249L
mutation was achieved following the analysis of procol-
lagen (I) intermediate variants present in the superna-
tant of primary fibroblasts derived from skin biopsies of
our patients. Compared to control cells we detected an
increased amount of pCα1(I) which is the variant retain-
ing the C-propeptide in the supernatant of patients'
cells, while the levels of free C-propeptide were severely
I
II
FIGURE 19.4
Clinical phenotype of two siblings with AR-OI due
to
BMP1
mutations. Note severe bone deformities in both affected
sibs, severe kyphoscoliosis and relatively large thorax, small abdomen
and large umbilical hernia
(modified from reference
29
).
FIGURE 19.3
Family pedigree showing parental consanguin-
ity. The proband is indicated with an arrow and affected patients are
shadowed in black.
