SERPINH1 and Osteogenesis Imperfecta - Osteogenesis Imperfecta - page 147

Each model peptide included one HSP47-binding
site represented by the important arginine (Arg) at the
Yaa position of a Xaa-Yaa-Gly triplet, termed “Yaa 0 ”. 13
The following was observed: (1) There was no signifi-
cant change in the HSP47 conformation upon collagen
binding. (2) In the leading and trailing CMP strands, the
important arginine at the position termed “Yaa 0 ” in the
Pro-Arg-Gly triplet forms a salt bridge with a conserved
aspartate residue Asp385 of HSP47. Mutation of Asp385
to asparagine drastically weakened the interaction.
(3) Two chaperones bound to one triple helix. (4) Each of
the two HSP47 molecules simultaneously formed numer-
ous contacts to either the leading or trailing strand and
to the middle strand of the collagen triple helix. Proper
HSP47 contacts could be made only if collagen is in a
proper folded, triple-helical conformation. (5) In addi-
tion to the salt bridge between Asp385 and the arginine
at Yaa 0 , the same collagen strand hydrogen bonds to the
side-chain of Arg222 via its main-chain carbonyl oxygen
atoms of Pro5 and Gly6, the former located at the so-
called “Yaa −3 ” position (i.e., three amino acids before the
important arginine residue). (6) There are also a number
of hydrophobic interactions, especially involving Leu381
and Tyr383. (7) With reference to the release of HSP47
in the Golgi, these authors suggested that protonation
of His-238 probably would lead to rearrangements that
alter HSP47 binding affinity.13 13
Of direct relevance to HSP47 in OI, these investiga-
tors also mapped the molecular location of the mutation
(L78P) in the patient reported below by Christiansen
et al., and found that replacement by proline, “with its
restricted Φ angle of about −60° and the absent polar
hydrogen,” would lead to a severely altered HSP47
structure. Widmer et al. propose that binding of HSP47
prevents local unfolding (micro denaturation) of newly
formed triple-helical regions at body temperature and
misalignment or improper triple-helix formation during
refolding.

child born to a consanguineous Saudi Arabian family
who were otherwise normal. 26
Prenatal ultrasound at about 28 weeks' gestation iden-
tified short bowed femora. A repeat scan at 36 weeks
again demonstrated short femora, low echogenicity of
the calvarium, and a relatively small chest. At birth he
was noted to have a triangular face, relative macroceph-
aly, bitemporal narrowing, blue sclerae, micrognathia
and relatively short limbs with bowing at the thighs.
A skeletal survey at the age of 1 month was consis-
tent with OI. There were multiple bone deformities and
fractures that involved the upper and lower extremities
and ribs, and generalized osteopenia ( Figure 16.2 ). He
later developed small opalescent teeth consistent with
dentinogenesis imperfecta. At the age of 1 year he devel-
oped bilateral renal stones with pelvi-ureteric junction
obstruction that ultimately required left nephrectomy. He
subsequently developed respiratory insufficiency requir-
ing repeated hospitalizations. At 3 years and 6 months he
developed sudden respiratory distress requiring resusci-
tation but he died soon afterwards.
Analyses of the COL1A1, COL1A2, CRTAP and
LEPRE1 genes were normal. Collagens synthesized by
cultured dermal fibroblasts had normal mobility on gel
electrophoresis and normal 3-hydroxylation of Pro-986.
Sequencing of SERPINH1 identified a homozygous mis-
sense mutation in SERPINH1 (c.233T>C, p.Leu78Pro).
Recessive inheritance was confirmed by finding that
the parents of the proband were heterozygous for the
mutation.
To summarize the effects of the SERPINH1 mutation
on collagen synthesis in this patient: (1) In vitro stud-
ies using dermal fibroblasts showed that the amount
of type I procollagen secreted and accumulated in the
medium after a 16-hour period was similar to that in
controls. (2) There was no increase in intracellular stor-
age. (3) Post-translational modification of type I pro-
collagen chains was normal on gel electrophoresis
( Figure 16.3 ). The rate of secretion of type I procolla-
gen trimers into the medium measured during 2-hour
pulse-chase studies was delayed by a short time when
compared to control ( Figure 16.3 ). Western blot and
immunocytochemical analysis showed that HSP47 pro-
tein was significantly decreased in patient cells. In con-
trol cells, type I procollagen was detected in both ER
and Golgi by immunofluorescence. In patient cells, type
I procollagen was less abundant in the ER and accumu-
lated in the Golgi.
In relation to these observations, in 1998, Kojima et al.
studied the intracellular transport of abnormal procolla-
gen molecules and the expression of HSP47 in fibroblasts
from a patient with lethal OI. Procollagen and HSP47
were also quantified by immunoprecipitation of normal
and OI fibroblasts radiolabeled with 35S-methionine.
By confocal microscopy, procollagen molecules were

SE RPINH1 MUTATIONS AND OI

In 2009, Drogemuller et al., noting the occurrence of
OI in several dog species, studied six dachshund fami-
lies with a congenital OI phenotype consistent with
autosomal recessive inheritance. 25 Based on extensive
SNP analysis, and comparing related segments of the
dog genome to human, these investigators defined
SERPINH1 on the dog chromosome 21 as the candi-
date gene related to the OI phenotype. Affected ani-
mals were homozygous for the mutation, carriers were
heterozygous.
To date, only one patient having an OI phenotype
and an SERPINH1 mutation has been reported. The
proband reported by Christiansen et al. was the first