Osteogenesis imperfecta (OI) is a clinically and genetically heterogeneous disorder characterized primarily by fragile bones that result in fracture and bone deformity. The clinical severity, presence of other phenotypic features, and variation in age of onset and type of OI are determined by the gene in which a mutation occurs and the nature and location of the mutation in the gene. About 95% of the pathogenic variants found in individuals with OI are found in the two type I collagen genes, COL1A1 and COL1A2 and account for all but a few of the dominant forms of OI. Now over a dozen additional genes have been identified which give rise to OI (or overlapping bone fragility phenotypes), including both dominant and recessive forms.
The Collagen Diagnostic Lab has traditionally recommended a tiered approach to establishing a genetic diagnosis of OI. Most commonly this began with COL1A1 and COL1A2 sequence analysis as the first step in testing for OI. While this stepwise approach is still appropriate for some clinical situations, the advent of next-generation sequencing technology has allowed us to perform sequence analysis of many genes simultaneously for minimal extra cost. For that reason, clinicians may want to consider testing a larger panel of OI and bone fragility-related genes if they suspect a patient has OI. This test guide will discuss these options in more detail.
Testing for OI: COL1A1 and COL1A2 genomic sequence analysis
About 95% of individuals with OI (perinatal lethal and non-lethal) that we have characterized are heterozygous for a pathogenic variant in one of the two type I collagen genes (COL1A1 and COL1A2). In these families the condition is dominantly inherited. While a significant portion of the pathogenic variants we identify are recurrent, in close to half, we identify new mutations.
In some cases, performing sequence analysis of COL1A1 and COL1A2 is a sufficient test for OI. One such scenario would be when a patient or family presents with a phenotype that is characteristic of a specific type of OI that is known to be caused by variants in COL1A1 or COL1A2 (e.g. OI type I). In such cases it would be appropriate to limit testing to COL1A1 and COL1A2, thus avoiding the cost of testing additional genes and the potential for identifying variants of uncertain significance in other genes.
Another general situation in which limiting testing to COL1A1 and COL1A2 makes sense is when the initial clinical suspicion of OI is low, and testing is being performed to rule out OI as a possible diagnosis. When the a priori risk of OI is low to begin with, a normal sequence analysis of COL1A1 and COL1A2 may be sufficient to rule out OI with a high degree of confidence. This is particularly relevant in the context of unexplained fractures in infancy and the consideration of abuse (discussed in detail further below).
Testing for OI: Sequencing a large panel of OI and bone fragility-related genes
Current sequencing technology allows us to test many genes simultaneously for only a minimal extra cost. The Collagen Diagnostic Lab currently offers an OI and Genetic Bone Disorders Panel of 30 genes (including COL1A1 and COL1A2) which encompasses dominant, recessive, and X-linked forms of OI, hypophosphatasia, and other bone fragility phenotypes that overlap with OI.
Utilizing this “one stop” approach allows clinicians to potentially avoid a prolonged (and expensive) diagnostic odyssey that may otherwise occur with stepwise testing. This is also a sensible approach when there is clinical suspicion for a heritable bone fragility disorder but the exact type is unclear.
Deletion/Duplication Analysis for OI
In rare instances, pathogenic copy number variants that are too large to detect by sequencing can be found in individuals with OI. Such variants likely represent approximately 1% or less of pathogenic variants in individuals with OI, and to date most have been reported in either COL1A1 or COL1A2 (although they can also occur in other OI-related genes). For this reason we recommend considering deletion/duplication analysis either as a next step if sequence analysis fails to identify a causative variant, or as a concurrent test with sequence analysis, but not as a first-line diagnostic test.
ADDITIONAL CONSIDERATIONS
Recessive OI
To date, there are 13 genes (CRTAP, LEPRE1, PPIB, FKBP10, SERPINH1, SERPINF1, PLOD2, OSX, ALPL, TMEM38B, BMP1, WNT1 and CREB3L1) determined to cause recessive OI or hypophosphatasia. The following indications may provide clues as to whether a patient could have a recessive form of OI:
- A severe to lethal OI phenotype is present
- Familial consanguinity is reported
- There are two or more affected children born to unaffected parents (although in the populations we have studied most recurrence results from parental mosaicism for dominant gene mutations)
- The affected individual is from a geographic or ethnic population known to have a “founder” mutation in one of the recessive genes. The populations include African Americans, Vietnamese, Burmese, Samoan and First Nation Canadians.
Because the clinical characteristics that result from pathogenic variants in these genes overlap, it is possible but impractical to choose a single gene tests or sequence in a tiered fashion and be sure that the candidate will be near the top of the list. In our experience, it is easier to sequence all the OI genes at once saving time and money. The OI and Genetic Bone Disorders Panel includes all recessive genes in which pathogenic variants have been identified and will be expanded as others are identified.
Research studies for new genes
If your patient has the clinical phenotype of OI and has had normal genomic DNA sequencing of all the known genes relevant to dominant and recessive forms of OI, duplication and deletion testing, and collagen screening and a genetic cause has not been identified we will discuss with you the option to expand the analysis in a search for mutations in yet unidentified genes. There is still a small group of individuals in whom a disease-causing mutation has not been found. DNA and/or cells from these families should be stored in preparation for identification of new genes and for whole exome/genome approaches to gene discovery in the near future.
Unexplained fractures and the consideration of abuse
There are about 400 infants born with OI in the US each year (an incidence of about 1/10,000). In the age group in which abuse that results in fracture is most frequent (0-3 years) about 25,000 children are identified each year with fractures. Thus we could expect that if there is no screening about 5% of those children with unexplained fractures would have OI, quite similar to the number we have identified in the course of studying children in whom the question of abuse had been raised.
To date, all infants in which the recessive forms of OI have been identified have had clearly recognized skeletal alterations either at birth or within a very short time thereafter. Among children with OI that results from mutations in type I collagen genes those with OI type II (the perinatal lethal form) and OI type III (the progressive deforming variety) can be recognized at birth from both their clinical presentation and the radiographic alterations. The blue sclerae of infants with OI type I may be difficult to appreciate and so the diagnosis could be missed and at an early age, some of the clinical features of OI type IV might not be seen (dental and scleral hue abnormalities).
Given these considerations, we think that in this context, analysis of type I collagen genes by DNA sequence determination is sufficient such that a normal result diminishes the likelihood that the child has OI to well below 1%. If the clinical indications of OI are still very strong, then analysis of the collagen produced by cultured cells should assist in the identification of the underlying alteration. In this context, we do not recommend sequence analysis of the recessive OI genes unless the family is from a small population group or there is clear evidence of parental consanguinity.
Clinical Classification of Osteogenesis Imperfecta
| OI Type (per OMIM) | Phenotypic Description | Gene | Inheritance | Biochemical |
|---|---|---|---|---|
| I | Fractures, normal stature, little or no deformity, blue sclerae, hearing loss | COL1A1 | dominant | 50% reduction in type I collagen synthesis |
| II | Severe lethal in neonatal period; multiple fractures, undermineralized bones, beaded ribs, short/bowed long bones and platyspondyly | COL1A1 or COL1A2 | dominant | Structural alteration in type I collagen chains; overmodification |
| III | Severe non-lethal OI identified in infancy, multiple fractures, bone deformity, grey or white sclerae, dentinogenesis imperfecta (DI), very short | COL1A1 or COL1A2 | dominant | Structural alteration in type I collagen chains; overmodification |
| IV | Multiple fractures, normal sclerae in adults, mild/moderate deformity, variable short stature, DI, some HL | COL1A1 or COL1A2 | dominant | Structural alteration in type I collagen chains; overmodification |
| V | Similar to OI IV plus hyperplastic callus formation, calcif. of interosseous membrane of forearm, anterior radial head dislocation | IFITM5 | dominant | dominant |
| VI | Moderate to severe phenotype | SERPINF1 | recessive | Normal |
| VII | Perinatal lethal or severe non-lethal phenotype with rhizomelic shortening and significant bowing | CRTAP | recessive | Structural alteration in type I collagen chains; overmodification |
| VIII | Perinatal lethal or severe non-lethal phenotype with gracile undermineralized bone and bulbous epiphyses | LEPRE1 | recessive | Structural alteration in type I collagen chains; overmodification |
| IX | Perinatal lethal or severe non-lethal phenotype | PPIB | recessive | Structural alteration in type I collagen chains; overmodification |
| X | Moderately severe OI | SERPINH1 | recessive | Normal |
| XI | Moderate to severe phenotype | FKBP10 | recessive | Normal |
| PLOD2 related | Bruck syndrome - congenital contractures and fractures | PLOD2 | recessive | Normal |
| CREB3L1 related | Severe OI - rare disorder | CREB3L1 | recessive | Apparently not expressed in fibroblasts |
References:
Osteogenesis Imperfecta – Diagnosis
Sillence DO et al. Genetic heterogeneity in osteogenesis imperfecta J Med Genet 1979 Apr;16(2):101-16.
Byers PH et al. Perinatal lethal osteogenesis imperfecta (OI type II): a biochemically heterogeneous disorder usually due to new mutations in the genes for type I collagen. Am J Hum Genet. 1988 Feb;42(2):237-48.
Wenstrup RJ et al. Distinct biochemical phenotypes predict clinical severity in nonlethal variants of osteogenesis imperfecta. Am J Hum Genet. 1990 May;46(5):975-82.
Byers PH. The Metabolic & Molecular Basis of Inherited Disease 8th Edition Volume IV. 2000 Disorders of Collagen Biosynthesis and Structure p 5241-85.
Osteogenesis Imperfecta – Recessive Forms
Glorieux FH et al. Osteogenesis imperfecta type VI: a form of brittle bone disease with a mineralization defect. J Bone Miner Res 2002 Jan;17(1):30-8.
Morello, R et al. CRTAP is required for prolyl 3- hydroxylation and mutations cause recessive osteogenesis imperfecta. Cell 2006 Oct 20;127(2):291-304.
Barnes AM et al. Deficiency of cartilage-associated protein in recessive lethal osteogenesis imperfecta. NEJM 2006 Dec 28;355(26):2757-64.
Cabral WA et al. Prolyl 3-hydroxylase 1 deficiency causes a recessive metabolic bone disorder resembling lethal/severe osteogenesis imperfecta. Nat Genet 2007 Mar;39(3):359-65.
Van Dijk FS et al. PPIB mutations cause severe osteogenesis imperfecta. Am J Hum Genet 2009 Oct;85(4):521-7.
Barnes AM et al. Lack of cyclophilin B in osteogenesis imperfecta with normal collagen folding. NEJM 2010 Feb 11;362(6):521-8.
Christiansen HE et al. Homozygosity for a missense mutation in SERPINH1, which encodes the collagen chaperone protein HSP47, results in severe recessive osteogenesis imperfecta. Am J Hum Genet 2010 Mar 12;86(3):389-98.
Alanay Y et al “American journal of human genetics.” Mutations in the gene encoding the RER protein FKBP65 cause autosomal-recessive osteogenesis imperfecta. Am J Hum Genet 2010 Apr 9;86(4):551-9.
Lapunzina P et al. Identification of a frameshift mutation in Osterix in a patient with recessive osteogenesis imperfecta. Am J Hum Genet 2010 Jul 9;87(1):110-4.
Shaheen R et al. Study of autosomal recessive osteogenesis imperfecta in Arabia reveals a novel locus defined by TMEM38B mutation. J Med Genet 2012 49:630-635.
Fahiminiya S et al. Mutations in WNT1 are a cause of osteogenesis imperfecta. J Med Genet 2013 May;50(5):345-8 .
Volodarsky M et al. A deletion mutation in TMEM38B associated with autosomal recessive osteogenesis imperfecta. Hum Mutat 2013 Apr;34(4):582-6.
Pyott SM et al. WNT1 mutations in families affected by moderately severe and progressive recessive osteogenesis imperfecta. Am J Hum Genet 2013 Apr 4;92(4):590-7.
Volodarsky M et al. A deletion mutation in TMEM38B associated with autosomal recessive osteogenesis imperfecta. Hum Mutat 2013 34:582-586.
Symoens S et al. Deficiency for the ER-stress transducer OASIS causes severe recessive osteogenesis imperfecta in humans. Orphanet J Rare Dis 8:154, 2013
Osteogenesis Imperfecta Type V
Glorieux FH et al. Type V osteogenesis imperfecta: a new form of brittle bone disease. J Bone Miner Res 2000 Sep;15(9):1650.
Arundel P et al. Evolution of the radiographic appearance of the metaphyses over the first year of life in type V osteogenesis imperfecta: clues to pathogenesis. J Bone Miner Res 2011 Apr;26(4):894-9.
Cho TJ et al. A single recurrent mutation in the 5′ UTR of IFITM5 causes osteogenesis imperfecta type V. Am J Hum Gen 2012 Aug;91:343-8.
Prenatal Diagnosis and Mode of Delivery:
Pepin M et al. Strategies and outcomes of prenatal diagnosis for osteogenesis imperfecta: a review of biochemical and molecular studies completed in 129 pregnancies. Prenat Diagn. 1997 Jun;17(6):559-70.
Cubert R et al. Osteogenesis imperfecta: mode of delivery and neonatal outcome. Obstet Gynecol. 2001 Jan;97(1):66-9.