Sillence's four types have both a clinical and a genetic meaning; the descriptions below are clinical and can be applied to several genetic types of OI. When used to refer to a genetic as well as a clinical type, it indicates that the clinical symptoms are indeed caused by mutations in the COL1A1 or COL1A2 genes, which are inherited in an autosomal dominant fashion. People with type I generally have a normal lifespan.
Type
Collagen is fatally defective at its C-terminus. In the rare cases of infants who survive their first year of life, severe developmental and motor delays are seen; neither of two infants studied in 2019, both aged around two years, had achieved head control, and both required a ventilator to breathe.
Type II is also known as the "lethal perinatal" form of OI, and is not compatible with survival into adulthood.
Type
Collagen quantity is sufficient, but is not of a high enough quality. features only found in type III are its progressively deforming nature Another differentiating factor between type III and IV is blue sclerae; in type III, infants commonly have blue sclerae that gradually turn white with age, but blue sclerae are not commonly seen in type IV, although they are seen in 10% of cases.
OI type III causes osteopenic bones that fracture very easily, sometimes even in utero, often leading to hundreds of fractures during a lifetime; While one of Sillence's required characteristics for type IV was having normal sclerae, which may make it difficult to turn the wrist. Cases of this type are caused by mutations in the IFITM5 gene on chromosome 11<nowiki/>p15.5. Type V is relatively common compared to other genetically defined types of OI—4% of OI patients at the genetics department of the Brazilian Hospital de Clínicas de Porto Alegre were found to have it.
- Type – With the same clinical features as type III, it is distinguished by bones which have an appearance similar to that seen in osteomalacia.
- Type – OI caused by a mutation in the gene LEPRE1 on chromosome 1<nowiki/>p34.2; clinically similar to OI types II and III, depending on affected individual.
- Type – OI caused by homozygous or compound heterozygous mutation in the PPIB gene on chromosome 15<nowiki/>q22.31.
- Type – OI caused by homozygous mutation in the SERPINH1 gene on chromosome 11q13.
- Type – OI caused by mutations in FKBP10 on chromosome 17q21. The mutations cause a decrease in the secretion of trimeric procollagen molecules. Other mutations in this gene can cause autosomal recessive Bruck syndrome, which is similar to OI.
- Type – OI caused by a frameshift mutation in SP7 on chromosome 12<nowiki/>q13.13. This mutation causes bone deformities, fractures, and delayed tooth eruption.
- Type – OI caused by a mutation in the bone morphogenetic protein 1 (BMP1) gene on chromosome 8<nowiki/>p21.3. This mutation causes recurrent fractures, high bone mass, and hypermobile joints.
- Type – OI caused by mutations in the TMEM38B gene on chromosome 9<nowiki/>q31.2. This mutation causes recurrent fractures and osteopenia, although the disease trajectory is highly variable.
- Type – OI caused by homozygous or compound heterozygous mutations in the WNT1 gene on chromosome 12q13.12. It is autosomal recessive. Family members who are heterozygous for OI XVI may have recurrent fractures, osteopenia and blue sclerae.
- Type – OI caused by homozygous mutation in the SPARC gene on chromosome 5<nowiki/>q33, causing a defect in the protein osteonectin, which leads to severe disease characterized by generalized platyspondyly, dependence on a wheelchair, and recurrent fractures.
- Type – OI caused by homozygous mutation in the FAM46A gene on chromosome 6<nowiki/>q14.1. Characterized by congenital bowing of the long bones, Wormian bones, blue sclerae, vertebral collapse, and multiple fractures in the first years of life.
- Type – OI caused by hemizygous mutation in the MBTPS2 gene on chromosome X<nowiki/>p22.12. Thus far, OI type XIX is the only known type of OI with an X-linked recessive pattern of inheritance, making it the only type that is more common in males than females. OI type XIX disrupts regulated intramembrane proteolysis, which is critical for healthy bone formation.
- Type – OI caused by homozygous mutation in the KDELR2 gene on chromosome 7<nowiki/>p22.1. Causes disease clinically similar to types II and III, thought to be related to inability of chaperone protein HP47 to unbind from collagen type I, as to do so it needs to bind to the missing ER lumen protein retaining receptor 2 protein encoded by KDELR2.
Genetics
thumb|An [[Collagen, type I, alpha 1|α1 type I collagen protein]]
Osteogenesis imperfecta is a group of genetic disorders, all of which cause bone fragility. OI has high genetic heterogeneity, that is, many different genetic mutations lead to the same or similar sets of observable symptoms (phenotypes).
The main causes for developing the disorder are a result of mutations in the COL1A1 and/or COL1A2 genes which are jointly responsible for the production of collagen type I. Approximately 90% of people with OI are heterozygous for mutations in either the COL1A1 or COL1A2 genes. There are several biological factors that are results of the dominant form of OI. These factors include: intracellular stress; abnormal tissue mineralization; abnormal cell-to-cell interactions; abnormal cell-matrix interactions; a compromised cell matrix structure; and, abnormal interaction between non-collagenous proteins and collagen.
Previous research led to the belief that OI was an autosomal dominant disorder with few other variations in genomes. However, with the lowering of the cost of DNA sequencing in the wake of 2003's Human Genome Project, autosomal recessive forms of the disorder have been identified. Recessive forms of OI relate heavily to defects in the collagen chaperones responsible for the production of procollagen and the assembly of the related proteins. Examples of collagen chaperones that are defective in patients with recessive forms of OI include chaperone HSP47 (Cole-Carpenter syndrome) and FKBP65. Mutations in these chaperones result in an improper folding pattern in the collagen 1 proteins, which causes the recessive form of the disorder. Defects in these proteins lead to defective bone mineralization which causes the characteristic brittle bones of osteogenesis imperfecta.
In the rare case of type XIX, first discovered in 2016, OI is inherited as an X-linked genetic disorder, with its detrimental effects resulting ultimately from a mutation in the gene MBTPS2. Genetic research is ongoing, and it is uncertain when all the genetic causes of OI will be identified, as the number of genes that need to be tested to rule out the disorder continue to increase. The cause is genetic mosaicism; that is, some of, or most of, the germ cells of one parent have a dominant form of OI, but not enough of their somatic cells do to cause symptoms or obvious disability in the parent—the parent's different cells have two (or more) sets of slightly different DNA. One possible deficiency arises from an amino acid substitution of glycine to a bulkier amino acid, such as alanine, in the collagen protein's triple helix structure. The larger amino acid side-chains lead to steric effects that creates a bulge in the collagen complex, which in turn influences both the molecular nanomechanics and the interaction between molecules, which are both compromised. Depending on both the location of the substitution and the amino acid being used instead, different effects are seen which account for the type diversity in OI despite the same two collagen genes being responsible for most cases. Replacements of glycine with serine or cysteine are seen less often in fatal type II OI, while replacements with valine, aspartic acid, glutamic acid, or arginine are seen more often.
At a larger scale, the relationship between the collagen fibrils and hydroxyapatite crystals to form bone is altered, causing brittleness. As X-rays are often insensitive to the comparatively smaller bone density loss associated with type I OI, DEXA scans may be needed. OI can also be detected before birth by using an in vitro genetic testing technique such as amniocentesis.
Genetic testing
To determine whether osteogenesis imperfecta is present, genetic sequencing of the most common problematic genes, COL1A1, COL1A2, and IFITM5, may be done; if no mutation is found yet OI is still suspected, the other 10+ genes known to cause OI may be tested.
Other differential diagnoses include rickets and osteomalacia, both caused by malnutrition, as well as rare skeletal/connective tissue syndromes such as Bruck syndrome, hypophosphatasia, geroderma osteodysplasticum, and Ehlers–Danlos syndrome. Treatment may include care of broken bones, pain medication, physical therapy, mobility aids such as braces or wheelchairs, and surgery.
Acute bone fracture care
Bone fractures are treated in individuals with osteogenesis imperfecta in much the same way as they are treated in the general population; OI bone heals at the same rate as non-OI bone.
Although oral bisphosphonates are more convenient and cheaper, they are not absorbed as well, and intravenous bisphosphonates are generally more effective, although this is under study. Some studies have found oral and intravenous bisphosphonates, such as oral alendronate and intravenous pamidronate, equivalent. In a 2013 double-blind trial of children with mild OI, oral risedronate increased bone mineral densities, and reduced nonvertebral fractures. However, it did not decrease new vertebral fractures. A Cochrane review in 2016 concluded that though bisphosphonates seem to improve bone mineral density, it is uncertain whether this leads either to a reduction in bone fractures or improvement in the quality of life of individuals with osteogenesis imperfecta.
Nutritional supplements
OI is a genetic disorder and is not caused by insufficient intake of any vitamin or mineral; supplementation cannot cure OI. Nevertheless, people with OI tend to be severely deficient in vitamin D at much higher rates than the general population, and the cause of this is not well understood. The severity of the deficiency and the likelihood of its occurrence is thought to be related to severity of OI. A unique concern of anesthesia in OI is perioperative fracture—fractures sustained due to patient transfer and airway access techniques that, while routine when a patient's bones are strong, may cause injury with brittle OI bones. As an example, due to a 1972 report of a humerus fracture from a sphygmomanometer cuff sustained in an OI patient during surgery, blood pressure monitoring protocols are often modified for patients with OI, with neonatal size cuffs and machine settings being used even in adults; further, the least deformed of the patient's limbs is preferred to receive the cuff.