Osteogenesis imperfecta (OI), often called brittle bone disease, is caused by genetic mutations that disrupt the body’s ability to make or use type I collagen, the protein that gives bones their strength and flexibility. About 85% of cases trace back to mutations in two specific genes, COL1A1 and COL1A2, which carry the instructions for building collagen. The condition affects roughly 1 in 10,000 to 13,500 people, and more than half of all cases result from brand-new mutations that appear spontaneously rather than being passed down from a parent.
How Collagen Goes Wrong
Type I collagen is the most abundant protein in bone. It’s built from three protein chains that wind around each other into a tight, rope-like structure called a triple helix. Every third amino acid in each chain must be glycine, the smallest amino acid, because only glycine is small enough to fit in the cramped interior of the helix. When a mutation swaps glycine for a larger amino acid, the helix can’t fold properly.
The consequences depend on the type and location of the swap. Studies on collagen folding show that replacing glycine with a larger amino acid lowers the protein’s thermal stability by an average of about 2°C, which may sound small but is enough to weaken the structure. Folding time also slows dramatically. In laboratory models, a single glycine substitution near the middle of the chain increased the time to fold from about 10 minutes to nearly an hour. A substitution closer to the point where folding begins was so disruptive that the chain couldn’t reach even 50% folding after 16 hours.
The location of a mutation along the gene matters as much as the type of substitution. Researchers have identified specific stretches of the COL1A1 and COL1A2 genes, called lethal regions, where glycine substitutions tend to produce the most severe or fatal outcomes. These regions correspond to sites where collagen molecules interact with other proteins in the bone matrix. In the COL1A2 gene, these lethal regions make up only about 31% of the chain, yet mutations within them account for a disproportionate share of the most serious cases. Some structural changes at these binding sites are tolerated better than others, which helps explain why two people with mutations in the same gene can have very different symptoms.
Quantitative vs. Qualitative Defects
Not all COL1A1 and COL1A2 mutations work the same way. They generally fall into two categories. Quantitative defects mean the body produces a normal collagen molecule but not enough of it. One copy of the gene is essentially silent, so cells make roughly half the usual amount of collagen. This typically leads to the mildest form of OI, with bones that fracture more easily than normal but retain a relatively typical shape.
Qualitative defects are different and usually more serious. Here, the mutated gene still produces collagen chains, but those chains are structurally abnormal. Because three chains must wind together into a single helix, one misshapen chain can drag down the entire molecule. The abnormal collagen gets incorporated into bone, weakening it from the inside. This is why qualitative mutations tend to cause moderate to severe disease, including bones that bow, a spine that curves, and fractures that happen with little or no force.
Inheritance Patterns
The most common forms of OI follow an autosomal dominant inheritance pattern, meaning a mutation in just one copy of the gene is enough to cause the condition. If one parent has OI caused by a dominant mutation, each child has a 50% chance of inheriting it. This pattern applies to the milder and moderate forms (types I and IV) as well as the severe forms (types II and III) when they run in families.
However, the majority of severe cases don’t run in families at all. Research analyzing 146 OI cases found that 56% were caused by de novo (spontaneous) mutations, meaning the genetic change happened for the first time in the affected child. Previous estimates placed this figure between 35% and 60%. This means parents with no personal or family history of OI can still have a child with the condition, and it also means that a couple who has one child with de novo OI generally faces a low (though not zero) risk of recurrence in future pregnancies.
Rarer forms of OI, classified as types VI through XVIII, follow an autosomal recessive pattern. In these cases, both parents carry one silent copy of a mutated gene without showing symptoms, and a child who inherits both copies develops the condition. One extremely rare form, type XIX, is X-linked, meaning the mutation sits on the X chromosome and primarily affects males.
Beyond Collagen: Other Genes Involved
While COL1A1 and COL1A2 mutations account for the vast majority of OI, at least 20 genes have now been linked to the condition. These non-collagen genes cause OI through several different mechanisms.
- Defective collagen processing. Some genes encode proteins that help modify or fold collagen after it’s built. When these helpers malfunction, the collagen itself may be normal in sequence but still ends up misshapen. Mutations in the genes for these processing proteins cause OI types VII through IX.
- Abnormal collagen transport and crosslinking. Other proteins act as chaperones, escorting collagen molecules out of the cell and helping them crosslink into strong fibers. Defects in these chaperones cause types X and XI.
- Faulty bone mineralization. Types V and VI result from mutations in genes called IFITM5 and SERPINF1, which affect how minerals are deposited into the collagen scaffold. The collagen may form correctly, but the bone still ends up weak because the mineralization process goes awry.
- Disrupted bone cell development. The most recently discovered OI genes, including WNT1, SP7, TMEM38B, and CREB3L1, don’t directly affect collagen at all. Instead, they interfere with the development or function of osteoblasts, the cells responsible for building new bone. Mutations in WNT1 are particularly notable: one mutated copy causes osteoporosis, while two mutated copies cause severe OI.
The OI Type Spectrum
OI was originally classified into four types by David Sillence in 1979, based on clinical severity and features visible on X-rays. That system has since expanded, though experts have debated how to organize the newer genetic discoveries. A widely used approach keeps the original clinical types (I through VI) for describing what the disease looks like, then adds the specific gene mutation as a separate label. This matters because different gene mutations can produce the same clinical picture, and knowing the exact genetic cause helps predict outcomes and guide family planning.
Type I is the mildest, with near-normal stature and fractures that often decrease after puberty. Type II is the most severe and frequently lethal before or shortly after birth. Type III causes progressive bone deformity and very short stature, while type IV falls in between, with moderate fractures and mild to moderate deformity. Type V is distinguished by unusual callus formation at fracture sites and calcification of the membrane between the radius and ulna in the forearm. Type VI is identified primarily through bone biopsy, which reveals a distinct mineralization defect.
How Genetic Causes Are Identified
Moderate to severe OI is often first suspected during a prenatal ultrasound between 18 and 24 weeks of pregnancy, when shortened or bowed limbs and fractures may be visible. If a family member already has OI, fetal DNA can be tested directly through chorionic villus sampling (around 10 to 12 weeks) or amniocentesis (around 15 to 20 weeks) to check for the known family mutation.
After birth, genetic testing has become the primary way to confirm a diagnosis and pinpoint the exact cause. Next-generation sequencing technology has made it possible to test many genes at once. A common approach is a gene panel that screens dozens of OI-related genes simultaneously. For cases where a panel doesn’t find the answer, whole exome sequencing can cast a wider net, analyzing nearly all protein-coding genes in the genome. The identified mutation is then classified using standardized guidelines to determine whether it’s the likely cause of the disease. Knowing the specific mutation helps families understand the inheritance pattern, predict severity, and make informed decisions about future pregnancies.