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Genetics & Biology: The EXT1 and EXT2 Genes

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Multiple Osteochondromas is caused by a mutation in the EXT1 or EXT2 genes, which disrupts healthy bone growth and creates bone bumps. A parent with the condition has a 50% chance of passing it to their child, though 29% of cases occur spontaneously without a family history.

Key Takeaways

  • • Multiple Osteochondromas is caused by a genetic mutation in either the EXT1 or EXT2 gene, which disrupts the normal scaffolding of bone growth.
  • • The condition is autosomal dominant, meaning a parent with the condition has a 50% chance of passing it on to each child.
  • • Approximately 29% of diagnoses are spontaneous (de novo) mutations, where neither parent carries the altered gene.
  • • EXT1 mutations generally result in a more severe presentation, including more bone bumps and higher risks of joint deformities, compared to EXT2 mutations.
  • • While clinical diagnosis via X-ray is often sufficient for care, genetic testing can provide a precise roadmap for monitoring and enable family planning options like IVF with PGT-M.

To understand Multiple Osteochondromas, it helps to look at the “instruction manual” for our bodies: our DNA. This condition is caused by a small change, or mutation, in one of two specific genes: EXT1 or EXT2 [1][2]. These genes are responsible for creating a vital building block for healthy bone growth. You can always refer back to the Home Page for a high-level overview.

The Building Blocks of Bone

The EXT1 and EXT2 genes work together like a specialized construction crew. Their job is to produce a substance called heparan sulfate [1][3].

Think of heparan sulfate as a necessary “scaffolding” or building block that helps regulate how bone cells grow and where they go. When one of these genes has a mutation, the body cannot make enough of this building block [2][4]. Without enough “scaffolding,” the signaling process that tells bones how to grow becomes disorganized. Instead of growing straight and smooth, some bone cells grow outward, forming the “bumps” known as osteochondromas [5][6].

How It Is Inherited

Multiple Osteochondromas follows a pattern called autosomal dominant inheritance [7][8]. This means:

  • One Copy is Enough: You only need one mutated copy of the gene (from either parent) to have the condition [3].
  • The 50% Rule: If a parent has the condition, there is a 50% chance with each pregnancy that the child will inherit the mutation [9].

Spontaneous (De Novo) Mutations

In many cases, the condition is not inherited from a parent. About 29% of cases are caused by a de novo (spontaneous) mutation [10]. This means the genetic change happened for the first time in the child, and neither parent carries the gene or has the condition. Once a person has a de novo mutation, they can still pass it on to their own future children with the same 50% chance.

Family Planning Options

The 50% inheritance statistic can feel daunting. However, modern medicine offers choices. For families or young adults planning for the future, a genetic counselor can discuss options like In Vitro Fertilization (IVF) with Preimplantation Genetic Testing for Monogenic/Single Gene Defects (PGT-M) [11][12]. This process allows doctors to screen embryos for the specific EXT1 or EXT2 mutation before pregnancy, ensuring the condition is not passed down.

EXT1 vs. EXT2: What’s the Difference?

While both genes cause the same condition, researchers have noticed that the specific gene involved can influence how the condition looks (the phenotype) [13][14].

Feature EXT1 Mutation EXT2 Mutation
Severity Often more severe [13] Often milder [14]
Number of Bumps Typically more numerous [15] Typically fewer [16]
Stature May be significantly shorter [13] Often closer to average height [13]
Complications Higher risk of joint deformities [16] Lower risk of deformities [14]

It is important to remember that these are general patterns. Even within the same family, two people with the exact same mutation can have very different experiences—one might have many bumps and the other only a few [7][17]. This is known as variable expressivity.

Why Genetic Testing Matters

It is worth noting that a standard clinical diagnosis—made by a doctor looking at the bumps and confirming them with X-rays—is usually enough to begin a proper care plan [18][15]. Formal genetic testing is not strictly mandatory for every patient.

However, identifying whether the mutation is in EXT1 or EXT2 via a blood or saliva test can help your medical team provide a more tailored “roadmap” for monitoring [19][12]. It is most frequently used to confirm borderline cases, or to provide the exact genetic information needed for family planning options like IVF [10][11].

Frequently Asked Questions

How is Multiple Osteochondromas inherited?
Multiple Osteochondromas follows an autosomal dominant inheritance pattern. This means it only takes one mutated copy of either the EXT1 or EXT2 gene to have the condition, and a parent with the mutation has a 50% chance of passing it to their child.
Can a child get Multiple Osteochondromas if neither parent has it?
Yes. About 29% of cases are caused by a spontaneous, or 'de novo', genetic mutation. In these situations, the genetic change happens for the first time in the child, and neither parent has the condition.
What is the difference between an EXT1 and EXT2 gene mutation?
While both mutations cause the condition, EXT1 mutations often lead to a more severe experience. Patients with an EXT1 mutation typically develop more bone bumps, may be shorter in stature, and have a higher risk of joint deformities compared to those with an EXT2 mutation.
Is genetic testing mandatory for diagnosing Multiple Osteochondromas?
A clinical diagnosis made by examining the bumps and confirming them with X-rays is usually enough to start medical care. However, formal genetic testing can help tailor your monitoring plan and provide exact genetic information for future family planning.
What family planning options exist to prevent passing on the condition?
Families can consult a genetic counselor to discuss options such as In Vitro Fertilization (IVF) with Preimplantation Genetic Testing (PGT-M). This advanced testing allows doctors to screen embryos specifically for the EXT1 or EXT2 mutation before pregnancy.

Questions for Your Doctor

  • • Has my child’s genetic testing identified an EXT1 or an EXT2 mutation?
  • • How does the specific type of mutation (like a 'nonsense' or 'missense' mutation) affect the predicted severity in our case?
  • • Since this is an autosomal dominant condition, should other family members be screened even if they don't have visible bumps?
  • • Can you explain how my child's current height and growth rate compare to the typical patterns seen with this genetic profile?
  • • Can you connect us with a genetic counselor to discuss options like IVF with PGT-M for our family planning?

Questions for You

  • • Is there anyone in our extended family (parents, grandparents, aunts, uncles) who had unexplained bone bumps or joint surgery at a young age?
  • • If this is a 'de novo' (spontaneous) mutation, how does that change how we talk to our child about their diagnosis?
  • • What are our main goals for genetic counseling—is it for understanding the current diagnosis or for future family planning?

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References

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    Chen XJ, Zhang H, Tan ZP, et al.

    Molecular medicine reports 2016; (14(5)):4687-4691 doi:10.3892/mmr.2016.5814.

    PMID: 27748933
  2. 2

    Hereditary Multiple Exostoses: Current Insights.

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    Orthopedic research and reviews 2019; (11()):199-211 doi:10.2147/ORR.S183979.

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    Hereditary Multiple Exostoses: New Insights into Pathogenesis, Clinical Complications, and Potential Treatments.

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    Current osteoporosis reports 2017; (15(3)):142-152 doi:10.1007/s11914-017-0355-2.

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    Journal of cellular biochemistry 2019; (120(10)):16362-16369 doi:10.1002/jcb.29155.

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    The pathogenic roles of heparan sulfate deficiency in hereditary multiple exostoses.

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    Matrix biology : journal of the International Society for Matrix Biology 2018; (71-72()):28-39 doi:10.1016/j.matbio.2017.12.011.

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    Aberrant perichondrial BMP signaling mediates multiple osteochondromagenesis in mice.

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    Venous malformation may be a feature of EXT1-related hereditary multiple exostoses: A report of two unrelated probands.

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    American journal of medical genetics. Part A 2023; (191(6)):1570-1575 doi:10.1002/ajmg.a.63158.

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    Osteochondromas: An Updated Review of Epidemiology, Pathogenesis, Clinical Presentation, Radiological Features and Treatment Options.

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    In vivo (Athens, Greece) 2021; (35(2)):681-691 doi:10.21873/invivo.12308.

    PMID: 33622860
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    Forearm Giant Osteochondromas in a Young Patient With Multiple Hereditary Exostoses: A Case Report.

    Sulaiman SR, Ismail HM, Al-Zubaidi SA, et al.

    Cureus 2025; (17(1)):e77295 doi:10.7759/cureus.77295.

    PMID: 39931628
  10. 10

    Mutation spectrum of EXT1 and EXT2 in the Saudi patients with hereditary multiple exostoses.

    Al-Zayed Z, Al-Rijjal RA, Al-Ghofaili L, et al.

    Orphanet journal of rare diseases 2021; (16(1)):100 doi:10.1186/s13023-021-01738-z.

    PMID: 33632255
  11. 11

    Identification of pathogenic mutations in 6 Chinese families with multiple exostoses by whole-exome sequencing and multiplex ligation-dependent probe amplification: Case series.

    Long X, Li Z, Huang Y, et al.

    Medicine 2019; (98(20)):e15692 doi:10.1097/MD.0000000000015692.

    PMID: 31096510
  12. 12

    Identification of a novel EXT2 frameshift mutation in a family with hereditary multiple exostoses by whole-exome sequencing.

    Yang M, Xie H, Xu B, et al.

    Journal of clinical laboratory analysis 2021; (35(9)):e23968 doi:10.1002/jcla.23968.

    PMID: 34403521
  13. 13

    A genotype-phenotype study of hereditary multiple exostoses in forty-six Chinese patients.

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    BMC medical genetics 2017; (18(1)):126 doi:10.1186/s12881-017-0488-2.

    PMID: 29126381
  14. 14

    Assessing the general population frequency of rare coding variants in the EXT1 and EXT2 genes previously implicated in hereditary multiple exostoses.

    Cousminer DL, Arkader A, Voight BF, et al.

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    PMID: 27616605
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    Advances in the pathogenesis and possible treatments for multiple hereditary exostoses from the 2016 international MHE conference.

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    Connective tissue research 2018; (59(1)):85-98 doi:10.1080/03008207.2017.1394295.

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  16. 16

    Correlation between mutated genes and forearm deformity in patients with multiple osteochondroma.

    Matsumoto K, Ishimaru D, Ogawa H, et al.

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  17. 17

    Imaging of benign complications of exostoses of the shoulder, pelvic girdles and appendicular skeleton.

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    Hereditary multiple exostoses: an educational review.

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  19. 19

    Chondrosarcoma transformation in hereditary multiple exostoses: A systematic review and clinical and cost-effectiveness of a proposed screening model.

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    Journal of bone oncology 2018; (13()):114-122 doi:10.1016/j.jbo.2018.09.011.

    PMID: 30591865

This page explains the genetics of Multiple Osteochondromas for educational purposes only. Always consult a certified genetic counselor or your medical team to discuss your specific genetic testing results and family planning options.

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