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PubMed This is a summary of 17 peer-reviewed journal articles Updated
Medical Genetics

Finding Answers: The Types and Genetics of Achondrogenesis

At a Glance

Achondrogenesis has three main types (IA, IB, and II), each caused by a different gene mutation. Identifying the specific type through genetic testing is crucial because it reveals the recurrence risk for future pregnancies, which ranges from a 25% chance to very low.

While a diagnosis of achondrogenesis is devastating for your current pregnancy, understanding the specific genetic “blueprint” of the condition is one of the most powerful tools you have for the future. Identifying the exact subtype provides essential clarity on the risk of this happening again in future pregnancies [1][2].

The Three Main Types of Achondrogenesis

The condition is classified into three types based on which specific gene is affected. Each gene plays a critical role in building a baby’s skeleton.

Type Gene Involved The Biological Problem Inheritance Pattern Recurrence Risk
Type IA (Houston-Harris) TRIP11 The “cellular post office” breaks down; proteins cannot be transported within cartilage cells [3][4]. Autosomal Recessive 25%
Type IB (Parenti-Fraccaro) SLC26A2 The body cannot properly transport sulfate, making the cartilage “glue” too weak to support bone growth [5][6]. Autosomal Recessive 25%
Type II (Langer-Saldino) COL2A1 The instructions for making Type II collagen are corrupted, causing severely malformed cartilage [7][8]. Autosomal Dominant (usually De Novo) Very Low

Understanding Inheritance and Recurrence Risk

The way a condition is passed down (its inheritance pattern) determines the recurrence risk, which is the mathematical chance it could happen in a subsequent pregnancy.

Autosomal Recessive (Types IA and IB)

In Types IA and IB, both parents are typically silent “carriers” of the condition [9]. This means you each have one healthy gene and one mutated gene. You have no symptoms at all because your healthy gene perfectly compensates for the mutated one.

  • The Risk: In every future pregnancy, there is a 25% (1 in 4) chance the baby will inherit the mutated gene from both parents and be affected by achondrogenesis [9][10].

De Novo Mutations (Type II)

Most cases of Type II are “de novo,” meaning the mutation happened entirely randomly in the egg or sperm just before or at the moment of conception. It was not inherited from either parent’s general DNA [11][12].

  • The Risk: The risk for future pregnancies is generally very low (often estimated at around 1%), as it was a random, lightning-strike event [11].
  • The Exception (Germline Mosaicism): Rarely, a parent may have the mutation hiding only in a percentage of their reproductive cells (egg or sperm), while the rest of their body is unaffected [13]. Because of this, doctors often suggest specialized genetic sequencing for parents to give them the most accurate risk assessment [11].

Confirming the Genetic Map

Because many severe skeletal dysplasias look similar on an ultrasound, doctors must identify the exact mutation through molecular diagnostic testing [14].

  1. Amniocentesis: If you are currently pregnant (often diagnosed during the 18-20 week anatomy scan), a sample of amniotic fluid can be taken to perform a microarray or whole-exome sequencing (WES) on the baby’s DNA [15][16].
  2. Postnatal Testing: If testing is not done during pregnancy, a small tissue or blood sample taken from the baby after birth can provide the exact same molecular answers [1].
  3. CVS (Chorionic Villus Sampling): Once your family’s specific mutation is known, CVS can be used in future pregnancies. Performed very early (between 10 and 13 weeks), it tests a small piece of the placenta to determine if the next baby is affected [17].

Once a mutation is definitively identified, families who wish to have more children may also explore Preimplantation Genetic Testing (PGT-M). This process is used alongside In Vitro Fertilization (IVF) to screen embryos and ensure only those without the achondrogenesis mutation are implanted.

Common questions in this guide

What are the three main types of achondrogenesis?
The condition is classified into Type IA (Houston-Harris), Type IB (Parenti-Fraccaro), and Type II (Langer-Saldino). Each type is caused by a different gene mutation that affects how a baby's skeleton and cartilage develop.
What is the risk of having another baby with achondrogenesis?
The recurrence risk depends heavily on the specific type. For Types IA and IB, there is a 25% chance in each future pregnancy because both parents carry the mutated gene. For Type II, the risk is generally very low (around 1%) as it usually occurs from a random mutation.
How do doctors diagnose the exact type of achondrogenesis?
Doctors confirm the exact type through molecular genetic testing to find the specific gene mutation. This can be done during pregnancy using amniocentesis or after birth using a small tissue or blood sample from the baby.
Can we test for achondrogenesis in future pregnancies?
Yes, once your family's specific genetic mutation is known, doctors can test for it in future pregnancies. Options include early diagnostic testing like Chorionic Villus Sampling (CVS) at 10-13 weeks, or Preimplantation Genetic Testing (PGT-M) combined with IVF.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Which specific gene mutation (TRIP11, SLC26A2, or COL2A1) has been identified, or what testing do we need to do to find out?
  2. 2.If the mutation is COL2A1 (Type II), what is the likelihood of germline mosaicism, and is there testing recommended for us as parents?
  3. 3.If we plan to have more children, are there specific fertility specialists you recommend who have experience with Preimplantation Genetic Testing (PGT)?
  4. 4.How soon in a future pregnancy could we perform diagnostic testing, such as Chorionic Villus Sampling (CVS)?

Questions For You

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References

References (17)
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    Antenatal Phenotype of Desbuquois Dysplasia.

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    Indian journal of pediatrics 2023; (90(1)):83-86 doi:10.1007/s12098-022-04386-8.

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    Diagnosis of Prenatal-Onset Achondrogenesis Type II by a Multidisciplinary Assessment: A Retrospective Study of 2 Cases.

    Wang W, Wu Q, Sun L, et al.

    Case reports in obstetrics and gynecology 2019; (2019()):7981767 doi:10.1155/2019/7981767.

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    The skeletal phenotype of achondrogenesis type 1A is caused exclusively by cartilage defects.

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    Suppressing UPR-dependent overactivation of FGFR3 signaling ameliorates SLC26A2-deficient chondrodysplasias.

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    Pathogenicity effects of a COL2A1 missense mutation (c.1594G>C) in cartilage development.

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    A Novel Splicing Variant of COL2A1 in a Fetus with Achondrogenesis Type II: Interpretation of Pathogenicity of In-Frame Deletions.

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    Novel missense COL2A1 variant in a fetus with achondrogenesis type II.

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This page explains the genetics and types of achondrogenesis for educational purposes. Always consult a genetic counselor or maternal-fetal medicine specialist for guidance on your specific risks and testing options.

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