What is GIST?
GIST stands for gastrointestinal stromal tumour — a rare type of tumour that grows in the wall of the digestive tract, most often the stomach or small intestine. It arises from specialised "pacemaker" cells that help move food through the gut.
Most GISTs are driven by a fault in one of two genes, KIT or PDGFRA. That discovery was a medical success story: targeted drugs such as imatinib were designed to switch those faulty genes off, and they transformed outcomes for the majority of GIST patients.
What is SDH-deficient GIST?
SDH-deficient GIST is a distinct, much rarer subtype — estimated to make up only a small fraction of all GISTs. Crucially, it is not driven by the usual KIT or PDGFRA faults. Instead, it is caused by the loss of an enzyme complex called succinate dehydrogenase (SDH).
SDH is part of the cell's energy machinery. When it stops working, the cell's chemistry is thrown off in ways that can drive cancer — a completely different mechanism from ordinary GIST. That difference matters enormously: imatinib — the standard first-line drug that transformed outcomes for common GIST — usually has little effect here. Some later-line, multi-target drugs can help some patients, but there is still no reliably effective therapy developed specifically for this subtype.
Who does it affect?
SDH-deficient GIST stands out because of who it tends to strike. Unlike common GIST, which usually appears in older adults, this subtype most often affects children and young adults, and is seen more frequently in females. It typically arises in the stomach, and can be multifocal — meaning more than one tumour at once.
It often behaves more slowly than common GIST, but it can still spread (most commonly to the liver or nearby lymph nodes), and it can persist for many years — which is exactly why better long-term treatment options are so needed.
Symptoms
Symptoms are not specific to this subtype and can be easy to miss. They may include:
- Vague abdominal pain or discomfort, or a feeling of fullness.
- Anaemia or fatigue, sometimes from slow bleeding into the gut.
- Nausea, or a lump that a doctor can feel.
- In some cases, no symptoms at all — the tumour is found incidentally on a scan done for another reason.
Because these signs are common to many conditions, a firm answer only comes from proper investigation and specialist review.
How it's diagnosed
Diagnosis usually involves imaging (such as a CT or MRI scan), a biopsy, and — importantly for this subtype — specialised laboratory testing. A pathologist can run a stain that reveals whether the SDH enzyme is present or lost, and genetic testing can identify changes in the SDH genes. Getting this right matters: because SDH-deficient GIST looks similar to common GIST under the microscope, it can be missed unless the specific tests are done.
How it's treated
For localized disease, surgery is the principal treatment. Because this subtype often behaves slowly — even after it has spread — decisions about extensive or repeated operations call for careful specialist judgement rather than automatic surgery. Beyond surgery, the picture is harder: the standard targeted drugs for common GIST are generally much less effective against this subtype. Some patients are offered other multi-target drugs, and researchers are actively studying newer approaches aimed squarely at the biology of SDH loss — including strategies drawn from metabolism and immunotherapy.
This is precisely the gap Mission Pegasus works in: taking the most promising scientific leads and pushing them toward real, tested treatment options.
Is it hereditary?
Sometimes. A meaningful proportion of SDH-deficient GISTs are linked to an inherited change in one of the SDH genes, which can also be associated with certain other rare tumours. Because of this, genetic counselling and germline testing are generally recommended for everyone diagnosed with SDH-deficient GIST — even when there is no known family history. Testing can guide monitoring for the other rare tumours linked to SDH gene changes and, where relevant, informs testing for family members. Not every case is inherited, and a specialist can explain what testing makes sense in each situation.
Why research matters
SDH-deficient GIST sits in a difficult spot: it is too rare to attract large, dedicated drug-development programmes, yet different enough that the existing treatments don't serve it well. Progress depends on connecting and testing the scientific leads that already exist — and doing it with urgency. That focused, catalytic work is the whole purpose of Mission Pegasus.
Sources & further reading
This guide is written for a general audience and reviewed against authoritative clinical and research sources. For the underlying detail, see:
- National Cancer Institute — Gastrointestinal Stromal Tumors Treatment (PDQ®), Patient Version
- National Cancer Institute — Childhood Gastrointestinal Stromal Tumors Treatment (PDQ®), Health Professional Version
- Florou et al. (2025), A Review of Genomic Testing and SDH-Deficiency in Gastrointestinal Stromal Tumors: Getting to the GIST, Cancer Medicine
- NIH Pediatric & Wildtype GIST Clinic
- The Life Raft Group — SDH-Deficient GIST resources
- ClinicalTrials.gov — current SDH-deficient GIST trials
Reviewed July 2026. This page is kept up to date as the science develops.