It seems that the incidence of genetic testing for medical reasons is becoming more and more common. What doesn't seem to be getting more common is a sensible evaluation of what those test results may mean. Medical opinion seems to be that, except for some obvious examples like the BRCA1 and BRCA2 genes, they are mostly irrelevant. They point to susceptibilities and potentials.
But some Interweb sources make it sound as if possession of a particular gene mutation (called a Single Nucleotide Polymorphism, or SNP) is almost a guarantee that you'll suffer the health consequences associated.
First, let's look at the one most commonly quoted on this forum - MTHFR. About 8% of the population are homozygous (two copies) of the C677>T mutation. This has been studied extensively, with lots of publications.
According to SNPedia - snpedia.com/index.php/Rs180... - this single gene has been associated with - autism, cancer (including gastric cancer, lung cancer, head and neck cancer, renal cancer), cleft lip and cleft palate, coronary artery disease, dementia, depression, hyperhomocysteinemia, migraine, neural tube defects, pre-eclampsia (gestational hypertension), schizophrenia, thrombosis and down syndrome.
If you want to believe the naturopaths then it's associated with (deep breath)
; Addictions: smoking, drugs, alcohol; Down syndrome; Miscarriages; Pulmonary embolisms; Depression in Post-Menopausal Women; Schizophrenia; Fibromyalgia; Chronic Fatigue Syndrome; Chemical Sensitivity; Parkinson’s; Irritable Bowel Syndrome; Pre-eclampsia; Stroke; Spina bifida; Esophageal Squamous cell carcinoma; Acute Lymphoblastic Leukemia; Vascular Dementia; Bipolar disorder; Colorectal Adenoma; Idiopathic male infertility; Blood clots; Rectal cancer; Meningioma; Glioma; Congenital Heart Defects; Infant depression via epigenetic processes caused by maternal depression; Deficits in childhood cognitive development; Gastric Cancer; Migraines with aura; Low HDL; High homocysteine; Post-menopausal breast cancer; Atherosclerosis; Oral Clefts; Type 1 Diabetes; Epilepsy; Primary Closed Angle Glaucoma; Alzheimer’s; Tetralogy of Fallot; Decreased telomere length; Potential drug toxicities: methotrexate, anti-epileptics; Cervical dysplasia; Increased bone fracture risk in post-menopausal women; Multiple Sclerosis; Essential Hypertension; Differentiated Thyroid Carcinoma; Prostate Cancer; Premature Death; Placental Abruption; Myocardial Infarction (Heart Attack); Methotrexate Toxicity; Nitrous Oxide Toxicity; Heart Murmurs; Tight Anal Sphincters; Tongue Tie; Midline Defects (many are listed above); Behcet’s Disease; Ischemic Stroke in Children; Unexplained Neurologic Disease; Asthma; Shortness of Breath; Bladder Cancer; Anecephaly
Phew! It's amazing any of us 8% are still around. SNPedia does make a point that many (most?) of those studies have been shown to be irreproducible.
What's really amazing is that I'm homozygous for no less that six mutations that have been associated with autism! But my best friend has Aspergers and she tells me that, despite some aspie-like traits, I'm most definitely neurotypical.
The problem is that there are hundreds of thousands of SNPs (I got 600,000 of mine tested). And a great way to get a paper published is to find a condition that's reasonably common and test a hundred people against those 600,000 SNPs. It's not surprising if you find a couple of SNPs that many of those hundred people have in common and that most of the controls don't have. Then somebody repeats the study and the correlation disappears.
The other thing that never gets mentioned is what does a 2x increased risk of condition X actually mean?
One of my genes - snpedia.com/index.php/Rs109... - is associated with a 1.17 times increase chance of MS. Now the normal chance of getting MS is about 1 in 1000. So this gene increases my chances to about 1 in 850 - Am I going to fret about it - no.
However, if I were female and had a bad BRCA1 gene then that could increase my chance of breast cancer up from 1 in 8, as in the normal population, to 1 in 2. That really would make me think. cancer.gov/about-cancer/cau...