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There's a condition called SYNGAP1-related intellectual disability which is caused by mutations to the SYNGAP1 gene.

I believe that this is called a monogenic disorder, while disorders that are caused by multiple genes are called multifactorial inheritance disorders. Other examples of monogenic diseases are apparently cystic fibrosis, sickle-cell anemia, and Huntington's disease. Examples of multifactorial inheritance disorders are heart disease, cancers, Alzheimer's, athritis.

The number of monogenic disorders are limited to the number of genes at $\approx20000$, while the potential number of multifactorial inheritance disorders is then $20000!$ or about $10^{77000}$. This upper bound is not very helpful since I would assume that most multifactorial disorders aren't going to affect anywhere near all genes.

How common is it that diseases/conditions are caused by the mutation to only a single gene? If I pick a random disease/disorder/condition how likely is it that it's caused by only a single gene?

Is there a list somewhere of all known monogenic disorders? If I search "monogenic" in OMIM there are 185 results, of which many don't appear to actually be monogenic. For example, the second result is obesity which is not monogenic.

If I look at Type 1 Diabetes there is only one gene-phenotype relationship which leads me to believe that it's monogenic, but the phenegene graphics shows three genes linked to type 1 diabetes: 600716, 147620, 142410. So not only are there multiple genes linked to diabetes, those multiple genes are also linked to multiple disorders.

The gene that causes sickle-cell is also connected to other disorders. Is this because different mutations in HBB lead to different phenotype disorders, where sickle-cell is one, but sickle-cell itself if just caused by that single gene mutation? I.e. sickle-cell is monogenic, but the responsible gene can mutate in other ways giving rise to other disorders?

Maybe I'm interpreting things in the graph wrong. But, what I'm looking for are gene, disorder pairs like SYNGAP1 where the one gene is linked to one and only disorder. I think what I'm looking for then is not just monogenic disorders, but also "monodisorder" genes.

Is there a way I can search OMIM for these?

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    $\begingroup$ Just a note that since different mutations in a single gene can sometimes cause different disorders, the possible number of disorders is not limited to the number of genes. On the other hand, many gene defects will result in non-viability before birth, so will not be seen in practice. There are also disorders caused by chromosomal abnormalities involving duplication/deletion of multiple genes or a chromosome. Apropos your final question, our knowledge is nowhere near complete, so genes that are today listed as "monodisorder" could easily have additional disorders recognized down the road. $\endgroup$
    – Armand
    Commented Aug 10, 2021 at 15:48
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    $\begingroup$ Re: " Is this because different mutations in HBB lead to different phenotype disorders, where sickle-cell is one, but sickle-cell itself if just caused by that single gene mutation? " In general, yes. There are often groups of related but somewhat different disorders for different mutations in the same part of a gene, for example see the beta thalassemias also caused by certain mutations in HBB. $\endgroup$
    – Armand
    Commented Aug 10, 2021 at 16:10

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