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X-Linked Diseases Meltdown

Author: Ed Friedlander MD, Posted on Saturday, November 12 @ 17:56:42 IST by RxPG  

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Pathology

X-linked dominant diseases: The only well-known one is familial vitamin-D resistant rickets, a renal phosphate-wasting syndrome. Manic-depression (nowadays, "bipolar disorder") probably has a locus here.

X-linked recessives: Expressing the phenotype requires one dose for hemizygous men, two for women.

Rules:

X-linked diseases:

-- affect all males with the gene

-- affect a woman only if (1) she had two affected X-chromosomes, i.e., she had an affected father and a carrier mother (possible if we're just dealing with color blindness, most unlikely if we're dealing with Duchenne's muscular dystrophy); (2) she suffers from really unfortunate lyonization; (3) the disease is expressed when individually lyonized cells are affected (i.e., G6PD deficiency, in which half the red cells hemolyze and half don't; or some cases of fragile X syndrome); (4) she has Turner's syndrome (XO) or testicular feminization (XY).
-- generally produce many affected family members, once the new mutation has been propagated.

The major X-linked disorders
Familiar proteins
-- Hemophilia A (factor VIII deficiency)
-- Hemophilia B (factor IX deficiency)
-- G6PD deficiency ("favism"; several alleles)
-- Lesch-Nyhan syndrome
-- Duchenne's muscular dystrophy (Jerry's kids)
-- Chronic granulomatous disease
-- Hunter's mucopolysaccharidosis
-- Fabry's disease
-- Common red-green color-blindness
-- Testicular feminization (common type)
-- Nephrogenic diabetes insipidus (hADH receptor in collecting duct)
[ -- Unnamed allele for monoamine oxidase A that correlates strikingly with horribly aggressive misbehavior; this new, major discovery caused a political flap...]
Being worked-out
-- Bruton's agammaglobulinemia
-- "David the Bubble Boy"'s immunodeficiency
-- Several other immunodeficiency syndromes
-- Some adrenoleukodystrophy genes
-- Some cases of agenesis of the corpus callosum (alexithymia)
-- Ehlers-Danlos type IX
-- Mencke's kinky hair (a real "kink" in copper metabolism)
Fragile X chromosome

Fabry's disease ("angiokeratoma corporis diffusum universale"; deficiency of the enzyme that breaks down ceramide trihexose). Glomeruli, maybe brain; lamellar bodies on EM.

Fragile X syndrome: About half of cases of familial mental retardation. One guy in 1500, mildly to moderately dim. Enormous testes. Difficulty counting things. Maxilla gets longer as you hit the teenaged years. Dr. Bell, the discoverer, was a famous lady astronomer before turning to medicine.


Note: Copyrights: "The Pathology Guy" - Ed Friedlander MD. Reproduced with permission.



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