Autosomal Dominant Disorders

2021-02-24 12:00 AM

Familial hypercholesterolemia is the most common inherited disorder (incidence1 in 500) and is due to a mutation in the low-density lipoprotein (LDL) receptor gene (LDLR) on chromosome 19.

AUTOSOMAL DOMINANT DISORDERS

Familial hypercholesterolemia is the most common inherited disorder (incidence1 in 500) and is due to a mutation in the low-density lipoprotein (LDL) receptor gene (LDLR) on chromosome 19. The mutations in the LDL receptor cause increased levels of circulating cholesterol, loss of feedback inhibition of HMG-coenzyme A (HMG-CoA) reductase, and increased phagocytosis of LDL by macrophages.

There are 5 major classes of mutation.

  • Class I: no LDL receptor synthesis
  • Class II: defect in transport out of the endoplasmic reticulum
  • Class III: defect in LDL receptor binding
  • Class IV: defect inability to internalize bound LDL
  • Class V: defect in the recycling of the LDL receptor

 Clinical features include elevated serum cholesterol (heterozygotes have elevations of 2–3 times the normal level and homozygotes have elevations of 5–6 times the normal level), skin xanthomas (collections of lipid-laden macrophages), xanthelasma around the eyes, and premature atherosclerosis (homozygotes often develop myocardial infarctions in late teens and twenties).

Marfan syndrome is due to a mutation of the fibrillin gene (FBN1) on chromosome15q21. Fibrillin is a glycoprotein that functions as a scaffold for the alignment of elastic fibres.

Clinical features include skeletal changes (tall, thin build with long extremities, hyperextensible joints, pectus excavatum [inwardly depressed sternum], and pectus carinatum [pigeon breast]) and abnormal eyes (ectopia lentis, characterized by bilateral subluxation of the lens). The cardiovascular system is also particularly vulnerable; it may show cystic medial degeneration of the media of elastic arteries with a loss of elastic fibres and smooth muscle cells with increased risk of dissecting aortic aneurysm (a major cause of death), dilatation of the aortic ring potentially leading to aortic valve insufficiency, and/or mitral valve prolapse.

Ehlers-Danlos syndrome (EDS) is a group of inherited connective tissue diseases that have in common a defect in collagen structure or synthesis. Clinically, the disease causes hyperextensible skin that is easily traumatized and hyperextensible joints secondary to effects on the joints and adjacent ligaments.

There are a number of variants with different modes of inheritance.

  • Kyphoscoliotic EDS: autosomal recessive form
  • Vascular variant EDS: an autosomal dominant form that causes rupture of vessels and bowel wall
  •  Classical EDS: an autosomal dominant form that causes a type V collagen defect; patients have a normal lifespan

Neurofibromatosis

Type 1 (von Recklinghausen disease) neurofibromatosis (90% of cases) has an incidence of 1 in 3,000. The condition is due to a mutation of the tumour suppressor gene NF1 located on chromosome 17 (17q11.2). The normal gene product (neurofibromin)inhibits p21 Ras oncoprotein.

  • Affected individuals characteristically have multiple neurofibromas and benign tumours of peripheral nerves that are often numerous and may be disfiguring. The plexiform variant of the neurofibromas is diagnostic.
  • Rarely (3%), malignant transformation of a neurofibroma may occur.
  • Other clinical features include pigmented skin lesions (6 or more “cafe-au-lait spots” that are light brown macules usually located over nerves); pigmented iris hamartomas (Lisch nodules); and increased risk of meningiomas and pheochromocytoma, an adrenal tumour that also occurs with von Hippel-Lindau disease and MEN 2.

Type 2 (bilateral acoustic) neurofibromatosis (10% of cases) has an incidence of 1 in 45,000.

  •  There is a mutated tumour suppressor gene NF-2(22q12.2) on chromosome 22.
  •  The normal gene product (merlin) is a critical regulator of contact-dependent inhibition of proliferation.
  •  Clinical features include vestibular schwannomas (acoustic neuromas), and increased risk of meningioma and ependymomas.