Handbook of Genetic Counseling/McCune Albright Syndrome

McCune Albright Syndrome

Introduction

• Establish rapport with small talk and introduce myself
• Briefly discuss the topics to be covered during the appointment
• Assess the concerns of the family and ask if they have any questions today

Elicit Family History Update

• Any changes in family history?
• Any new births or plans for pregnancy?

Definition

• McCune Albright syndrome is a disorder characterized by polyostotic fibrous dysplasia accompanied by café au lait spots and endocrine disorders esp. precocious puberty in girls. The patient is fracture prone, and deformity and shortening of the bones may develop.

Genetic Etiology

• Mode of inheritance:
  • Thought to be sporadic although there have been a few familial occurrences.
  • Believed to be a somatic mutation disorder
• Molecular Genetics:
  • Identification of activating mutations in the GNAS1 gene
• G proteins are involved in signal transduction pathways that affect the production of cAMP
• An overactive cAMP pathway stimulates growth and function of the gonads, adrenal cortex, specific pituitary cells, osteoblasts, and melanocytes
  • Mutations in the gene render it functionally constitutive.
  • Proposal that this a disorder of mosacicism (nonmocaicism is thought to be a lethal condition)
• Chromosome location
  • Chromosome 20

Incidence

• Described in 1936-1937 by McCune and Albright respectively- since then >100 cases have been reported
• The frequency of diagnosis in females vs. males is 3:2

Clinical Features

• Endocrine Abnormalities:
  • Precocious puberty occurs in over 50% of the female cases and has been reported for males as well
    • Menstruation, breast development, and pubic hair growth can occur during the first months of life
    • Usually caused by ovarian cysts that secrete estrogen into the blood stream
    • Many women with MAS are fertile in adulthood
  • Hyperthyroidism:
    • 50% of patients with MAS have thyroid gland abnormalities
    • Enlargement=goiter
    • Pituitary thyroid-stimulating hormone (TSH) levels are low in these patients, while thyroid hormone levels may be normal or elevated
    • Excessive secretion of growth hormone has been seen in a few patients
• Most develop course facies, enlarged hands/feet, and arthritis characteristic of acromegaly
• Associated with gigantism and gynecomastia
  • Adrenal Enlargement:
    • Rarely seen
    • Excessive secretion of the adrenal hormone cortisol caused by the hyperfunction of the anterior pituitary
    • Results in cushing syndrome
• Weight gain, obesity of the face and trunk, skin fragility and cessation of growth in children
• Treatment: removal of adrenal glands or use of drugs that block cortisol synthesis
• Bone Disease (polyostotic fibrous dysplasia):
  • Abnormal fibrous tissue growth in many bones
  • In affected areas, normal bone is replaced by irregular masses of fibroblast cells leads to deformity and/or thickening of the bones
    • When this occurs in weight bearing bones such as the femur, limping, deformity, and fractures can result
    • Many children have unequal arm/leg length even in the absence of fractures
  • Regions of fibrous dysplasia are also common in the skull and jaw bones
    • May result in facial asymmetry
  • Severity of bone involvement varies
  • Know known hormonal or medical treatment has been proven effective.
  • Some surgical procedures such as grafting, pinning, and casting have been used to correct fracture and deformity (skull and jaw changes are often corrected surgically to improve appearance)
• Skin abnormalities:
  • Flat areas of increased pigment may be found = café au lait spots
  • These spots are usually present at birth, and they usually do not become more extensive
  • The pattern of pigmentation is unique
    • Starts or ends abruptly at the midline on the abdomen in front or at the spine in the back
    • A few have spots confined to small areas such as the nape of the neck or crease of the buttocks

Natural history

• Pigmentation is usually evident in infancy
• Bone dysplasia may progress throughout childhood
• The sexual precocity is often unusual, with menstruation occurring before breast and pubic hair development.
• Precocious puberty may lead to short stature

Testing

• Currently, testing for a mutation in the GNAS1 gene is done for research purposes.
• It is not a routine test that can be ordered.

Differential Diagnosis

• Hemihypertrophy, a sporadic condition, has as its most significant feature- limb length asymmetry and sometimes, facial asymmetry. Between 5-8% of these children will develop an intra-abdominal tumor, either Wilms tumor of the kidney or hepatoblastoma.

Psychosocial Assessment

• How are things at home?
• Does mom work outside of the home?
• Financial concerns?
• How is the family adjusting?

Support Groups

• The Paget Foundation

120 Wall Street

New York NY 10005

2125095335

8002372438

e-mail: pagetfdn@aol.com

Home Page: http://www.paget.org

• MAGIC Foundation for Children's Growth

1327 North Harlem Avenue

Oak Park IL 60302

7083830808

8003624423

e-mail: mary@magicfoundation.org

Home Page: http://www.magicfoundation.org

• NIH/National Institute of Child Health and Human Development

31 Center Dr

Bethesda MD 20892

3014965133

References

• OMIM
• Jones KL (1997). Smith's Recognizable Patterns of Human Malformation. Philadelphia: W.B. Saunders Company.
• www.magicfoundation.org/mas

Notes

The information in this outline was last updated in 2002.