Genetics & Counseling
March 3rd, 2009 |Identification of the ABCD1 gene
Genetic linkage with glucose-6-phosphate dehydrogenase (G6PD) pointed the X-ALD locus to the extremity of the long arm of the X-chromosome, Xq28. In 1993, the X-ALD gene (ABCD1) was identified using positional cloning strategies. The ABCD1 gene is approximately 20 kb long and contains ten exons.
ALDP belongs to the ATP-binding cassette (ABC) superfamily of transmembrane transporter proteins. The protein consists of 745 amino acids and contains a membrane domain with six transmembrane segments in the amino-half and an ATP-binding domain in the carboxy-half of the protein. Immunocytochemical studies demonstrated that ALDP is a peroxisomal membrane protein, which is in agreement with the observed biochemical abnormality in X-ALD patients (deficient peroxisomal beta-oxidation of saturated very long-chain fatty acids (VLCFA).
Mutations have been found in all X-ALD patients thoroughly examined. Complementation studies in fibroblasts derived from X-ALD patients demonstrated that expression of normal ABCD1 gene in patient cells restores VLCFA metabolism. Furthermore, stable expression of ALDP in X-ALD fibroblasts corrects VLCFA levels to normal levels.
The function of ALDP and its role in relation to either VLCFA metabolism was recently unveiled. ALDP is a transporter of VLCFA-CoA esters across the peroxisomal membrane. A deficiency in ALDP impairs VLCFA transport and two consequences: 1) reduced peroxisomal VLCFA beta-oxidation and 2) increased VLCFA-CoA esters in the cytosol which are substrate for further chain elongation to longer fatty acids.
Immunocytochemical studies have shown that in 70% of all X-ALD patients ALDP cannot be detected by immunofluorescence. All mutations other than missense mutations disrupt the stability of ALDP.
Genetic counseling
X-ALD is inherited in an X-linked recessive manner. The ABCD1 gene is the only gene associated with X-ALD.
All daughters of an affected male are carriers; none of his sons will be affected. A female who is a carrier has a 50% chance of transmitting the ABCD1 mutation with each pregnancy. Sons who inherit the mutation will be affected; daughters who inherit the mutation are carriers.
The range of phenotypic expression in X-ALD and the prognosis of an affected male is unpredictably variable and can NOT be predicted through levels of VLCFA in plasma or cultured skin fibroblasts, the residual VLCFA beta-oxidation activity present in X-ALD skin fibroblasts, the family history or the nature of the mutation identified in the ABCD1 gene of the patient. The same mutation can be associated with each of the known clinical phenotypes. Mild phenotypes may be associated with large deletions that abolish formation of the gene product, and severe phenotypes occur with missense mutations in which normal amounts of ALDP protein is produced. Widely varying clinical phenotypes often co-occur in a single kindred or sibship. The most common ABCD1 mutation, a two base pair deletion in exon 5 found in approximately 10% of X-ALD families, has been associated with all X-ALD phenotypes. The group from Dr. Johannes Berger has identified a family in which six affected members had five different phenotypes.
Segregation analysis suggests that the phenotypic variability is due to an autosomal modifier gene. However, unidentified environmental factors may also be involved, as indicated by phenotypic variability in a set of monozygotic twins.
It is important for couples at risk to be aware that widely varying phenotypes often coexist in the same kindred or sibship. Thus, families that have experienced the relatively mild phenotypes need to be advised that affected offspring may display the severe phenotype.
Mode of inheritance
Parents of a male or female proband: About 93% of index cases have inherited the ABCD1 mutation from one parent; at most, 7% of individuals with X-ALD have de novo mutations.
It is appropriate to measure plasma VLCFA concentration in the mothers of both affected males and carrier females and in the fathers of carrier females. When the disease-causing mutation has been identified in an affected family member, mutation analysis of the ABCD1 gene can be used in the evaluation of the parents.
Sibs of a proband: The risk to sibs depends upon the genetic status of the parents, which can be clarified by pedigree analysis, VLCFA measurement, and molecular genetic testing.
If the proband’s mother is a carrier, the chance of transmitting the disease-causing mutation in each pregnancy is 50%. Male sibs who inherit the mutation will be affected; female sibs who inherit the mutation will be carriers.
If the proband’s father has a disease-causing mutation in the ABCD1 gene, all of the female sibs will be carriers and none of the male sibs will be affected.
If neither parent is a carrier, the risk to sibs of a proband is low.
Offspring of a proband: Affected males transmit the ABCD1 mutation to all of their daughters and none of their sons.
Carrier females have a 50% chance of transmitting the ABCD1 mutation in each pregnancy. Sons who inherit the mutation will be affected; daughters who inherit the mutation are carriers and will usually not be seriously affected.
Other family members of a proband: Depending upon their gender, family relationship, and the carrier status of the proband’s parents, the proband’s aunts and uncles and their offspring may be at risk of being carriers or of being affected.
Evaluation of at-risk family members is important for management and genetic counseling but is often implemented insufficiently. Several factors may contribute to insufficient evaluation:
- Establishing the diagnosis in an affected individual with severe disability may be devastating to a family. Immediate concerns may overshadow the timely testing of family members.
- Molecular genetic testing has been clinically available for a short time; but its availability may not yet be generally known.
- Insurance companies may not cover the cost of testing at-risk family members.
- Some at-risk family members may choose not to be tested because they fear that a positive result might impair their ability to obtain or retain medical insurance coverage.
- Individuals may have incomplete knowledge about at-risk family members and may not wish to inform them about the risk.
Prevalence of X-ALD
The prevalence is estimated to be between 1:20,000 and 1:50,000, and appears to be approximately the same in all ethnic groups. The minimum frequency of hemizygotes identified in the United States is estimated to be 1:21,000 and that of hemizygotes plus heterozygotes 1:16,800.
