Genetic Analysis in the Methylmalonic Acidurias

Genetic analysis in the Methylmalonic Acidurias

By David S. Rosenblatt, MD

Professor of Human Genetics, Medicine, Paediatirics and Biology

McGill University 687 Pine Ave West H5-63 Montreal, Canada H3A IAI

Phone (514) 842-1231 x 5571

Fax (514) 843-1712

E-mail MC74@Musica.McGill.CA

Extracted from OAA News December 1997 with permission

 

My laboratory often receives cultured fibroblasts (skin tissue) from patients around the world who excrete methylmalonic acid in their urine.  The approach that we use combines the disciplines of biochemical, somatic cell, and molecular genetics.

Clinically, there are a number of important questions that we want answered before we undertake our diagnostic studies.  The first relates as to whether there are elevated levels of the chemical homocystine in the blood or urine in addition to methylmalonic acid.  The second is whether the levels of the metabolites (homocystine and methylmalonic acid) in the blood and urine goes down in the patients after treatment with vitamin B12 (Cobalirnin).  Usually we prefer that hydroxycobalamin (a specific form of vitamin B 12) be used to see if there is a response.  Of course we want to know about other patients in the family who have had methylmalonic acid excretion in the urine.

It is not uncommon for us to receive skin cells from infants who have transient excretion of methylmalonic acid because they were exclusively breast fed by mothers who themselves were vitamin B12 deficient.  These infants will have abnormally low levels of vitamin B12 in their blood.  They also may excrete homocystine in their urine.  These children improve very quickly on vitamin B12 treatment, but may have long-term effects if not treated early.  They do not have a genetic disease and study of their cultured cells will be normal.

  The tests described below are used to determine what form of MMA the patient has and to see which type of treatment is desired, low protein diet and/or vitamin B12 supplementation.  For patients who excrete both homocystine and methylmalonic acid in their urine, we look in cultured skin cells at the incorporation of radio labelled propionic acid (see note A) as a marker for the methylmalonyl coA mutase enzyme (functional deficiency of the mutase results in methylmalonic aciduria) and at the incorporation of radio labelled methyltetrahydrofolate (see note B) as a marker for the methionine synthase enzyme (functional deficiency of the methionine synthase results in homocystinuria).  Patients with a pure methylmalonic aciduria usually have low incorporation of propionate and normal incorporation of methyltetrahydrofolate.

If the incorporation of propionate is low in cultured cells, we re-do the study in the presence of vitamin B12 in the culture medium.  Patients with cblA and cbIB types of vitamin B12 responsive methylmalonic aciduria will increase their propionate uptake in the presence of the vitamin, but not to normal levels.  Patients with muto methylmalonic aciduria do not increase their levels of propionate uptake in the presence of vitamin B 12, and patients with muto- methylmalonic aciduria have a slight increase of propionate uptake in the presence of vitamin B 12.

The classification of patients into mut, cblA or cbIB types of methylmalonic aciduria is based on complementation studies.  Briefly, cells from an undiagnosed patient with low propionate incorporation are grown together in culture with cells from patients with known diagnoses in the presence of a chemical (PEG) which causes cells to fuse together.  Cells from a patient from a different complementation class will correct the defect in propionate incorporation in the patient, but cells from a patient with the same defect will not.

These studies in cultured cells can be useful in predicting outcomes in groups of patients.  For example, as a group the cblA patients are the mildest followed by the cbIB, the mutant then the muto patients.

The genes responsible for the cblA and cbIB forms of methylmalonic aciduria are not known.  However patients with mut methylmalonic aciduria have mutations in the mut gene on the short arm of chromosome 6. Many of the patients with mut- methylmalonic aciduria have mutations in the vitamin B12 binding region of the mutase gene.  A few more than 20 mutations have been identified to date.  With the exception of two mutations (one in 6 Japanese patients and one in 5 African or African-American patients), most mutations are not seen in multiple patients with methylmalonic aciduria. (In other words, most patients with methylmalonic aciduria have unique mutations).  When both mutations are known in a family, molecular analysis can be done for carrier detection or prenatal diagnosis.  This is still done only by a few research laboratories.  For most families, prenatal diagnosis is still performed by looking at propionate incorporation in cultured amniotic fluid cells.

It is to be hoped that, as we know more ~ the mutations causing methylmalonic aciduria, we will be in a better position to adjust therapy and improve outcomes.

Note A: Propionic acid is connected to a compound called succinyl coA by the enzyme methylmalonyl coA mutase and this then goes on to make energy for the body.  If this enzyme does not function properly, then the propionic acid will change into methylmalonic acid, which accumulates in blood and urine.

Note B: Methyltetrahydrofolate is needed to activate the enzyme methionine synthase, which converts homocystine to the protein methionine.  If this process does not work, then the homocystine will accumulate ate in the blood and urine.

References:

Shevell MI, Matias--uk N, Ledley FD and Rosenblatt DS; Varying neurological phenotypes among muto and mut- patients with methylmalonyl coA mutase deficiency.  Am J. Med.  Genet. 45 "616-624, 1993

Ledley FD and Rosenblatt DS, Mutations in mut methylmalonic academia. clinical and enzymatic correlations, Hum.  Mut. 9: 1-6, 1997.

  6   December, 1997

   

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