Clinical features of PMD
Pelizaeus-Merzbacher disease (PMD), named after two German physicians who first described its most important clinical features, is a rare condition caused by mutations affecting the gene for proteolipid protein 1 (PLP1, formerly called PLP). The PLP1 gene lies on the X chromosome so that most affected individuals are males who inherit the mutant or abnormal gene from their mothers. Rarely, females can have symptoms. Clinically, Pelizaeus-Merzbacher disease usually begins during infancy and signs of the disease may be present at birth or in the first few weeks of life. The first recognizable sign is a form of involuntary movement of the eyes called nystagmus. The eye movements can be circular, as if the child is looking around the edge of a large circle, or horizontal to-and-fro movements. The nystagmus tends to improve with age. Some infants have stridor (labored and noisy breathing). Infants may show hypotonia (lack of muscle tone; floppiness) at first, but most eventually, over several years, develop spasticity (a type of increased muscle tone or stiffness of the muscles and joints). Motor and intellectual milestones are delayed, however the intellectual delay is often more apparent than real, if care and time are taken to evaluate the children. Most PMD individuals learn to understand speech, but verbal output can vary from normal speech to almost complete mutism. Head and trunk control may be a problem and wavering or tremor of the upper body (titubation)when sitting is common. Trouble with coordination (ataxia) is also common, and dexterity of the arms and fingers is usually reduced. Vision is usually reduced to some degree, probably from the effects of the myelin abnormality, but also from the nystagmus as well.
Although the following terms are somewhat artificial, they are used in many textbooks and medical reports. Connatal PMD refers to the most severe form of the disease, with neurological signs, such as nystagmus, stridor and hypotonia, being noticeable from birth to within the first few weeks of life. Seizures may occur only in these children. These children usually are unable to talk or walk, although they may comprehend quite well. The Classical PMD syndrome is the most commonly seen form of the disease. Nystagmus usually begins in the first 2 months to 6 months. Later, delay in the usual developmental milestones, such as rolling over, sitting up, standing, walking and speech are seen. Muscle tone may be hypotonic, although this is not as noticeable as in the connatal child. Most of these children do learn to talk, although they may have slurred speech (dysarthria). Some of these children learn to walk with assistance, such as walkers, but most are not able to. Virtually all PMD patients have ataxia. We now know that some mutations of the PLP1 gene may result in a less severe syndrome, called spastic paraparesis (weakness and stiffness of the legs) or SPG2, where the major sign is gait difficulty due to weakness and spasticity of the legs. One family has been reported with a mutation that causes tremor and/or attention deficit disorder as the major abnormalities. Peripheral nerve myelin is usually not affected, however we have discovered that in the rare families whose mutations prevent the synthesis of any PLP1 (the PLP1 null syndrome) have a mild peripheral myelin disorder, but have less severe overall neurologic difficulties.
The clinical diagnosis generally includes the clinical findings listed above along with a family history consistent with X chromosome transmission (that is, being passed down by mothers, and never being passed from an affected father to his son). The most useful screening test after the neurologic examination and family history, is a brain magnetic resonance imaging (MRI) scan, which is a very sensitive test for leukodystrophies (diseases of the white matter), most reliable if it is done after one or two years of age (the times when the major white matter pathways in the brain are developing). Other tests to exclude other leukodystrophies such as the lysosomal storage diseases (such as metachromatic leukodystrophy, Salla disease and Krabbe disease) and adrenoleukodystrophy should also be done. Evoked potentials testing is also helpful and should show abnormal central conduction but normal or near normal peripheral conduction. The definitive test is demonstration of a pathologic mutation of the PLP gene.
There are two major aspects of the disease that are important to really understand it. The first is the genetics of PMD and the second relates to the effect of PLP1 mutations on the nervous system. First I'll describe the genetics. PMD occurs when there is a change (or mutation) in the body's "blueprint" material. These blueprint materials, called genes, control the way a body is made, what it looks like, and how it works. Most genes come in pairs. One gene of each pair comes from the mother's egg and the other from the father's sperm. In the tens of thousands of gene pairs, sometimes one will be changed. The mutation may be inherited or may happen by itself. Sometimes a mutated gene will not cause problems. Other times a gene with a mutation will cause the body not to work correctly, and that a person will have a genetic condition such as PMD. Genes are carried on chromosomes. Most individuals have 46 chromosomes in each cell in their body. The chromosomes come in 23 pairs with the first 22 pairs being identical in males and in females. The last pair is the sex chromosomes; females have two X chromosomes, while males have one X and one Y chromosome. The chromosome can be thought of like a bookcase and the gene as a book located on the bookcase. DNA (deoxyribonucleic acid) which is the basic component of the gene, is like the letters in the book. Genetic information is stored, and passed down from generation to generation, in the form of the precise sequence of DNA letters or bases.
Since the gene for PMD is located on the X chromosome, the disease typically affects only boys or men in a family. Technically, this is called X linked inheritance. Remember that females have two X chromosomes while males have one X and one Y chromosome. If there is a gene on the X chromosome which is not working properly, males will be affected more often than females, since females likely have a gene on the other X chromosome which does work properly and this usually compensates for the defective X chromosome. Females who carry the gene for PMD therefore typically are not affected since the PLP gene on the other X chromosome is normal. Males with PMD are usually not able to have children, so the disease when it occurs in several generations is passed on by women who are carriers for the PMD mutation. Women who carry the PMD gene have a 50% or 1 in 2 chance of passing it on to their sons and their daughters. These odds are the same for every pregnancy. What happened in one pregnancy does not in any way influence the odds for the next pregnancy. Sons who inherit the gene would be affected, whereas daughters would be carriers. If a daughter did not inherit the PMD gene, then she would not pass PMD on to her children.
Basic molecular biology
Deoxyribonucleic acid (DNA), which carries the instructions that instruct cells to make proteins, is made up of four chemical bases or letters, abbreviated C, T, G, and A (for cytosine, thymidine, guanine and adenine). A DNA molecule is simply a long chain of these bases strung together. The information is the sequence of bases. This is like all the information stored in a book in the order of specific letters of the alphabet, or the information on a computer disk represented by a long string of zeroes and ones. In fact, each chromosome is basically a single molecule of DNA. The largest human chromosome (the first) has about 120,000,000 bases.
A mutation (any alteration of the DNA) that affects only a single base (one letter) is called a point mutation. Other types of mutations can occur as well, including insertions (additions of DNA into a gene), deletions (removal of part of a gene), and duplications where entire genes are present in one or more additional copies. The gene responsible for PMD is the proteolipid protein 1 gene (PLP1) and it is located on the X chromosome.
The types of mutations that are known to cause PMD fall into two general categories: point mutations and duplications. In just the past few years it has been discovered that most PMD is caused by duplications (or rarely triplication or even quintuplication) of the entire PLP1 gene. This seems to be the case for PMD families around the world and we still do not understand why it occurs. The duplications appear to account for about 50 to as much as 75 % of those families with PMD. We currently believe that the duplication results in too much otherwise normal proteolipid protein being made. Furthermore, this excessive PLP1 is toxic to the cells (called oligodendrocytes) trying to make myelin. There can be quite a lot of difference in the neurologic difficulties between families with duplications. One reason for this may be due to the differences in the size of the duplication in different families. While we believe that members of the same family will have the same size duplication, there is know to be a big difference in duplication size between different families. The smallest duplications known are around 100,000 DNA bases in length, but the biggest ones found so far are around 5 million bases. The PLP1 gene is about 30,000 bases long. Other factors that may explain the differences in families are what genes other than PLP1 are also duplicated, and whether some of these genes that come before or after PLP on the X chromosome are mutated by the duplication. Further research will be needed to understand the variability between families (and even within families) affected by PMD.
PLP1 point mutations
Point mutations are usually mistakes in the gene where one of the bases or 'letters' is replaced by the wrong one (technically called a base substitution). Depending upon where the letter is and what it is replaced by, the mutation could result in:
- No effect
- One amino acid in the protein encoded by the gene is replaced by the wrong amino acid (amino acid substitution).
Depending on the place and nature of the amino acid substitution, these mutations can have mild or severe effects. PLP1 with just one wrong amino acid at a critical location is toxic to myelin forming cells, just as is overabundance of normal PLP1 (and may even be more toxic than overabundance)
- The protein is prematurely terminated (ends at the wrong place)
- Disturbance in the regulation of the gene
- Disturbance in splicing of the gene
Mutations can also result in the gain or loss of more than one base. If this occurs in the region of the gene that codes for protein then this might not only result in the gain or loss of one or more amino acids in the protein, but also might cause the protein to be completely disturbed after the place the mutation occurs because the machinery that decodes the genetic information into protein (called ribosomes) gets out of register with the proper code and just makes scrambled protein after the mutation site.
Since there are only 4 letters in the genetic alphabet and they are read in words 3 letters long, there are 64 possible genetic words or codons possible. Of these 64 codon possibilities, 61 of them code for one of 20 possible amino acids. The remaining 3 codons are called termination codons and tell the protein synthesis machinery to stop making protein. Notice that there are more codon possibilities than there are amino acids. Some amino acids have more than one codon that can encode them, whereas others have only one or two codon possibilities. Proteins are simply chains of amino acids hooked together like beads on a chain.
To make a simple analogy, take the following simple sentence:
The red fox ran far and sat.
Now if one of the letters is mistyped, like what happens with a base substitution mutation, the meaning of the sentence changes:
The red sox ran far and sat.
These missense mutations may sometimes not be harmful or cause mild disease, but if they occur at an important location in the protein can be quite harmful.
If, as in the case of a base deletion, all the words get jumbled up after the mutation, because the protein synthesis machinery has to read the code three letters at a time (these are called frame shift mutations):
The red oxr anf ara nds at.
The severity of this type of mutation depends mostly upon where the mutation is located. If the frame shift occurs at the end of the gene, it may not cause severe problems, whereas a mutation near the beginning of the gene will typically have severe consequences.
Although not strictly point mutations, the effects of mutations that delete or insert a small number (for example two to a couple of dozen) of bases, are similar to what happens with single base mutations.
Many PLP1 mutations have been identified. Most of these point mutations are unique to a specific family. Since these are unique mutations, it is not easy to predict for a PMD patient with one of these mutations what will happen over the course of his life, especially if there is no prior history of the disease in the family. A major goal of genetic research on PMD focuses on the clinical signs caused by specific mutations in PLP1. This is called genotype-phenotype correlation.
To make matters even more complicated, we now know that most genetic information coding for proteins is broken up into chunks that are separated, sometimes by very large distances, from each other. These chunks are called exons, and the DNA segments that separate the exons are called introns. The genetic information in the nucleus of a cell is first transcribed to molecules of ribonucleic acid (RNA), then the introns are removed from the RNA to generate the messenger RNA (mRNA) molecules that have all the protein coding information nicely spliced together. The mRNA then leaves the nucleus to serve as the blueprint for the protein synthesis machinery in the cytoplasm (the rest of the cell that surrounds the nucleus) of the cell.
We know that the PLP1 gene is broken up into 7 exons, and it turns out that one of the exons (the third one) sometimes is partially spliced out, resulting in a protein that looks like PLP, but is missing 35 amino acids in the middle of the protein. The smaller protein is called DM20. There are some PMD causing mutations that affect how the PLP1 mRNA is spliced together.
We also know that in addition to the regions that code for protein, there are regions of genes that regulate their expression. In order for the right proteins to be made in the right organs and in the right amounts, there are many processes that have to be regulated very precisely. One important type of regulation occurs in the nucleus, which has to decide which genes to turn on and which to turn off, and by how much. Some DNA sequences that lie near but usually outside of the protein coding regions function to regulate gene expression or transcription into RNA. Mutations that change these regulatory sequences can have drastic affects on the gene, and might result in the protein being made in too high or too low an amount, or to be made in the wrong organ or at the wrong time of life.
PLP1 and myelin
PMD is one of the leukodystrophies, disorders that affect the formation of the myelin sheath, the fat and protein covering--which acts as an insulator--on neural fibers (axons) in the central nervous system or CNS, which is the brain and spinal cord. About 75 % of myelin is made up of fats and cholesterol and the remaining 25 % is protein. PLP1 constitutes about half of the protein of myelin and is its most abundant constituent other than the fatty lipids. New experiments indicate that about half or more of affected individuals have a duplication of an otherwise normal PLP1 gene. Thus, it appears that the presence of too much PLP1 in oligodendrocytes, the cells that make myelin in the central nervous system, is harmful. The point and other small mutations usually cause the substitution of one of the amino acids for another somewhere in the protein or prevent PLP1 from reaching its full length. This probably results in the protein being unable to fold into the correct shape or to interact with other myelin constituents. These mutant proteins are toxic to oligodendrocytes and prevent them from making normal myelin.
Unfortunately, there is currently no cure for Pelizaeus-Merzbacher disease, nor is there a standard course of treatment. Gene therapy and cell transplantation are being explored as possible therapies. For now, however, treatment is symptomatic and supportive, and may include medication for seizures and the stiffness or spasticity that most PMD patients have. Physical therapy can be helpful in maintaining strength and joint flexibility, and occupational therapy is helpful in maximizing the abilities of a PMD patient. Braces or walkers may enable a child to walk. If speech or swallowing is impaired, a speech/swallowing therapist should be able to provide important guidelines to make speech more understandable and to prevent choking. Orthopedic surgery may help reduce contractures, or locked joints, that can result from spasticity. A physical medicine specialist (also known as physiatrist or rehab doctor) may be the most effective physician in evaluating a child's needs and coordinating all the different therapists. A developmental pediatrician should also evaluate each child to assess his abilities and help to design an educational curriculum to maximize his learning and potential. It is important in these developmental assessments to factor in the longer time it takes a PMD child to process information, and also to factor in the motor limitations most kids with PMD have. Periodic developmental assessments should be done to monitor each child's progress.
Once a PLP1 gene mutation is identified in a family, it is possible to test family members for the mutation and to provide prenatal diagnosis for parents who have a risk of transmitting this disorder. Such testing, especially for a couple planning a family, or for a woman who wants to know whether she is a carrier, should be done under the guidance of a medical geneticist and/or genetic counselor. Carrier testing is usually deferred until the female is 18 years of age. It is now possible to do preimplantation genetic testing (PGD) for PMD, but this is often not covered by health insurance.
The prognosis for those with Pelizaeus-Merzbacher disease varies. Some mutations are more severe than others and may result in death during childhood, but most live into adulthood. Survival into the sixties has been seen. The course of the disorder is usually very slow, with some individuals reaching a plateau and remaining stable for years. However, some do worsen over time, for reasons that we do not understand, and will need further research.
A international group of clinicians and researchers working on Pelizaeus-Merzbacher disease and proteolipid protein has been organized to promote research to facilitate understanding of disease pathogenesis and development of specific treatments and, we hope, a cure. In North America, please contact James Garbern for more information.
These articles, available from a medical library, are sources of in-depth information on Pelizaeus-Merzbacher disease:
Boulloche, J. and Aicardi, J. Pelizaeus-Merzbacher disease: clinical and nosological study. Journal of Child Neurology 1:233-9 (1986) [Abstract].
Cailloux, F. et al. Genotype phenotype correlation in inherited brain myelination defects due to proteolipid protein gene mutations. European Journal of Human Genetics 8:837-845 (2000) [Abstract].
Cambi, F. et al. Refined genetic mapping and proteolipid protein mutation analysis in X-linked pure hereditary spastic paraplegia. Neurology 46:1112-7 (1996) [Abstract].
van der Knaap, M and Falk, J. The reflection of histology in MR imaging of Pelizaeus-Merzbacher disease.
AJNR Am J Neuroradiol. 10(1):99-103 (1989). [Abstract].
Garbern, J. PLP1-related disorders, Genereviews (2004).
Garbern, J. Pelizaeus-Merzbacher disease, eMedicine (2005).
Garbern, J., Cambi, F., Shy, M. and Kamholz, J. The Molecular Pathogenesis of Pelizaeus-Merzbacher disease. Archives of Neurology 56:1210-1214, (1999) [Abstract].
Garbern, J., Cambi, F. et al. Proteolipid protein is necessary in peripheral as well as central myelin. Neuron 19:205-218 (1997) [Abstract].
Gencic S, Abuelo D, Ambler M, Hudson LD. Pelizaeus-Merzbacher disease: an X-linked neurologic disorder of myelin metabolism with a novel mutation in the gene encoding proteolipid protein.
Am J Hum Genet. 1989 Sep;45(3):435-42 (1989) [Abstract].
Gow, A. and Lazzarini, R. A cellular mechanism governing the severity of Pelizaeus-Merzbacher disease. Nature Genetics 13:422-428 (1996) [Abstract].
Hudson LD, Puckett C, Berndt J, Chan J, Gencic S. Mutation of the proteolipid protein gene PLP in a human X chromosome-linked myelin disorder. Proc Natl Acad Sci U S A. 86:8128-31 (1989) [Abstract]
Inoue, K et al. A duplicated PLP gene causing Pelizaeus-Merzbacher disease detected by comparative multiplex PCR. Am J Hum Genet. 59:32-9 (1996) [Abstract].
Mimault, C. et al. Proteolipoprotein gene analysis in 82 patients with sporadic Pelizaeus-Merzbacher disease: duplications, the major cause of the disease, originate more frequently in male germ cells, but point mutations do not. American Journal of Human Genetics 65:360-369 (1999) [Abstract].
Seitelberger, Franz, Urbanits, S. and Nave, K.-A. Pelizaeus-Merzbacher disease. Handbook of Clinical Neurology, vol. 22 (66) new series, H. Moser, ed. Elsevier Science, Amsterdam, (1996).
Trofatter JA, Dlouhy SR, DeMyer W, Conneally PM, Hodes ME. Pelizaeus-Merzbacher disease: tight linkage to proteolipid protein gene exon variant. Proc Natl Acad Sci U S A. 86:9427-30 (1989) [Abstract].
Wolf NI, Sistermans EA, Cundall M, Hobson GM, Davis-Williams AP, Palmer R, Stubbs P, Davies S, Endziniene M, Wu Y, Chong WK, Malcolm S, Surtees R, Garbern JY, Woodward KJ. Three or more copies of the proteolipid protein gene PLP1 cause severe Pelizaeus-Merzbacher disease.
Brain. 128:743-51 (2005) [Abstract]
Woodward, K. and Malcolm, S. Proteolipid protein gene: Pelizaeus-Merzbacher disease in humans and neurodegeneration in mice. Trends in Genetics, 5:4:125-128 (1999) [Abstract].
Yool, DA, Edgar, JM, Montague, P and Malcolm, S. The proteolipid protein gene and myelin disorders in man and animal models. Human Molecular Genetics 9:987-992 (2000) [Abstract].
Additional information is available from the following organizations and individuals:
Ms. Patti Daviau
525 S. Harris
Indianapolis, IN 46222
Ms. Laura Spear
2 John James Audobon
Marlton, NJ 08053
The PMD Foundation, Inc.
The Myelin Project
European Leukodystrophy Association
Nat. Org. for Rare Disorders (NORD)
P.O. Box 8923
New Fairfield, CT 06812-1783
Hunter's Hope Foundation
PO Box 643
Orchard Park, NY 14127
Toll Free: 1-877-984-HOPE
Association for Neuro-Metabolic Disorders
c/o 5223 Brookfield Lane
Sylvania, OH 43560
United Leukodystrophy Foundation
2304 Highland Drive
Sycamore, IL 60178
Nat. Tay-Sachs & Allied Diseases Assoc.
2001 Beacon St., Ste.
204 Brookline, MA 02146
The National Human Genome Research Institute has a great deal of information on a wide variety of genetics topics that you might find useful.
The Public Broadcasting System has a nice site that help explain genetics: The Human Genome
The National Center for Biotechnology Information has excellent online textbooks.