MLD is directly caused by a deficiency of the enzyme arylsulfatase A and is usually characterized by enzyme activity which is less than 10% of human controls. Without this enzyme, sulfatides build up in many tissues of the body, eventually destroying the myelin sheath of the nervous system. The myelin sheath is a fatty covering that protects nerve fibers. Without it, the nerves in the brain (central nervous system - CNS) and the peripheral nerves (peripheral nervous system - PNS) which control, among other things the muscles related to mobility, cease to function properly.
A recent study contended sulfatide is not completely responsible for MLD because it is nontoxic. It has been suggested lysosulfatide, sulfatide which has had its acyl group removed, plays a role because of its cytotoxic properties in vitro.
MLD has an autosomal recessive inheritance pattern. The inheritance probabilities per birth are as follows:
If both parents are carriers:
25% (1 in 4) children will have the disorder
50% (2 in 4) children will be carriers (but unaffected)
25% (1 in 4) children will be free of MLD - unaffected child that is not a carrier
If one parent is affected and one is free of MLD:
0% (0) children will have the disorder - only one parent is affected, other parent always gives normal gene
100% (4 in 4) children will be carriers (but unaffected)
If one parent is a carrier and the other is free of MLD:
50% (2 in 4) children will be carriers (but unaffected)
50% (2 in 4) children will be free of MLD - unaffected child that is not a carrier
In addition to these frequencies there is a 'pseudo'-deficiency that affects 7%-15% of the population. People with the pseudo deficiency do not have any MLD problems unless they also have affected status. With the current diagnostic tests, Pseudo-deficiency reports as low enzyme levels but sulfatide is processed normally so MLD symptoms do not exist. This phenomenon wreaks havoc with traditional approaches to Newborn Screening so new screening methods are being developed.
The incidence of metachromatic leukodystrophy is estimated to occur in 1 in 40,000 to 1 in 160,000 individuals worldwide. There is a much higher incidence in certain genetically isolated populations, such as 1 in 75 in Habbanites (a small group of Jews who immigrated to Israel from southern Arabia), 1 in 2,500 in the western portion of the Navajo Nation, and 1 in 8,000 among Arab groups in Israel.
Symptoms and forms
Like many other genetic disorders that affect lipid metabolism, there are several forms of MLD, which are late infantile, juvenile, and adult.
In the late infantile form, which is the most common form of MLD (50-60%), affected children begin having difficulty walking after the first year of life, usually at 15–24 months. Symptoms include muscle wasting and weakness, muscle rigidity, developmental delays, progressive loss of vision leading to blindness, convulsions, impaired swallowing, paralysis, and dementia. Children may become comatose. Untreated, most children with this form of MLD die by age 5, often much sooner.
Children with the juvenile form of MLD (onset between 3 and 10 years of age) usually begin with impaired school performance, mental deterioration, and dementia and then develop symptoms similar to the late infantile form but with slower progression. Age of death is variable, but normally within 10 to 15 years of symptom onset although some juveniles can live for several decades or longer after onset.
The adult form commonly begins after age 16 as a psychiatric disorder or progressive dementia. Adult-onset MLD progresses more slowly than the late infantile and juvenile forms, with a protracted course of a decade or more.
Palliative care can help with many of the symptoms and usually improves quality of life and longevity.
Carriers have low enzyme levels compared to their family population ("normal" levels vary from family to family) but even low enzyme levels are adequate to process the body's sulfatide.
There is currently no treatment or cure for MLD. Children with advanced juvenile or adult onset and late infantile patients displaying symptoms receive treatment limited to pain and symptom management. Presymptomatic late infantile MLD patients, as well as those with juvenile or adult MLD that are either presymptomatic or displaying mild to moderate symptoms, have the option of bone marrow transplantation (including stem cell transplantation), which is under investigation to see if it may slow down progression of the disease or stop its progression in the central nervous system. However, results in the peripheral nervous system have been less dramatic, and the long-term results of these therapies have been mixed.
A team of international researchers and foundations organized in 2008 to form an International MLD Registry to create and manage a shared repository of knowledge, including the natural history of MLD. This consortium consists of scientific, academic and industry resources. This registry never became operational.
Bone marrow and stem cell transplant therapies
Two different approaches to gene therapy are currently being researched for MLD.
Gene therapy with an autologous stem cell transplant - Italian researchers at the San Raffaele Telethon Institute tested a novel approach combining gene therapy with a stem cell transplant. Recruiting for the Phase I/II Clinical Trial formally started on March 24, 2010 after approval from the Italian Authorities. Recruiting the initial cohort of 8 patients was completed in mid-March 2013. The trial was to test the efficacy and safety of autologous (using the patient's own cells) hematopoietic stem cell transplantation (HSCT) after genetic modification to deliver a super-therapeutic (over-expressing) ARSA enzyme to the nervous system by the route of the blood cells. Using the patient's own stem cells with genetic correction should reduce or eliminate the complications of graft vs. host disease and provide a long term solution to proper ARSA expression in MLD patients. Bench and animal tests showed positive results. The researchers published 2-year outcomes for the first three patients in July 2013. Results were described as promising.
Inclusion criteria are pre-symptomatic late infantiles and both pre- and early-symptomatic juveniles. See details on inclusion criteria and the trial protocol here.
The trial was single center in Milano, Italy. All costs were to be paid by the researchers. This was a 3-year study. In March 2013, the last of the 8 primary trial patients started therapy. A limited compassionate access period was underway.(Current March 2013)
Intracerebral Gene therapy - A Phase I/II Clinical Trial started recruiting in Paris in late March, 2013 for an Intracerebral Gene Therapy clinical trial where special "vectors" carrying genetically modified material are directly injected into a dozen sites in the brain. The hope is that the corrected cells and the enzyme they produce will then diffuse into surrounding areas of the brain. Extensive work in the lab and some encouraging ALD studies provided the basis for this trial.
Recruiting 5 late infantiles between the ages of 6 and 48 months for a two-year study
Must have first shown symptoms in the last 12 months
Additional trial inclusion criteria and information can be found here
1st symptoms before age 30 months, currently 7 years old or younger
Ambulatory - be able to stand up alone and walk 10 steps while holding only one hand.
Additional clinical trial information & inclusion criteria, can be found on the MLD Foundation website here and at the Clinical Trials.gov site.
The clinical trial is a 38 week multi-site European study of 18 children in three different dosing cohorts. The 'no treatment' placebo arm was removed from the trial in June 2012.
Patients must go to one of three trial site for their every other week enzyme infusions: Copenhagen, Paris or Tübingen, Germany.
HGT-1110 has orphan product status in both Europe and the United States.
History: Shire suspended development of the Metazyme intravenous ERT product in 2010. It was in clinical trial when it was acquired from Zymenex in 2008 (subsequently renamed HGT-1111 by Shire) after it was shown to not have sufficient efficacy by a Phase I/II clinical trial in Europe. The initial study completed September 2008 and the extension study completed October 2010 with the cessation of product supply to trial participants.
Biomarin South (formerly Zacharon before being acquired by Biomarin in January 2013) from San Diego has initiated a drug discovery program for MLD. This program is based on using assays which measure sulfatide accumulation in cultured fibroblasts as a means to discover and develop small molecule drugs for MLD. (This approach differs from other approaches which have measured enzyme activity to discover effective drugs.) As of July 2011, Zacharon has begun adapting the assays it developed for other lysosomal storage diseases so that they can be employed to discover and develop drugs for MLD. (current March 2013)
The Cooper Health System (New Jersey) sponsored a clinical trial underway to determine the safety and efficacy of a Vitamin K antagonist (Warfarin) in treating Metachromatic Leukodystrophy (MLD) in 2009. No results are known to have been published.(current March 2013)
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^Poeppel P, Habetha M, Marcão A, Büssow H, Berna L, Gieselmann V (March 2005). "Missense mutations as a cause of metachromatic leukodystrophy, Degradation of arylsulfatase A in the endoplasmic reticulum". FEBS J.272 (5): 1179–88. doi:10.1111/j.1742-4658.2005.04553.x. PMID15720392.
^Biffi A, Lucchini G, Rovelli A, Sessa M (October 2008). "Metachromatic leukodystrophy: an overview of current and prospective treatments". Bone Marrow Transplant. 42 Suppl 2: S2–6. doi:10.1038/bmt.2008.275. PMID18978739.
^Biffi, A.; Montini, E.; Lorioli, L.; Cesani, M.; Fumagalli, F.; Plati, T.; Baldoli, C.; Martino, S.; Calabria, A.; Canale, S.; Benedicenti, F.; Vallanti, G.; Biasco, L.; Leo, S.; Kabbara, N.; Zanetti, G.; Rizzo, W. B.; Mehta, N. A. L.; Cicalese, M. P.; Casiraghi, M.; Boelens, J. J.; Del Carro, U.; Dow, D. J.; Schmidt, M.; Assanelli, A.; Neduva, V.; Di Serio, C.; Stupka, E.; Gardner, J.; Von Kalle, C. (2013). "Lentiviral Hematopoietic Stem Cell Gene Therapy Benefits Metachromatic Leukodystrophy". Science341 (6148): 1233158. doi:10.1126/science.1233158.edit