² Department of Neurology, Thomas Jefferson University, and Jefferson Medical College, Philadelphia, PA, USA
³ Department of Pediatric Neurology and Developmental Medicine, University Children’s Hospital, Tübingen, Germany

MLD and GLD

Metachromatic leukodystrophy (MLD) and globoid cell leukodystrophy (Krabbe disease, GLD) are autosomal recessively inherited disorders caused by the deficiency of arylsulfatase A (ASA) and galactosylceramidase (GALC), respectively. Since both enzymes depend on the assistance of activator proteins saposin B (ASA) and A (GALC), respectively, clinically indistinguishable diseases can also be caused by the deficiencies of these activator proteins (Figure 9.1). The pathological hallmark of both diseases is a loss of oligodendrocytes and thus myelin leading to multiple progressive and finally lethal neurologic symptoms [1, 2].

Case studies

Late-infantile MLD

A 3-year-old girl had an unrevealing family history, pregnancy and birth; her early development also was normal – she learned to sit within 7 months, crawled at 8 months, stood up with help at 10 months. Thereafter, however, motor development stagnated. At 18 months she learned to walk but was unstable. Language development was normal until then (first words at 12 months, two word combinations at 18 months). Eight months later, LD was diagnosed after demonstration of ASA deficiency. At 27 months she had a febrile infection and deteriorated thereafter, lost walking 1 month later, developed swallowing problems; she could no longer sit at 33 months and her speech became dysarthric; at 35 months she could no longer grasp, and also stopped speaking ; at 3 years, head control was lost as well as interest in her surroundings. She survived several years in this severely impaired condition. Her gross motor course is illustrated in Figure 9.2.

Adult MLD

A woman presented with clumsiness, dysfunctional and bizarre behaviour, memory deficits, and apathy at the age of 37 years. She had been working as a housewife and was described as well-organized and intelligent. At the age of 32 years she showed progressive apathy and loss of interest in daily living routines, including care of her three children. Memory functions deteriorated dramatically. She could recall neither the names nor birthdays of her relatives. She developed bladder incontinence. At the age of 37 she fell down stairs and developed a subdural haematoma. In the course of diagnostic procedures in a neurosurgery department she underwent MRI which revealed typical leukodystrophic alterations. Biochemical and molecular studies confirmed the diagnosis of MLD. Upon neurologic examination at that time she revealed no abnormalities, especially no clinical signs of neuropathy. Muscle tone and gait were normal. Cognitive and mental functions were severely impaired, with fluctuating vigilance and loss of concentration. Immediate and delayed verbal recall was impaired. She showed frontal signs with grasping and perseveration. Follow-up examinations at the age of 38 and 40 years revealed further cognitive decline but still no motor impairment.

Infantile GLD

At 4½ months of age a female infant came to our attention because of a 2-month history of apathy and poor motor development. At this time the infant was very irritable, crying almost 24 hours a day. The child was experiencing generalized twitching of the extremities and was hyper-extending the head. The hands were kept tightly clenched. She had unexplained elevations in her body temperature. By 6 months of age she had no purposeful movement and appeared to be blind. At 8 months of age seizures became frequent. She died at 9 months of age after a high fever that could not be controlled. This case is typical of many patients with the infantile form of GLD. The diagnosis of GLD was confirmed by the finding of very low GALC activity. While some live longer due to improved symptomatic care, the clinical regression in untreated infantile-onset GLD patients is rather typical.

Adult GLD

A woman, currently 48 years of age, was considered normal until about 28 years of age when she suffered the onset of lower extremity paresis with episodes of tripping and clumsiness on walking. Magnetic resonance imaging done at this time showed symmetrical white matter lesions affecting the posterior limb of the internal capsules, thalami, corona radiate, centrum semiovale and subependymal white matter (especially about the occipital horns). She underwent physiotherapy to help her legs but continued to experience spastic paresis with a clumsy gait and difficulty on rising from a squatting or sitting position. There was no obvious intellectual impairment. She is married, continues to work part time and is the mother of a healthy daughter born when she was 43 years old. Her sister, who is 1½ years younger, was considered normal until 4 to 5 years of age when she developed progressive weakness in all extremities. She experienced rapid mental deterioration and onset of seizures. She is currently wheelchair-bound and significantly mentally disabled. EMG studies performed when the younger sister was 37 years of age showed severe sensorimotor peripheral polyneuropathy with axonal and demyelinating features including median motor distal latency prolongation, median motor conduction velocity slowing, and prolongation of the median F-response. Similar studies performed at the same time in the older sister showed that the abnormalities were far less severe than in the more symptomatic younger sister. These sisters have the same low GALC activity and the same mutations in the GALC gene. Clearly other genetic and/ or environmental factors play a role in modifying disease onset and progression in the later-onset forms of Krabbe disease.

Figure 9.1 Enzymes and activator proteins involved in degradation of sulfatide and galactosylceramide. Sulfatide is hydrolyzed by arylsulfatase A (ASA) to yield galactosylceramide and sulfate. The activator protein saposin B solubilizes sulfatide and presents it to the enzyme. Sulfatide accumulates in the absence of either protein. This step is defective in metachromatic leukodystrophy (MLD). Galactosylceramide is cleaved into galactose and ceramide by galactosylceramidase (GALC). This enzyme depends on the assistance of saposin A. A deficiency of either of these proteins causes globoid cell leukodystrophy (GLD) (Krabbe disease).

Figure 9.2 Natural course of MLD. Typical course of gross motor function in a child with late-infantile MLD (see case history oflate-infantile MLD) described by a gross motor function classification, the GMFC-MLD [9] : first abnormalities at 18 months (walksunstable); once level 2 (walking only with aids) is attained, gross motor function rapidly deteriorates and at the age of 3 years, there isno gross motor function anymore.

Epidemiology

In the European population the incidence of MLD and GLD is estimated to be about 0.6 in 100,000 live newborns for each disorder [3]. Activator protein deficiencies are much rarer. Their incidence has not been exactly determined. In general, the diseases occur in all ethnicities. Higher incidences of MLD are found in Habbanite Jews (1:75), Christian Arabs (1:8,000), Eskimos and Navajo Indians. A higher frequency of GLD has been described in certain Druze and Arab villages in Israel (1:170) [4]. To initiate therapy before symptoms are obvious, New York State instituted a newborn screening test for Krabbe disease using dried blood spots in August 2006. Since that time over 1.2 million newborns have been tested, and 4 were found to have infantile Krabbe disease based on a combination of mutation analysis and follow-up enzyme testing.

Genetics

The genes for ASA and GALC are located on 22q13.31-qter (ASA) and 14q31 (GALC), respectively. Whereas the ASA is a small gene in which the 8 exons encompass only about 3 kb of genomic sequence, the 17 exons of the GALC gene span 56 kb.

More than 150 mutations are known in the ASA gene. Among Caucasians, only a few of them are frequent (c.459 + 1 g > a, p.426P > L, p.179I > S) accounting for ~ 60% of all alleles, whereas many of the other mutations were only found in a few, mostly single, families (Table 9.1). More than 110 mutations in the GALC gene are known. One mutation (p.57 G > S) in the GALC gene is common in southern Italy. Japanese patients display a different spectrum of genetic defects in both disorders. In the ASA gene this is a p.99 G > R substitution and for the GALC gene it is a deletion/insertion mutation c.683_694del12insCTC. Table 9.1 summarizes the most common disease-causing mutations found in patients with MLD and GLD.

In both diseases there is a limited genotype/phenotype correlation. In MLD, homozygosity for mutations which do not allow the synthesis of any functional enzyme, always cause the severe late infantile form of disease. The presence of one allele associated with residual enzyme activity mitigates the disease to the juvenile form, whereas homozygosity for two alleles with residual ASA activity in most cases will result in a late juvenile or even adult form of MLD (Figure 9.3). Thus, residual enzyme activity which, in late-onset patients, is in the range of 2–4% of normal, is one determinant of clinical severity. However, clinical variability in late-onset patients – even in siblings with identical ASA genotype – is considerable, showing that other genetic or epigenetic factors influence the disease course substantially. Therefore, a precise prediction of the individual course of disease based on genotype analysis is impossible.

Data on genotype/phenotype correlations in GLD are more limited than in MLD, but the data available suggests that correlations in GLD and MLD follow similar rules. The finding of certain mutations in the GALC gene, either homozygous or heterozygous with known mutations, can predict a severe phenotype in a newborn individual. The presence of other known mutations in the GALC gene will predict a later-onset form of GLD. Late-onset MLD patients frequently present initially with psychiatric symptoms rather than motor problems. This phenotype correlates with a certain genotype. Patients presenting psychiatrically are frequently heterozygous for a null allele and the p.179I > S amino acid substitution [5]. The molecular basis for this peculiar genotype/phenotype correlation remains unclear.

Table 9.1 Most common mutations and polymorphisms found in patients with MLD and GLD.