C. P. Morris

Long-term clinical progress in bone marrow transplanted mucopolysaccharidosis type I patients with a defined genotype

SummaryTwo mucopolysaccharidosis type I (MPS-I) patients, subjected to bone marrow transplantation (BMT) more than 10 years ago, have recently had their α-L-iduronidase genotypes defined. Both patients, homozygous for the relatively common W402X mutation, received BMT when they were 14 and 11 months of age, and are now 12 and 14 years old, respectively. Untreated MPS-I patients, homozygous for W402X, have an extremely severe clinical phenotype with rapid clinical deterioration and death before 6 years of age. The 12-year-old patient, with limited mobility, is coping well at school, while the other patient is wheelchair-bound with severe disability in his lower limbs, and attends a school for the physically handicapped. Both patients have less than normal intelligence with slowly continuing losses. A third MPS-I patients, diagnosed at the age of 6 months, was felt, prior to BMT at 14 months, to have a severe phenotype. Twelve years post-BMT, he is ambulatory, albeit with restricted movement, and has normal intelligence. This patient did not have a defined MPS-I genotype and had α-L-iduronidase protein and activity consistent with a less severe outcome than the first two patients. We conclude that BMT has significantly slowed down the clinical regression of the W402X phenotype. We propose that if further gains are to be made, BMT should be performed within the first few months of life. Early diagnosis is therefore essential.

Population frequency of the arylsulphatase A pseudo-deficiency allele

SummaryThe enzymatic diagnosis of metachromatic leukodystrophy is complicated by the frequent occurrence of the pseudo-deficiency of arylsulphatase A (ASA) enzyme activity. An A to G nucleotide transition in the first polyadenylation signal of the ASA gene results in the loss of its major mRNA species and a greatly reduced level of enzyme activity. This nucleotide change (nucleotide 1620 of the ASA cDNA) is the cause of ASA pseudo-deficiency and is closely linked to another A to G transition (nucleotide 1049), within the ASA gene, which changes Asn350 to serine but which does not affect ASA activity. The distribution of these 2 nucleotide changes has been investigated in 73 unrelated individuals from the Australian population. The two transitions were found together on 14 (9.6%) out of 146 chromosomes. The transition at nucleotide 1620 was not found alone; however, the other transition was found alone on 7 (4.8%) out of the 146 chromosomes. The carrier frequency of the ASA pseudo-deficiency mutation in Australia is thus estimated to be about 20%.

Huntington disease-linked locus D4S111 exposed as the alpha-L-iduronidase gene.

Abstractα-l-Iduronidase (IDUA) has been intensively studied due to its causative role in mucopolysaccharidosis type I (Hurler, Scheie and Hurler/Scheie syndromes). The recent cloning of a human IDUA cDNA has resulted in a reevaluation of the chromosomal location of this gene. Previously assigned to chromosome 22, IDUA now has been localized to 4p16.3, the region of chromosome 4 associated with Huntingtons disease (HD). The existence of a battery of cloned DNA, physical map information, and genetic polymorphism data for this region has allowed the rapid fine mapping of IDUA within the terminal cytogenetic band of 4p. IDUA was found to be coincident with D4S111, an anonymous locus displaying a highly informative multiallele DNA polymorphism. This map location, 1.1×106 bp from the telomere, makes IDUA the most distal cloned gene assigned to 4p. However, it falls within a segment of 4p16.3 that has been eliminated from the HD candidate region, excluding a role for IDUA in this disorder.

Mucopolysaccharidosis type I (Hurler syndrome): linkage disequilibrium indicates the presence of a major allele.

SummaryTwo polymorphisms exist in the α-l-iduronidase (IDUA) gene, the gene that is defective in mucopolysaccharidosis type I (MPS I), viz. aKpnI polymorphism and a variable number of tandem repeats (VNTR) polymorphism with three common alleles. The analysis of allele and haplotype frequencies for these two polymorphisms in the normal population and in MPS I patients revealed the presence of linkage disequilibrium. The frequency of the 2,2 (VNTR,KpnI) allele in MPS I patients was 57% compared with only 37% in the normal population. The implications for the presence of a major MPS I allele and the ability to predict patient phenotype are discussed.

Gene diagnosis and carrier detection in hunter syndrome by the iduronate-2-sulphatase cDNA probe

Hunter disease (McKusick 309900) is an X-chromosomal mucopolysaccharidosis due to deficiency of the lysosomal enzyme iduronate-2-sulphatase (IDS; EC 3.1.6.13). Diagnosis is based on both the typical clinical features of patients and the lack/reduction of IDS activity. Female carriers show no symptoms of the disease. In the past, several different assays were elaborated for measuring enzyme activity in carriers but none of them proved to be suitable for detecting heterozygotes reliably (Zlotogora and Bach 1984)

Linkage homogeneity near the fragile X locus in normal and fragile X families

The fragile X syndrome locus, FRAXA, is located at Xq27. Until recently, few polymorphic loci had been genetically mapped close to FRAXA. This has been attributed to an increased frequency of recombination at Xq27, possibly associated with the fragile X mutation. In addition, the frequency of recombination around FRAXA has been reported to vary among fragile X families. These observations suggested that the genetic map at Xq27 in normal populations was different from that in fragile X populations and that the genetic map also varied within the fragile X population. Such variability would reduce the reliability of carrier risk estimates based on DNA studies in fragile X families. Five polymorphic loci have now been mapped to within 4 cM of FRAXA--DXS369, DXS297, DXS296, IDS, and DXS304. The frequency of recombination at Xq26-q28 was evaluated using data at these loci and at more distant loci from 112 families with the fragile X syndrome. Two-point and multipoint linkage analyses failed to detect any difference in the recombination fractions in fragile X versus normal families. Two-point and multipoint tests of linkage homogeneity failed to detect any evidence of linkage heterogeneity in the fragile X families. On the basis of this analysis, genetic maps derived from large samples of normal families and those derived from fragile X families are equally valid as the basis for calculating carrier risk estimates in a particular family.

PCR detection of two RFLPs in exon I of the α-L-iduronidase (IDUA) gene

Two polymorphisms were detected within exon I of the a-l-iduronidase (IDUA) gene both of which create restriction endonuclease sites and one of which changes an amino acid. The polymorphisms may be detected by digesting the same 245-bp polymerase chain reaction product. The polymorphisms can be used diagnostically in families with IDUA deficiency (mucopolysaccharidosis type I) and Huntington disease, which is closely linked to the IDUA locus.

An 86-bp VNTR within IDUA is the basis of the D4S111 polymorphic locus.

The gene for the lysosomal glycosidase [alpha]-L-iduronidase (EC 3.2.1.76; gene symbol, IDUA) is located on human chromosome 4 at 4p16.3, about 1100 kb from the telomere. Deficiency of IDUA results in the autosomal recessive disorder mucopolysaccharidosis type I. The authors have recently isolated the IDUA cDNA and the IDUA gene. The IDUA gene spans about 19 kb and has been shown to be coincident with the highly polymorphic multialliele locus D4S111, which is used in Huntington disease diagnosis. Polymorphisms at D4S111 are detected by p157.9, a plasmid subclone of the cosmid A157.1, which contains the IDUA gene. To fully characterize a repetitive element found in the second intron of the IDUA gene, they isolated and sequenced a 2.2-kb PstI restriction fragment that is entirely contained within the second of this 2.2-kb fragment and shows that it contains seven copies of an 86-bp repeat and that the polymorphism detected by p157.9 at D4S111k is a VNTR resulting from variation in the number of this repeat. 7 refs., 1 fig.

Arylsulfatase A lysosomal enzyme Map position 22q13.3

Metachromat ic l eukodys t rophy is an au tosomal recessive d i sorder of myel in metabol i sm caused by a deficiency of the lysosomal enzyme arylsul fa tase A (ARSA) [1]. The ARSA gene has p rev ious ly been local ized to chromosome 22 between 22q13 and qter us ing h u m a n rodent hybr id clones [2]. The human ARSA cDNA probe conta ined the whole coding region and consis ted of a 2.0-kb insert in pBluescript [3]. Fluorescence in situ hybr id iza t ion was as descr ibed in [4], except that no prereassocia t ion was necessary and chromosomes were s ta ined before analysis with both p r o p i d i u m iod ide (as counters ta in) and DAPI (for chromosome identification). Twenty metaphases from a normal male all showed specific label l ing of one or both chromat ids of chromosome 22 in the region ex tending from dis ta l 22q13.31 to 22q13.33. Two non-specific backg round dots were observed. A similar result was shown in a second normal male (15 h igh-resolu t ion metaphases) , conf irming the distal 22q13.3 posi t ion. 1. Gieselmann Vet al. (1995) Hum Mutat 4: 233-242. 2. Geurts van Kissel AHM et al. (1980) Cytogenet Cell Ganet 28: 169-!72. 3. Stein C et al. (1989) J Biol Chem 264: 1252-1259. 4. Callen DF et al. (1990) Ann Gfnft 33: 219-221.Metachromat ic l eukodys t rophy is an au tosomal recessive d i sorder of myel in metabol i sm caused by a deficiency of the lysosomal enzyme arylsul fa tase A (ARSA) [1]. The ARSA gene has p rev ious ly been local ized to chromosome 22 between 22q13 and qter us ing h u m a n rodent hybr id clones [2]. The human ARSA cDNA probe conta ined the whole coding region and consis ted of a 2.0-kb insert in pBluescript [3]. Fluorescence in situ hybr id iza t ion was as descr ibed in [4], except that no prereassocia t ion was necessary and chromosomes were s ta ined before analysis with both p r o p i d i u m iod ide (as counters ta in) and DAPI (for chromosome identification). Twenty metaphases from a normal male all showed specific label l ing of one or both chromat ids of chromosome 22 in the region ex tending from dis ta l 22q13.31 to 22q13.33. Two non-specific backg round dots were observed. A similar result was shown in a second normal male (15 h igh-resolu t ion metaphases) , conf irming the distal 22q13.3 posi t ion. 1. Gieselmann Vet al. (1995) Hum Mutat 4: 233-242. 2. Geurts van Kissel AHM et al. (1980) Cytogenet Cell Ganet 28: 169-!72. 3. Stein C et al. (1989) J Biol Chem 264: 1252-1259. 4. Callen DF et al. (1990) Ann Gfnft 33: 219-221.