The genes for X-linked ocular albinism (OA1) and microphthalmia with linear skin defects (MLS): cloning and characterization of the critical regions

Two new patients with focal dermal hypoplasia: A novel PORCN variant and insights on the diagnostic considerations

Mutations in the PORCN gene cause an X-linked dominant condition; focal dermal hypoplasia (FDH), characterized by atrophic skin, pigmented skin lesions in addition to several ocular and skeletal malformations. FDH is rare with around 275 cases reported so far from diverse ethnic groups. Herein, we provide a report of two new patients with FDH from Egypt. In addition to the typical clinical manifestations of the disease, infrequently reported clinical findings in the form of broad metaphysis, bilateral short broad femurs, and dermal sinus over the sacrum were seen in Patient 1 and partial fusion of labia majora, ventral hernia, and bladder extrophy were present in Patient 2. Two heterozygous protein-truncating PORCN mutations were identified in our patients, a known nonsense c.370C>T p.(Arg124Ter) and a novel frameshift c.375delG p.(Ala126HisfsTer3). Segregation analyses confirmed that the two mutations were "de novo" and not inherited from any of the parents. Our study expands the clinical and mutational spectrum of focal dermal hypoplasia and emphasizes the importance of investigating the different body systems and organs for the early management of patients.

Linear Skin Defects with Multiple Congenital Anomalies (LSDMCA): An Unconventional Mitochondrial Disorder

Mitochondrial disorders, although heterogeneous, are traditionally described as conditions characterized by encephalomyopathy, hypotonia, and progressive postnatal organ failure. Here, we provide a systematic review of Linear Skin Defects with Multiple Congenital Anomalies (LSDMCA), a rare, unconventional mitochondrial disorder which presents as a developmental disease; its main clinical features include microphthalmia with different degrees of severity, linear skin lesions, and central nervous system malformations. The molecular basis of this disorder has been elusive for several years. Mutations were eventually identified in three X-linked genes, i.e., HCCS, COX7B, and NDUFB11, which are all endowed with defined roles in the mitochondrial respiratory chain. A peculiar feature of this condition is its inheritance pattern: X-linked dominant male-lethal. Only female or XX male individuals can be observed, implying that nullisomy for these genes is incompatible with normal embryonic development in mammals. All three genes undergo X-inactivation that, according to our hypothesis, may contribute to the extreme variable expressivity observed in this condition. We propose that mitochondrial dysfunction should be considered as an underlying cause in developmental disorders. Moreover, LSDMCA should be taken into consideration by clinicians when dealing with patients with microphthalmia with or without associated skin phenotypes.

Clinical spectrum of females with HCCS mutation: from no clinical signs to a neonatal lethal form of the microphthalmia with linear skin defects (MLS) syndrome

BACKGROUND

Segmental Xp22.2 monosomy or a heterozygous HCCS mutation is associated with the microphthalmia with linear skin defects (MLS) or MIDAS (microphthalmia, dermal aplasia, and sclerocornea) syndrome, an X-linked disorder with male lethality. HCCS encodes the holocytochrome c-type synthase involved in mitochondrial oxidative phosphorylation (OXPHOS) and programmed cell death.

METHODS

We characterized the X-chromosomal abnormality encompassing HCCS or an intragenic mutation in this gene in six new female patients with an MLS phenotype by cytogenetic analysis, fluorescence in situ hybridization, sequencing, and quantitative real-time PCR. The X chromosome inactivation (XCI) pattern was determined and clinical data of the patients were reviewed.

RESULTS

Two terminal Xp deletions of ≥ 11.2 Mb, two submicroscopic copy number losses, one of ~850 kb and one of ≥ 3 Mb, all covering HCCS, 1 nonsense, and one mosaic 2-bp deletion in HCCS are reported. All females had a completely (>98:2) or slightly skewed (82:18) XCI pattern. The most consistent clinical features were microphthalmia/anophthalmia and sclerocornea/corneal opacity in all patients and congenital linear skin defects in 4/6. Additional manifestations included various ocular anomalies, cardiac defects, brain imaging abnormalities, microcephaly, postnatal growth retardation, and facial dysmorphism. However, no obvious clinical sign was observed in three female carriers who were relatives of one patient.

CONCLUSION

Our findings showed a wide phenotypic spectrum ranging from asymptomatic females with an HCCS mutation to patients with a neonatal lethal MLS form. Somatic mosaicism and the different ability of embryonic cells to cope with an OXPHOS defect and/or enhanced cell death upon HCCS deficiency likely underlie the great variability in phenotypes.

Mother and daughter with a terminal Xp deletion: implication of chromosomal mosaicism and X-inactivation in the high clinical variability of the microphthalmia with linear skin defects (MLS) syndrome

The microphthalmia with linear skin defects (MLS or MIDAS) syndrome is a rare X-linked dominant inherited disorder with male lethality, associated with segmental aneuploidy of the Xp22.2 region in most of the cases. However, we recently described heterozygous sequence alterations in a single gene, HCCS, in females with MLS. Beside the classical MLS phenotype, occasional features such as sclerocornea, agenesis of the corpus callosum, and congenital heart defects can occur. Although the majority of cases are sporadic, mother-to-daughter transmission has been observed and a high intra- and interfamilial phenotypic variability exists. We describe an asymptomatic mother and her daughter presenting with the typical features of MLS syndrome. By cytogenetic analysis both females were found to have a terminal Xp deletion with the breakpoint in Xp22.2, mapping near to or within the MSL3L1 gene which is located centromeric to HCCS. FISH analysis revealed that the mother is a mosaic with 45,X(11)/46,X,del(X)(p22.2)(89), while in all cells of the MLS-affected daughter a hybridization pattern consistent with a 46,X,del(X)(p22.2) karyotype was detected. By haplotype analysis we identified the paternal X chromosome of the mother to carry the terminal Xp deletion. X-inactivation studies showed a completely skewed pattern in mother and daughter with the deleted X chromosome to be preferentially inactivated in their peripheral blood cells. We suggest that both chromosomal mosaicism as well as functional X chromosome mosaicism could contribute to the lack of any typical MLS feature in individuals with a heterozygous MLS-associated mutation. The 45,X cell population, that most likely is also present in other tissues of the mother, might have protected her from developing MLS. Nonetheless, a non-random X-inactivation pattern in favor of activity of the wild-type X chromosome in the early blastocyte could also account for the apparent lack of any disease sign in this female.

Microphthalmia with linear skin defects (MLS) syndrome evaluated by prenatal karyotyping, FISH and array comparative genomic hybridization

OBJECTIVE

To explore the utility of comparative genomic hybridization to BAC arrays (array CGH) for prenatal diagnosis of microphthalmia and linear skin defects syndrome.

METHODS

We used karyotype analysis, FISH and array CGH to investigate an X;Y translocation. Replication studies were done on cultured amniocytes and lymphoblasts.

RESULTS

We describe a severe case of MLS syndrome that presented prenatally with multiple anomalies including cystic hygroma, microphthalmia, intrauterine growth restriction and a complex congenital heart defect. Cytogenetic analysis of amniocytes revealed an unbalanced de novo translocation between chromosomes X and Y [karyotype 46,X,der(X)t(X;Y)(p22.3;q11.2).ish der(X)(DXZ1+,DMD+,KAL-,STS-,SRY-),22q11.2 (Tuple1 x 2)]. MLS diagnosis was made at birth and the prenatal karyotype was confirmed. Replication studies showed the derivative X chromosome was the inactive X. Array CGH confirmed the X and Y imbalances seen in the karyotype and also showed twelve BACs in the MLS region were deleted as a result of the translocation. FISH with BAC clones verified the array findings and placed the X breakpoint in Xp22.2, resulting in the amended karyotype, 46,X,der(X)t(X;Y)(p22.2;q11.2).ish der(X)(DXZ1+,DMD+,KAL-,STS-,SRY-),22q11.2(Tuple1 x 2) arr cgh Xp22.33p22.2(LLNOYCO3M15D10 -->GS1-590J6)x 1,Yq11.222q23(RP11-20H21-->RP11-79J10)x 1.

CONCLUSION

The sensitivity of array CGH was valuable in detecting monosomy of the MLS critical region. Array CGH should be considered for the prenatal diagnosis of this syndrome.

Focal dermal hypoplasia with exuberant fat herniations and skeletal deformities

Focal dermal hypoplasia or Goltz syndrome is a rare congenital and mesoectodermal dysplasia with multisystemic involvement. Although the genetic alterations responsible for focal dermal hypoplasia are not fully known, there is predominance in affected females, suggesting dominant X-linked inheritance. Besides the skin, other structures frequently involved are the skeletal system, eyes, teeth, hair, and nails. Skeletal abnormalities are predominantly observed in the hands and feet. We report a 9-year-old girl who had typical linear skin atrophy on the trunk, exuberant "fat herniations," several skeletal abnormalities, and exuberant "lobster claw" deformity. In addition, she had the typical longitudinal striations in femur metaphyses. With regard to family history, her mother had one male stillbirth with several deformities. This typical focal dermal hypoplasia patient is considered valuable in light of the affected male stillbirth and parents with nonaffected phenotypes that together provides evidence for mother-to-daughter spontaneous transmission.

Microphthalmia with linear skin defects (MLS) syndrome: Clinical, cytogenetic, and molecular characterization of 11 cases

The microphthalmia with linear skin defects (MLS) syndrome (MIM 309801) is a severe and rare developmental disorder, which is inherited as an X-linked dominant trait with male lethality. In the vast majority of patients, this syndrome is associated with terminal deletion of the Xp22.3 region. Thirty-five cases have been described to date in the literature since the first description of the syndrome in the early 1990s. We now report on the clinical, cytogenetic, and molecular characterization of 11 patients, 7 of whom have not been described previously. Seven of these patients have chromosomal abnormalities of the short arm of the X-chromosome, which were characterized and defined by fluorescence in situ hybridization (FISH) analysis. Intriguingly, one of the patients displays an interstitial Xp22.3 deletion, which to the best of our knowledge is the first reported for this condition. Finally we report on the identification and molecular characterization of four cases with clinical features of MLS but apparently normal karyotypes, verified by FISH analysis using genomic clones spanning the MLS minimal critical region, and with genome-wide analysis using a 1 Mb resolution BAC microarray. These patients made it possible to undertake mutation screening of candidate genes and may prove critical for the identification of the gene responsible for this challenging and intriguing genetic disease.

Aicardi syndrome

Aicardi syndrome (AS) is characterized by a triad of callosal agenesis, infantile spasms and chorioretinal 'lacunae'. It occurs only in individuals with two X chromosomes and is not familial. The outcome of AS is severe, with a high early mortality, considerable morbidity and a generally poor developmental outcome. However, the spectrum of AS seems broader than previously defined with a small proportion of the affected girls only moderately or mildly retarded. Several novel and important features should be added to the classic triad. The brain malformation is complex with cortical migration abnormalities, often cystic formations and sometimes choroid plexus papillomas; the eye anomalies, often feature a coloboma in addition to the lacunae, and focal seizures rather than spasms, are common. AS has been reported in 2 boys, both with an XXY complement, supporting the hypothesis of an X-linked gene lethal early in pregnancy for male conceptuses. A locus at Xp22.3 has been suggested but has not been confirmed. Treatment is only symptomatic.

Mechanisms regulating the development of the corpus callosum and its agenesis in mouse and human

The development of the corpus callosum depends on a large number of different cellular and molecular mechanisms. These include the formation of midline glial populations, and the expression of specific molecules required to guide callosal axons as they cross the midline. An additional mechanism used by callosal axons from neurons in the neocortex is to grow within the pathway formed by pioneering axons derived from neurons in the cingulate cortex. Data in humans and in mice suggest the possibility that different mechanisms may regulate the development of the corpus callosum across its rostrocaudal and dorsoventral axes. The complex developmental processes required for formation of the corpus callosum may provide some insight into why such a large number of human congenital syndromes are associated with agenesis of this structure.

Presence of filamin in the astrocytic inclusions of Aicardi syndrome

Aicardi syndrome affects only females and has been hypothesized to be an X-linked dominant male-lethal disorder. Because no familial cases can be studied for genetic linkage analysis, the mutated gene has remained elusive. With the goal of selecting genes for mutation analysis by a functional candidate approach, a detailed pathologic analysis of two brains from deceased Aicardi syndrome patients was performed. The presence of micrencephaly, absent or hypoplastic corpus callosum, polymicrogyria, heterotopia, ventriculomegaly, intracerebral cyst, and intracytoplasmic eosinophilic inclusions was confirmed in glial fibrillary acidic protein-positive astrocytes in the cortex and heterotopias, but not in white matter. The inclusions demonstrated strong immunolabeling with antibodies to filamin and vimentin but weak labeling with antibodies to proteins S100 and microtubule-associated protein 1. These findings suggested that an underlying defect in the cytoskeleton, which involves filamin, may cause this condition. Because the filamin A gene in Xq28 is mutated in another disorder with heterotopia, familial bilateral periventricular heterotopia, mutation analysis of filamin A in Aicardi syndrome patients was pursued. No mutations were found, and the full-length protein was expressed in both brain samples. Future studies will focus on investigation of X-linked genes that may affect function of filamin or other cytoskeletal proteins.

Xp22.3 microdeletion in a 19-year-old girl with clinical features of MLS syndrome

We describe a 19-year-old girl who has clinical features of microphthalmia with linear skin defects (MLS) syndrome caused by a microdeletion of Xp22.3. In addition to the classical ocular abnormalities and linear skin defects she has other features not previously described. She was previously reported in this journal in 1990 as poikiloderma congenitale, but her true diagnosis of an Xp22.3 microdeletion was clarified when fluorescent in situ hybridization (FISH) analysis indicated that one of her X chromosomes had a microdeletion including the KAL gene. We describe this patient with an Xp22.3 microdeletion to heighten awareness among dermatologists of this syndrome and to underscore the difficulties in diagnosing MLS syndrome.

Recurrent digital fibroma, focal dermal hypoplasia, and limb malformations

Recurrent digital fibroma of infancy generally is considered a sporadic tumor of childhood. We describe the case of a mother with recurrent digital fibroma at a young age who gave birth to a daughter with focal dermal hypoplasia, coloboma of the iris and eyelids, anal atresia, and extensive limb malformations. When the infant was 3 months old, fibromas started to appear at the fingertips. The cases of three additional patients are described, with a similar combination of multiple digital fibromas, pigmented marks on the temporal region, and limb malformations. One of these patients has consanguineous parents. The clinical findings overlap partially with Gorlin-Goltz syndrome, which has been renamed by some authors "microphthalmia with linear skin defects" (MLS). Since the skin signs are clearly different, however-more like those of Setleis syndrome ("forceps mark" temporal dysplasia)-the patients described here seem to have a new combination of congenital malformations. Deletion of distal Xp, known to occur in some MLS patients, was not detected using cosmids in fluorescence in situ hybridization. This pattern of digital fibroma with congenital malformations seems to represent a new syndrome.

Characterization of a novel chromo domain gene in xp22.3 with homology to Drosophila msl-3

The Drosophila male-specific lethal (MSL) genes regulate transcription from the male X chromosome in a dosage compensation pathway that equalizes X-linked gene expression in males and females. The members of this gene family, including msl-1, msl-2, msl-3, mle, and mof, encode proteins with no sequence homology. However, mutations in each of these genes produce a similar phenotype: sex-specific lethality of male embryos caused by the failure of mutants to increase transcription from the single male X chromosome. The MSL gene products assemble into a multiprotein transcriptional activation complex at hundreds of sites along the chromatin of the X chromosome. Here we report the isolation and characterization of a human gene, named MSL3L1, that encodes a protein with significant homology to Drosophila MSL-3 in three distinct regions, including two putative chromo domains. MSL3L1 was identified by database queries with genomic sequence from BAC GS-590J6 (GenBank AC0004554) in Xp22.3 and was evaluated as a candidate gene for several developmental disorders mapping to this region, including OFD1 and SED tarda, as well as Aicardi syndrome and Goltz syndrome.

X-linked late-onset sensorineural deafness caused by a deletion involving OA1 and a novel gene containing WD-40 repeats

We have identified a novel gene, transducin (beta)-like 1 (TBL1), in the Xp22.3 genomic region, that shows high homology with members of the WD-40-repeat protein family. The gene contains 18 exons spanning approximately 150 kb of the genomic region adjacent to the ocular albinism gene (OA1) on the telomeric side. However, unlike OA1, TBL1 is transcribed from telomere to centromere. Northern analysis indicates that TBL1 is ubiquitously expressed, with two transcripts of approximately 2.1 kb and 6.0 kb. The open reading frame encodes a 526-amino acid protein, which shows the presence of six beta-transducin repeats (WD-40 motif) in the C-terminal domain. The homology with known beta-subunits of G proteins and other WD-40-repeat containing proteins is restricted to the WD-40 motif. Genomic analysis revealed that the gene is either partly or entirely deleted in patients carrying Xp22.3 terminal deletions. The complexity of the contiguous gene-syndrome phenotype shared by these patients depends on the number of known disease genes involved in the deletions. Interestingly, one patient carrying a microinterstitial deletion involving the 3' portion of both TBL1 and OA1 shows the OA1 phenotype associated with X-linked late-onset sensorineural deafness. We postulate an involvement of TBL1 in the pathogenesis of the ocular albinism with late-onset sensorineural deafness phenotype.

Characterization and physical mapping in human and mouse of a novel RING finger gene in Xp22

Microphthalmia with linear skin defects (MLS) is an X-linked dominant male-lethal syndrome caused by different deletions of chromosome Xp22. Through the screening of cDNA libraries with the cross-species conserved marker 61B3-R (DXS1141), we identified a new gene at the telomeric breakpoint of the MLS critical region, which encodes a transcript containing a RING finger domain. This novel gene was independently cloned by another group and found to be mutated in Opitz syndrome. In this study we characterized the expression pattern of this gene, identified various splice variants, delineated its exon-intron boundaries, and determined that it is not mutated in either Aicardi or Goltz syndrome, two X-linked dominant conditions with phenotypes that overlap with that of MLS syndrome. This novel RING finger gene is expressed throughout mouse embryonic development, with the highest levels of expression in E7-E11. FISH and hybridization to mouse YACs confirmed human and mouse synteny in the order of this gene and other genes in the MLS critical region; however, this gene spans the boundary of the pseudoautosomal region in mouse but not in humans.

Reticulolinear aplasia cutis congenita of the face and neck: a distinctive cutaneous manifestation in several syndromes linked to Xp22

A distinct form of aplasia cutis congenita presenting as linear facial skin defects has been described under a variety of names as Xp deletion syndrome. MIDAS (microphthalmia, dermal aplasia and sclerocornea) syndrome, MLS (microphthalmia and linear skin defects) and Gazali-Temple syndrome. The syndrome is lethal in males, and its severity in females varies from a relatively mild residual facial scarring with short stature to lethal developmental organ malformations. A new case with peculiar ultrastructural findings is presented. A review of the literature suggests that these associations represent a series of contiguous-gene syndromes.

Cloning and characterization of a novel rho-type GTPase-activating protein gene (ARHGAP6) from the critical region for microphthalmia with linear skin defects

Microphthalmia with linear skin defects syndrome (MLS) is an X-linked dominant, male-lethal disorder associated with chromosomal rearrangements that result in deletions of the distal short arm of the X chromosome. In an effort to isolate expressed sequences from the 500-kb MLS critical region in Xp22.3, exons were trapped from 14 overlapping cosmids. Using exon connection followed by cDNA library screening, we identified a 2.4-kb contig of cDNA library screening 170 kb of genomic sequence in the MLS deletion region. Northern analysis of this cDNA detected a prominent approximately 4.2-kb transcript and a less abundant approximately 6-kb transcript in all tissues examined, with additional transcripts in skeletal muscle. Sequence analysis revealed a coding region of 601 amino acids contained in 12 exons, with a splice variant isoform of 495 amino acids. The predicted protein sequence of the gene, named ARHGAP6, contains homology to the GTPase-activating (GAP) domain of the rhoGAP family of proteins, which has been implicated in the regulation of actin polymerization at the plasma membrane in several cellular processes. The possible role of the ARHGAP6 protein in the pathogenesis of MLS is discussed.

Opitz G/BBB syndrome, a defect of midline development, is due to mutations in a new RING finger gene on Xp22

Opitz syndrome (OS) is an inherited disorder characterized by midline defects including hypertelorism, hypospadias, lip-palate-laryngotracheal clefts and imperforate anus. We have identified a new gene on Xp22, MID1 (Midline 1), which is disrupted in an OS patient carrying an X-chromosome inversion and is also mutated in several OS families. MID1 encodes a member of the B-box family of proteins, which contain protein-protein interaction domains, including a RING finger, and are implicated in fundamental processes such as body axis patterning and control of cell proliferation. The association of MID1 with OS suggests an important role for this gene in midline development.

Intragenic TaqI restriction fragment length polymorphism (RFLP) in CICN4, between the loci for X-linked ocular albinism (OA1) and microphthalmia with linear skin defects syndrome (MLS)

A TaqI RFLP was detected within the ClCN4 gene, which lies between the loci for OA1 and MLS. There were no observed recombinations between this RFLP and the OA1 mutation in three informative families. Thus, the marker will be useful for genetic counseling in OA1.

Cloning of the gene for ocular albinism type 1 from the distal short arm of the X chromosome

Ocular albinism type 1 (OA1) is an X-linked disorder characterized by severe impairment of visual acuity, retinal hypopigmentation and the presence of macromelanosomes. We isolated a novel transcript from the OA1 critical region in Xp22.3-22.2 which is expressed at high levels in RNA samples from retina, including the retinal pigment epithelium, and from melanoma. This gene encodes a protein of 424 amino acids displaying several putative transmembrane domains and sharing no similarities with previously identified molecules. Five intragenic deletions and a 2 bp insertion resulting in a premature stop codon were identified from DNA analysis of patients with OA1, indicating that we have identified the OA1 gene.

Development and physical analysis of YAC contigs covering 7 Mb of Xp22.3-p22.2

A total of 54 YAC clones have been isolated from the region of Xp22.2-p22.3 extending from the amelogenin gene locus to DXS31. Restriction analysis of these clones in association with STS contenting and end clone analysis has facilitated the construction of 6 contigs covering a total of 7 Mb in which 20 potential CpG islands have been located. Thirty new STSs have been developed from probe and YAC end clone sequences, and these have been used in the analysis of patients suffering from different combinations of chondrodysplasia punctata, mental retardation, X-linked ichthyosis, and Kallmann syndrome. The results suggest that (1) the gene for chondrodysplasia punctata must lie between the X chromosome pseudoautosomal boundary (PABX) and DXS1145; (2) a gene for mental retardation lies between DXS1145 and the sequence tagged site GS1; and (3) the gene for ocular albinism type 1 lies proximal to the STS G13. The CpG islands within the YAC contigs constitute valuable markers for the potential positions of genes. Genes found associated with any of these potential CpG islands would be possible candidates for the disease genes mentioned above.

Identification and characterization of the gene causing type 1 spinocerebellar ataxia

Spinocerebellar ataxia type 1 (SCA1) is a neurodegenerative disorder caused by expansion of a CAG trinucleotide repeat. In this study, we describe the identification and characterization of the gene harbouring this repeat. The SCA1 transcript is 10,660 bases and is transcribed from both the wild type and SCA1 alleles. The CAG repeat, coding for a polyglutamine tract, lies within the coding region. The gene spans 450 kb of genomic DNA and is organized in nine exons. The first seven fall in the 5' untranslated region and the last two contain the coding region, and a 7,277 basepairs 3' untranslated region. The first four non-coding exons undergo alternative splicing in several tissues. These features suggest that the transcriptional and translational regulation of ataxin-1, the SCA1 encoded protein, may be complex.

Microphthalmia with linear skin defects (MLS) syndrome: clinical, cytogenetic, and molecular characterization

The microphthalmia with linear skin defects (MLS) syndrome (MIM 309801) is a severe developmental disorder observed in XX individuals with distal Xp segmental monosomy. The phenotype of this syndrome overlaps with that of both Aicardi (MIM 304050) and Goltz (MIM 305600) syndromes, two X-linked dominant, male-lethal disorders. Here we report the clinical, cytogenetic, and molecular characterization of 3 patients with this syndrome. Two of these patients are females with a terminal Xpter-p22.2 deletion. One of these 2 patients had an aborted fetus with anencephaly and the same chromosome abnormality. The third patient is an XX male with Xp/Yp exchange spanning the SRY gene which results in distal Xp monosomy. The extensive clinical variability observed in these patients and the results of the molecular analysis suggest that X-inactivation plays an important role in determining the phenotype of the MLS syndrome. We propose that the MLS, Aicardi, and Goltz syndromes are due to the involvement of the same gene(s), and that different patterns of X-inactivation are responsible for the phenotypic differences observed in these 3 disorders. However, we cannot rule out that each component of the MLS phenotype is caused by deletion of a different gene (a contiguous gene syndrome).

Mapping and cloning of the critical region for the spinocerebellar ataxia type 1 gene (SCA1) in a yeast artificial chromosome contig spanning 1.2 Mb

The gene responsible for spinocerebellar ataxia type 1 (SCA1) has been localized to a 6.7-cM region between the centromeric marker D6S109 and the telomeric marker D6S89. We screened two yeast artificial chromosome (YAC) libraries using sequence-tagged sites at D6S89 and at newly identified markers in 6p22-p23. Fifty YAC clones were identified and 34 insert termini were isolated from some of these YACs for detailed overlap mapping and long-range restriction analysis. A large YAC contig estimated to span 2.5 Mb was developed and genetic analysis in five large SCA1 kindreds using highly informative dinucleotide repeat polymorphisms mapped to this contig allowed the identification of D6S274 as the closest centromeric flanking marker for SCA1. Long-range restriction analysis determined the size for the critical SCA1 region, as defined by the two flanking markers D6S274 and D6S89, to be 1.2 Mb. This region is spanned by a minimum set of four nonchimeric YAC clones. The development of a 2.5-Mb YAC contig in 6p22-p23 provides valuable reagents for characterization of this genomic region and for the cloning of the SCA1 gene.