GFAP mutations, age at onset, and clinical subtypes in Alexander disease.

OBJECTIVE: To characterize Alexander disease (AxD) phenotypes and determine correlations with age at onset (AAO) and genetic mutation. AxD is an astrogliopathy usually characterized on MRI by leukodystrophy and caused by glial fibrillary acidic protein (GFAP) mutations. METHODS: We present 30 new cases of AxD and reviewed 185 previously reported cases. We conducted Wilcoxon rank sum tests to identify variables scaling with AAO, survival analysis to identify predictors of mortality, and χ(2) tests to assess the effects of common GFAP mutations. Finally, we performed latent class analysis (LCA) to statistically define AxD subtypes. RESULTS: LCA identified 2 classes of AxD. Type I is characterized by early onset, seizures, macrocephaly, motor delay, encephalopathy, failure to thrive, paroxysmal deterioration, and typical MRI features. Type II is characterized by later onset, autonomic dysfunction, ocular movement abnormalities, bulbar symptoms, and atypical MRI features. Survival analysis predicted a nearly 2-fold increase in mortality among patients with type I AxD relative to those with type II. R79 and R239 GFAP mutations were most common (16.6% and 20.3% of all cases, respectively). These common mutations predicted distinct clinical outcomes, with R239 predicting the most aggressive course. CONCLUSIONS: AAO and the GFAP mutation site are important clinical predictors in AxD, with clear correlations to defined patterns of phenotypic expression. We propose revised AxD subtypes, type I and type II, based on analysis of statistically defined patient groups.

Population data suggest that deletions of 1p36 are a relatively common chromosome abnormality.

Monosomy 1p36 is a relatively common chromosome deletion. Deletion of this chromosome band can be difficult to visualize using routine cytogenetic banding techniques. The use of fluorescence in situ hybridization (FISH) with telomere region-specific probes has aided in the diagnosis of patients. In this study we ascertained 62 patients with deletions of 1p36 from 61 families and collected information regarding previous chromosome analyses, mode of ascertainment, clinical indication, age at diagnosis, and parental ages. The majority of deletions occur on the maternally derived chromosome. We identified terminal deletions, interstitial deletions, derivative chromosomes, and complex rearrangements. We correlated the type of rearrangement with the parental origins. Almost 50% of the patients had at least one chromosome analysis interpreted as normal. Retrospectively, 98% of deletions could be identified by routine chromosome analysis with careful attention to chromosome 1p36. Clinical indications were variable, with developmental delay/mental retardation being the most common. Increased maternal serum alpha fetoprotein (MSAFP) was detected in four of the five prenatally diagnosed cases. Maternal age at the time of birth of the affected child was significantly lower than the general United States population mean. We suggest a multistep approach for the diagnosis and clinical evaluation in cases of monosomy 1p36.

Loss of the potassium channel beta-subunit gene, KCNAB2, is associated with epilepsy in patients with 1p36 deletion syndrome.

PURPOSE: Clinical features associated with chromosome 1p36 deletion include characteristic craniofacial abnormalities, mental retardation, and epilepsy. The presence and severity of specific phenotypic features are likely to be correlated with loss of a distinct complement of genes in each patient. We hypothesize that hemizygous deletion of one, or a few, critical gene(s) controlling neuronal excitability is associated with the epilepsy phenotype. Because ion channels are important determinants of seizure susceptibility and the voltage-gated K(+) channel beta-subunit gene, KCNAB2, has been localized to 1p36, we propose that deletion of this gene may be associated with the epilepsy phenotype. METHODS: Twenty-four patients were evaluated by fluorescence in situ hybridization with a probe containing KCNAB2. Clinical details were obtained by neurologic examination and EEG. RESULTS: Nine patients are deleted for the KCNAB2 locus, and eight (89%) of these have epilepsy or epileptiform activity on EEG. The majority of patients have a severe seizure phenotype, including infantile spasms. In contrast, of those not deleted for KCNAB2, only 27% have chronic seizures, and none had infantile spasms. CONCLUSIONS: Lack of the beta subunit would be predicted to reduce K(+) channel-mediated membrane repolarization and increase neuronal excitability, suggesting a possible relation between loss of this gene and the development of seizures. Because some patients with seizures were not deleted for KCNAB2, there may be additional genes within 1p36 that contribute to epilepsy in this syndrome. Hemizygosity of this gene in a majority of monosomy 1p36 syndrome patients with epilepsy suggests that haploinsufficiency for KCNAB2 is a significant risk factor for epilepsy.

Terminal osseous dysplasia and pigmentary defects: clinical characterization of a novel male lethal X-linked syndrome.

We describe a new syndrome of distal limb anomalies and pigmentary skin defects in 10 females of a large, four-generation pedigree. The family was ascertained through a 4-month-old infant girl with multiple anomalies, including hypertelorism, iris colobomas, low-set ears, midface hypoplasia, punched-out pigmentary abnormalities over the face and scalp, generalized brachydactyly, and digital fibromatosis. No affected males were identified in this pedigree. Affected females had a lower than normal male-to-female ratio of liveborn offspring, and some of them also had a history of several miscarriages. These findings, together with a significant variability in the phenotype of the affected females, suggest that this condition is inherited in an X-linked dominant fashion, with prenatal male lethality, and that X-inactivation plays an important role in the phenotypic expression of the disease. The syndrome has been described twice in the literature, but only in sporadic cases; it was therefore not recognized as a mendelian entity. Because the most consistent findings are anomalies of the distal skeleton of the limbs and localized pigmentary abnormalities of the skin, we named the syndrome "terminal osseous dysplasia with pigmentary defects." This condition, though rare, can be added to the small group of male lethal X-linked dominant disorders in humans.

Molecular and clinical characterization of a patient with duplication of 1p36.3 and metopic synostosis.

Chromosome 1p duplications are rare. There have been only 11 reported cases of isolated 1p duplication, all of which were proximal, interstitial duplications. We present a patient with a terminal duplication of 1p (1p36.3). To our knowledge, this is the first such reported case. Our patient presented with metopic synostosis, rectal stenosis, atrial septal defect, and mildly delayed gross motor development. Molecular characterization using microsatellite marker analysis and fluorescence in situ hybridization (FISH) revealed an area of duplication between p58 and D1S2893, approximately 13 cM in size. We compare our patient's clinical findings with the clinical phenotype found in patients with the corresponding deletion of 1p36.3 and discuss the role of gene dosage in other deletion/duplication syndromes.

Molecular refinement of the 1p36 deletion syndrome reveals size diversity and a preponderance of maternally derived deletions.

The deletion of chromosome 1p36 is a newly recognized, relatively common contiguous gene deletion syndrome with a variable phenotype. The clinical features have recently been delineated and molecular analysis indicates that the prevalence of certain phenotypic features appears to correlate with deletion size. Phenotype/genotype comparisons have allowed the assignment of certain clinical features to specific deletion intervals, significantly narrowing the regions within which to search for candidate genes. We have extensively characterized the deletion regions in 30 cases using microsatellite markers and fluorescence in situ hybridization analyses. The map order of 28 microsatellite markers spanning the deletion region was obtained by a combination of genotypic analysis and physical mapping. The deletion region was divided into six intervals and breakpoints were found to cluster in mainly two regions. Molecular analysis of the deletions showed that two patients had complex re-arrangements; these cases shared their distal and proximal breakpoints in the two common breakpoint regions. Of the de novo deletions ( n = 28) in whichparental samples were available and the analysis was informative ( n = 27), there were significantly morematernally derived deletions ( n = 21) than paternally derived deletions ( n = 6) (chi1(2) = 8.35, P < 0.0001). Phenotype/genotype correlations and refinements of critical regions in our naturally occurring deletion panel have delineated specific areas in which to focus the search for the causative genes for the features of this syndrome.