SORL1 deficiency in human excitatory neurons causes APP-dependent defects in the endolysosome-autophagy network.

Dysfunction of the endolysosomal-autophagy network is emerging as an important pathogenic process in Alzheimer's disease. Mutations in the sorting receptor-encoding gene SORL1 cause autosomal-dominant Alzheimer's disease, and SORL1 variants increase risk for late-onset AD. To understand the contribution of SORL1 mutations to AD pathogenesis, we analyze the effects of a SORL1 truncating mutation on SORL1 protein levels and endolysosome function in human neurons. We find that truncating mutation results in SORL1 haploinsufficiency and enlarged endosomes in human neurons. Analysis of isogenic SORL1 wild-type, heterozygous, and homozygous null neurons demonstrates that, whereas SORL1 haploinsufficiency results in endosome dysfunction, complete loss of SORL1 leads to additional defects in lysosome function and autophagy. Neuronal endolysosomal dysfunction caused by loss of SORL1 is relieved by extracellular antisense oligonucleotide-mediated reduction of APP protein, demonstrating that PSEN1, APP, and SORL1 act in a common pathway regulating the endolysosome system, which becomes dysfunctional in AD.

Large genomic deletions and duplications in the BRCA1 gene identified by a novel quantitative method.

We applied a novel method to detect single or multiple exon deletions and amplifications in the BRCA1 gene. The test, called multiplex ligation-dependent probe amplification (MLPA), uses probes designed to hybridize adjacently to the target sequence. After ligation, the joined probes are amplified and quantified. Our two diagnostic laboratories have tested in the recent years 805 families by conventional PCR-based techniques, and found 116 BRCA1 and 28 BRCA2 mutation-positive families. Using MLPA, we have tested the remaining 661 noninformative breast cancer families and identified five distinct BRCA1 germ-line mutations in five families: a deletion of exon 8, a deletion of exons 20-22, a duplication of exon 13 and exons 21-23, respectively, and a triplication, encompassing exons 17-19. Genomic deletions of BRCA1 constitute a substantial fraction of mutations in Dutch breast cancer families. If MLPA had been included in our initial BRCA1 testing, 33 families with a deletion or duplication would have been identified, representing 27% of the total 121 BRCA1 mutation-positive families. The MLPA test for BRCA1 ensures a sensitive and comprehensive high-throughput screening test for genomic rearrangement and can easily be implemented in the molecular analysis of BRCA1.

Hypomyelination and congenital cataract: broadening the clinical phenotype.

OBJECTIVE: To further delineate the clinical spectrum of hypomyelination and congenital cataract (HCC), a rare autosomal recessive white matter disorder due to deficiency of a membrane protein, hyccin, encoded by FAM126A. DESIGN: Case reports and literature review. SETTING: University hospital. PATIENTS: Nine additional patients with HCC. RESULTS: Cataract was congenital in 5 patients; it was found at 4, 5, and 7 months in 3 patients, and only a mild lens opacity was noted at age 3 years in the remaining patient. Neurologic presentation was at birth in 1 child, was characterized by developmental delay at the end of the first year of life in 7 patients, and was characterized by sudden motor regression in the second year of life in the remaining patient. Three patients were able to walk with support only, 5 achieved the ability to walk without support, and the remaining patient was not able to stand at age 2 years. Mental retardation was present in all patients. Peripheral neuropathy was present in the 8 patients who underwent neurophysiological investigations. Brain magnetic resonance imaging showed hypomyelination associated with periventricular white matter abnormalities in all patients and brainstem pyramidal tract involvement in 8. Molecular analysis depicted 3 novel mutations and the previously reported IVS5 + 1G>T mutation. CONCLUSIONS: Our study broadens the clinical spectrum of HCC. The clinical variability ranges from severe early-onset neurologic impairment to a milder phenotype. In contrast to this clinical variability, the peculiar magnetic resonance pattern of hypomyelination combined with increased periventricular white matter water content allows distinction of HCC from other forms of hypomyelinating leukoencephalopathies.