Screening for GFAP rearrangements in a cohort of Alexander disease and undetermined leukoencephalopathy patients

Refining the concept of GFAP toxicity in Alexander disease

Background

Alexander disease is caused by dominantly acting mutations in glial fibrillary acidic protein (GFAP), the major intermediate filament of astrocytes in the central nervous system.

Main body

In addition to the sequence variants that represent the origin of disease, GFAP accumulation also takes place, together leading to a gain-of-function that has sometimes been referred to as “GFAP toxicity.” Whether the nature of GFAP toxicity in patients, who have mixtures of both mutant and normal protein, is the same as that produced by simple GFAP excess, is not yet clear.

Conclusion

The implications of these questions for the design of effective treatments are discussed.

Alexander disease

Alexander disease is a rare and generally fatal disorder of the central nervous system, originally defined by the distinctive neuropathology consisting of abundant Rosenthal fibers within the cytoplasm and processes of astrocytes. More recently, mutations in GFAP, encoding glial fibrillary acidic protein, the major intermediate filament protein of astrocytes, have been identified in nearly all patients. No other genetic causes have yet been identified. The precise mechanisms by which mutations lead to disease are poorly understood. Despite the genetic homogeneity, there are a wide range of clinical phenotypes. The genetic issues and the approach to diagnosis are the prime consideration in this chapter.

QMPSF is sensitive and specific in the detection of NPHP1 heterozygous deletions

Background:

Nephronophthisis, an autosomal recessive nephropathy, is responsible for 10% of childhood chronic renal failure. The deletion of its major gene, NPHP1, with a minor allele frequency of 0.24% in the general population, is the most common mutation leading to a monogenic form of childhood chronic renal failure. It is challenging to detect it in the heterozygous state. We aimed to evaluate the sensitivity and the specificity of the quantitative multiplex PCR of short fluorescent fragments (QMPSF) in its detection.

Methods:

After setting up the protocol of QMPSF, we validated it on 39 individuals diagnosed by multiplex ligation-dependent probe amplification (MLPA) with normal NPHP1 copy number (n=17), with heterozygous deletion (n=13, seven parents and six patients), or with homozygous deletion (n=9). To assess the rate of the deletions that arise from independent events, deleted alleles were haplotyped.

Results:

The results of QMPSF and MLPA correlated perfectly in the identification of 76 heterozygously deleted and 56 homozygously deleted exons. The inter-experimental variability of the dosage quotient obtained by QMPSF was low: control, 1.05 (median; range, 0.86−1.33, n = 102 exons); heterozygous deletion, 0.51 (0.42−0.67, n = 76 exons); homozygous deletion, 0 (0−0, n = 56 exons). All patients harboring a heterozygous deletion were found to carry a hemizygous mutation. At least 15 out of 18 deletions appeared on different haplotypes and one deletion appeared de novo.

Conclusions:

The cost- and time-effective QMPSF has a 100% sensitivity and specificity in the detection of NPHP1 deletion. The potential de novo appearance of NPHP1 deletions makes its segregation analysis highly recommended in clinical practice.

Stroke-like onset of brain stem degeneration presents with unique MRI sign and heterozygous NMNAT2 variant: a case report

Background

Acute-onset neurodegenerative diseases in older patients are rare clinical cases, especially when the degeneration only affects specific regions of the nervous system. Several neurological disorders have been described in which the degeneration of brain parenchyma originates from and/or primarily affects the brain stem. Clinical diagnosis in these patients, however, is often complicated due to a poor understanding of these diseases and their underlying mechanisms.

Case presentation

In this manuscript we report on a 73-year-old female who had experienced a sudden onset of complex neurological symptoms that progressively worsened over a period of 2 years. Original evaluation had suggested a MRI-negative stroke as underlying pathogenesis. The combination of patient’s medical history, clinical examination and exceptional pattern of brain stem degeneration presenting as “kissing swan sign” in MR imaging was strongly suggestive of acute onset of Alexander’s disease. This leukoencephalopathy is caused by GFAP (glial fibrilary acidic protein) gene mutations and may present with brain stem atrophy and stroke-like onset of symptoms in elderly individuals. However, a pathognomonic GFAP gene mutation could not be identified by Sanger sequencing.

Conclusions

After an extended differential diagnosis and exclusion of other diseases, a definite diagnosis of the patient’s condition presently remains elusive. However, whole-exome sequencing performed from patient’s blood revealed 12 potentially disease-causative heterozygous variants, amongst which several have been associated with neurological disorders in vitro and in vivo – in particular the axon degeneration-related NMNAT2 gene.