Linkage and apparent heterogeneity in proximal spinal muscular atrophies

Spinal Muscular Atrophy Modeling and Treatment Advances by Induced Pluripotent Stem Cells Studies

Spinal Muscular Atrophy (SMA) is a neurodegenerative disease characterized by specific and predominantly lower motor neuron (MN) loss. SMA is the main reason for infant death, while about one in 40 children born is a healthy carrier. SMA is caused by decreased levels of production of a ubiquitously expressed gene: the survival motor neuron (SMN). All SMA patients present mutations of the telomeric SMN1 gene, but many copies of a centromeric, partially functional paralog gene, SMN2, can somewhat compensate for the SMN1 deficiency, scaling inversely with phenotypic harshness. Because the study of neural tissue in and from patients presents too many challenges and is very often not feasible; the use of animal models, such as the mouse, had a pivotal impact in our understanding of SMA pathology but could not portray totally satisfactorily the elaborate regulatory mechanisms that are present in higher animals, particularly in humans. And while recent therapeutic achievements have been substantial, especially for very young infants, some issues should be considered for the treatment of older patients. An alternative way to study SMA, and other neurological pathologies, is the use of induced pluripotent stem cells (iPSCs) derived from patients. In this work, we will present a wide analysis of the uses of iPSCs in SMA pathology, starting from basic science to their possible roles as therapeutic tools.

Microtubule-associated protein 1B, a growth-associated and phosphorylated scaffold protein

Microtubule-associated protein 1B, MAP1B, is one of the major growth associated and cytoskeletal proteins in neuronal and glial cells. It is present as a full length protein or may be fragmented into a heavy chain and a light chain. It is essential to stabilize microtubules during the elongation of dendrites and neurites and is involved in the dynamics of morphological structures such as microtubules, microfilaments and growth cones. MAP1B function is modulated by phosphorylation and influences microtubule stability, microfilaments and growth cone motility. Considering its large size, several interactions with a variety of other proteins have been reported and there is increasing evidence that MAP1B plays a crucial role in the stability of the cytoskeleton and may have other cellular functions. Here we review molecular and functional aspects of this protein, evoke its role as a scaffold protein and have a look at several pathologies where the protein may be involved.

Prenatal prediction in families with autosomal recessive proximal spinal muscular atrophy (5q11.2-q13.3): molecular genetics and clinical experience in 109 cases

Prenatal prediction in families at risk for autosomal recessive proximal spinal muscular atrophy (SMA) mainly of type I is often requested due to the high incidence and the fetal outcome of the disease. So far, only indirect genotype analysis can be performed in SMA families, since the gene has not yet been identified. We present our experience of 109 prenatal diagnoses obtained in 91 families by use of single- and multi-locus polymorphic microsatellites of the region 5q11.2-q13.3. The marker combinations and specific features of the closest microsatellites are described in detail. From 137 requests for prenatal prediction of SMA between October 1991 and August 1994, 28 families were excluded, mostly because the clinical diagnosis was uncertain or doubtful. Others had to be classified as 'SMA-variants' or showed autosomal dominant transmission of SMA. Of the 109 prenatal diagnoses performed, 29 fetuses were diagnosed to be at high risk (> 99 per cent) of developing the disease, while in seven additional pregnancies no exact prediction could be made due to a recombination event in one parental haplotype. Altogether, recombinations between closely flanking markers were observed in 14 cases. In 35 cases, the parents decided to terminate the pregnancy. Of the remaining pregnancies, 32 could be followed beyond term. All infants were reported to develop normally without signs of SMA. Two children were born with transverse reduction defects of one hand, which was most likely related to early chorionic villus sampling at 9 and 10 weeks' gestation. No further abnormalities could be detected. The limits of indirect genotype analysis and the problems of diagnostic accuracy and heterogeneity of proximal SMA are discussed.

Apparent SMA I unlinked to 5q

A proband with a clinical picture indistinguishable from SMA type I is described. The parents are second cousins. On DNA analysis it appeared that the proband and his healthy 2 year old sib had inherited the same haplotypes for DNA markers flanking the SMA locus on 5q. This supports non-linkage of SMA to chromosome 5q in this family. The consanguinity of the parents raises the possibility of a second locus for autosomal recessive SMA type I outside the 5q12-13 region. This may have implications for genetic counselling after prenatal diagnosis in consanguineous families. Furthermore, this case illustrates the importance of the inclusion of all healthy sibs in prenatal DNA studies for SMA type I.