Congenital muscular dystrophy type 1A (MDC1A) is one of the main subtypes of early-onset muscle disease, caused by disease-associated variants in the laminin-α2 (LAMA2) gene. MDC1A usually presents as a severe neonatal hypotonia and failure to thrive. Muscle weakness compromises normal motor development, leading to the inability to sit unsupported or to walk independently. The phenotype associated with LAMA2 defects has been expanded to include milder and atypical cases, being now collectively known as LAMA2-related muscular dystrophies (LAMA2-MD). Through an international multicenter collaborative effort, 61 new LAMA2 disease-associated variants were identified in 86 patients, representing the largest number of patients and new disease-causing variants in a single report. The collaborative variant collection was supported by the LOVD-powered LAMA2 gene variant database (https://www.LOVD.nl/LAMA2), updated as part of this work. As of December 2017, the database contains 486 unique LAMA2 variants (309 disease-associated), obtained from direct submissions and literature reports. Database content was systematically reviewed and further insights concerning LAMA2-MD are presented. We focus on the impact of missense changes, especially the c.2461A > C (p.Thr821Pro) variant and its association with late-onset LAMA2-MD. Finally, we report diagnostically challenging cases, highlighting the relevance of modern genetic analysis in the characterization of clinically heterogeneous muscle diseases.

Familial amyloid polyneuropathy with substitution of methionine for valine at position 30 in the TTR gene is the most common type of hereditary transthyretin amyloidosis. Although several authors have previously reported a size-dependent fiber loss, predominantly involving unmyelinated and small diameter myelinated fibers, the mechanisms of nerve fiber loss have not been fully understood. In this study we establish the morphometric pattern of peripheral neuropathy in familial amyloid polyneuropathy patients and asymptomatic mutation carriers in the biopsies from our archive and correlated the pathological findings with clinical features. Ninety-eight patients with familial amyloid polyneuropathy and thirty-seven asymptomatic mutation carriers (TTR Val30Met mutation), aged between 17 and 84 years, that underwent sural nerve biopsy between 1981 and 2017 at Centro Hospitalar Universitário do Porto were studied. Thirty-one controls were included for comparison. The median age at nerve biopsy was 26.0 [(interquartile range = 23.5-39.5] years for asymptomatic mutation carriers, 45.0 [35.0-60.0] years for familial amyloid polyneuropathy patients and 44.0 [30.0-63.0] years for controls. The median duration between nerve biopsy and symptoms onset was 7.0 [3.3-11.8] years (range: 1 to 27 years) in the asymptomatic carriers. Most patients were in an earlier disease stage (93% with a polyneuropathy disability scale ≤ 2). Patients had loss of small and myelinated fibers compared to both asymptomatic carriers and controls (p<0.001), whereas asymptomatic carriers showed loss of small myelinated fibers when compared to controls (p<0.05). The loss of myelinated fibers increased with disease progression (p<0.001), and patients in more advanced clinical stage showed more frequent amyloid deposition in the nerve (p=0.001). There was a positive correlation between large myelinated fiber density and time to symptoms onset in the asymptomatic carriers that developed early-onset form of the disease (r = 0.52, p<0.01). Additionally, asymptomatic carriers with amyloid deposition already present in sural nerve biopsies developed symptoms earlier than those with no amyloid (p<0.01). In conclusion, this study confirms that small fiber size loss is an initial event in familial amyloid polyneuropathy, already present in asymptomatic gene carriers, starting several years before symptoms onset. We show for the first time that large myelinated fibers loss and amyloid deposition are pathological features that correlate independently with short period to symptoms onset for asymptomatic carriers that developed early-onset form of the disease. These findings are therapeutically relevant, as it would allow for a better interpretation of the role of disease-modifying agents in transthyretin familial amyloid polyneuropathy.

Intellectual disability (ID) can be caused by non-genetic and genetic factors, the latter being responsible for more than 1700 ID-related disorders. The broad ID phenotypic and genetic heterogeneity, as well as the difficulty in the establishment of the inheritance pattern, often result in a delay in the diagnosis. It has become apparent that massive parallel sequencing can overcome these difficulties. In this review we address: (i) ID genetic aetiology, (ii) clinical/medical settings testing, (iii) massive parallel sequencing, (iv) variant filtering and prioritization, (v) variant classification guidelines and functional studies, and (vi) ID diagnostic yield. Furthermore, the need for a constant update of the methodologies and functional tests, is essential. Thus, international collaborations, to gather expertise, data and resources through multidisciplinary contributions, are fundamental to keep track of the fast progress in ID gene discovery.