Retrospective Cohort Analysis of Clinical, Molecular, and Histopathologic Characteristics of 275 Patients With Nemaline Myopathy

Abstract

Background and objectives: Nemaline myopathy (NM) is a congenital myopathy with a wide severity spectrum, from severe, generalized muscle weakness and respiratory failure in the neonatal period to mild, distal weakness in young adulthood. Eleven genes have been definitively established to cause the condition. Although some recurrent variants have been identified, the overall correlation of genotype with clinical severity in NM remains poor. This poses challenges when counseling families about prognosis after a diagnosis of NM is made. Better clinical and molecular predictors of outcome are needed for clinical trial readiness.

Methods: A retrospective cohort analysis of 275 patients with a histopathologic diagnosis of NM and/or pathogenic variants in NM-associated genes was performed to identify relationships between early clinical findings and long-term outcomes, including need for respiratory and feeding support.

Results: Early clinical predictors of long-term outcomes were identified: patients with hypotonia at birth had increased odds of requiring gastrostomy tubes, and patients with respiratory distress at birth had increased odds of requiring both gastrostomy tubes and invasive ventilation. Individuals with ACTA1-NM were more likely to require feeding tubes and invasive ventilation in the first year of life compared with those with NEB-related NM, but the odds of requiring invasive ventilation were similar after the first year of age. Additional clinical information is presented by genotype and severity class.

Discussion: Neonatal findings of individuals with NM are correlated with long-term clinical outcomes, and there are some relationships between genotype and disease severity. Prospective longitudinal studies are needed to confirm these findings and evaluate for additional early clinical predictors of prognosis.

Figure 1. Ages at Onset and Diagnosis by Genotype and Clinical Severity Scores

Age at onset and age at diagnosis by genotype (A) and 2000 NM Classification Scheme (B). The vertical lines represent the interquartile range, and the horizontal line represents the median. (A) The median age at onset was 0.01 years for patients with NEB-NM (n = 95, IQR 0.01–0.5), ACTA1-NM (n = 77, IQR 0.01–0.07), TPM2-NM (n = 9, IQR 0.01–2.7), and TPM3-NM (n = 17, IQR 0.01–0.28). The median age at diagnosis for patients with NEB-NM was 2.0 years (n = 92, IQR 0.42–5.0), ACTA1-NM was 0.35 years (n = 74, IQR 0.1–3.5), TPM2-NM was 13.0 years (n = 7, IQR 2.5–50.0), and TPM3-NM was 1.3 years (n = 17, IQR 0.75–5.7). (B) The median age at onset was 0.01 years for patients with severe NM (n = 60, IQR 0.01–0.01), intermediate NM (n = 58, IQR 0.01–0.01), and typical/congenital NM (n = 112, IQR 0.01–0.25); 3.0 years for those with mild/juvenile NM (n = 22, IQR 2.0–4.25); and 46.0 years for those with adult-onset NM (n = 12, IQR 42–58). The median age at diagnosis for patients with severe NM was 0.08 years (n = 59, IQR 0.04–0.2), intermediate NM was 0.6 years (n = 57, IQR 0.76–8.75), typical/congenital NM was 2.6 years (n = 104, IQR 0.76–8.75), mild/juvenile NM was 11.0 years (n = 23, IQR 4.0–39.0), and adult-onset NM was 56.0 years (n = 12, IQR 45–62.8). IQR = interquartile range.

Figure 2. Survival and Time-to-Event Analyses of Censored Data by Diagnosis and Clinical Findings

(A) Survival of the overall cohort of patients with available information (dark line, n = 263), compared with survival of the general US population (gray line). Dashed lines represent 96% CI. Of survivors past the age of 60, 1 died of complications related to thyroid cancer and another passed in their sleep because of unknown causes. (B) Survival by genotype: no molecular cause identified (solid dark line, n = 50), NEB (dashed dark line n = 95), ACTA1 (solid gray line, n = 76), and TPM3 (dashed gray line, n = 18), among patients with available information. Of 15 patients with TPM3-NM still alive at last known follow-up, age at last follow-up ranged from 1 year to 36 years with an average of 15.6 years. Pairwise comparisons (using Benjamini-Hochberg correction to adjust for multiple testing) showed that the group without molecular diagnosis (18 of total 50 patients died) and those with NEB-NM (13 of 95 patients died) showed a difference in survival, with the NEB-NM group having higher rates of survival (p = 0.0076). (C) Survival by degree of respiratory assistance before 1 year of age: no respiratory assistance (solid dark line, n = 134), invasive respiratory assistance (dashed line, n = 51), and continuous positive airway pressure or bilevel positive airway pressure (dashed/dotted line, n = 11), among patients with available information. This analysis was based on 196 patients (5 excluded because of missing last follow-up/death time). (D) Survival by finding of respiratory failure at birth: no respiratory failure (dark line, n = 209) and presence of respiratory failure (gray line, n = 54), among patients with available information. Two hundred sixty-three patients were included in analyses (5 with missing information and 4 terminated or stillborn patients were excluded). (E) Time to complete or partial loss of ambulation of the cohort, in patients with available information (n = 212). Additional 53 patients had missing age at loss of ambulation or last follow-up and were excluded from analyses. A total of 153 patients were included in the analyses. Dashed lines represent 96% CI. These analyses omit 3 cases of TNNT1-NM.

Figure 3. Odds Ratio Analyses

Odds ratios, modeled using the Fisher exact test, for relationships between clinical findings and long-term outcomes (A), genotype and clinical findings (B), and birth findings and genotype (C). Outcomes for each reported association are indicated on Y axes. Dots represent odds ratio, and lines represent 95% CI. Because multiple comparisons were conducted, the expected number of significant results by chance was compared with the number observed. With 18 comparisons and alpha set at 0.05, 0.9 comparisons would be expected to reject the null hypothesis (no difference in odds) by chance. The analysis found 7 comparisons whose 95% CIs exceeded the odds ratio of one, rejecting the null hypothesis. GT = gastrostomy tube.

Figure 4. Clinical Severity Scores Broken Down by Clinical Category and Genotype

The Amburgey scoring system related to the 2000 NM Classification Scheme (A) and genotype (B) (NEB-NM n = 80, median = 1.5, mean = 4.65; ACTA1-NM n = 53, median = 0, mean = 2.76; TPM2-NM n = 9, median = 0, mean = 3; TPM3-NM n = 14, median = 1, mean = 3.79; TNNT1-NM n = 3, median = 0, mean = 6.33; KLHL40-NM n = 0; KLHL41-NM n = 2, median = 6.5, mean = 3.5; LMOD3-NM n = 2, median = 3, mean = 3; CFL2-NM n = 2, median = 0, mean = 0). *p = 0.048, ****p < 0.0001, ns = not significant. The blue vertical lines represent the interquartile range, and the horizontal line represents the median. NM = nemaline myopathy.

Figure 5. Concordance Analysis of Alternate NM Classification Schemes

Comparison of the 2000 NM Classification Scheme and the updated Sewry NM Classification Scheme proposed in 2019. The 2 schemes are significantly correlated (p 0.0001). Color legend and numbers in each box indicate the number of overlapping cases for each severity class. NM = nemaline myopathy.

Figure 6. Clinical Presentations at Birth Among Infants Eventually Categorized According to the 2000 NM Classification Scheme

The relationship between birth respiratory distress (A) and birth hypotonia (B) with the 2000 NM Classification Scheme. NM = nemaline myopathy.