Type 1 Gaucher Disease: Significant disease manifestations in “asymptomatic” homozygotes identified by prenatal carrier screening

Introduction

Type 1 Gaucher disease (GD), is the most common lysosomal storage disease and is particularly prevalent in the Ashkenazi Jewish (AJ) population with an estimated frequency of ~1 in 1000.¹ This autosomal recessive inborn error of metabolism results from the deficient activity of the degradative enzyme, acid β-glucosidase (β-Glu), and the lysosomal accumulation of its glycosphingolipid substrate, glucosylceramide (GL-1), primarily in the monocyte-macrophage cells of the liver, spleen, and bone marrow.¹ Clinical manifestations include hepatosplenomegaly, anemia, thrombocytopenia, and significant bone disease.¹ Enzyme replacement therapy (ERT) with a macrophage-targeted recombinant β-Glu has proven to be safe and effective in ameliorating, and even preventing, disease manifestations.^2-5

The carrier frequency of GD in the AJ population has been estimated to be 1 in 14 to 18.^6-8 Four Acid β-Glucosidase (GBA) gene mutations (N370S, 84GG, L444P and IVS2+1) account for ~95% of the disease-causing lesions in AJ patients.^6,9 The most common mutation, N370S, a missense mutation that retains about 14% of wild-type expressed activity in vitro,^10,11 is present in over 70% of the AJ Type 1 GD patients ^12,13 who are either homoallelic or heteroallelic with another GD mutation.^12,13 Based on data from the Gaucher Registry,¹⁴ it is estimated that ~56% of AJ GD patients are N370S homozygotes. The clinical spectrum of N370S homozygotes in the AJ population is highly variable, ranging from severely affected children with hepatosplenomegaly, pancytopenia and bone disease to adults who are asymptomatic until the 8^th decade of life. ^15-17Previous studies have characterized the age at diagnosis and severity of manifestations in diagnosed N370S homozygotes.^12,16,17 Although some N370S homozygotes have early-onset disease, and are treated by ERT in childhood or adolescence, most are diagnosed later.¹⁷ About 32% of N370S homozygotes in the Gaucher Registry were diagnosed by age 20.¹⁷ However, the median age at diagnosis of over 1100 N370S homozygotes in the registry was 29 years,¹⁴ consistent with the slowly progressive course of the disease. In contrast, the median age at diagnosis for over 550 GD patients having the next most frequent N370S/L444P genotype was 16 years.¹⁴

Based on heterozygote frequencies of 1 in 15 to 18 for GD reported by prenatal carrier screening programs for Jewish genetic diseases,^7,18,19 it is estimated that 1 in 900 to 1300 AJ individuals is homozygous for GD. It has been suggested that only one-third of N370S homozygotes come to medical attention and that about two-thirds remain asymptomatic throughout life.^6,20,21 That Type 1 GD is a “low penetrant” disorder, similar to hemochromatosis,^22,23 has led to discussions on the value of including GD in the disease panel for prenatal carrier screening in the AJ population.^{24,25, 26}

Is homozygosity for the N370S genotype truly “low penetrant”²⁴ with the majority of patients remaining asymptomatic throughout life, or will these individuals eventually become symptomatic, and require therapeutic intervention, after significant, and possibly irreversible disease has occurred? Early recognition and characterization of the natural history of N370S homozygotes would provide data for improved genetic counseling, and informed medical care, including the rationale for early therapeutic intervention. To date however, there have been no systematic studies to determine the frequency of AJ N370S homozygotes or to evaluate their clinical manifestations and progression, particularly in those that report no symptoms. Since prenatal carrier screening programs for diseases prevalent in the AJ population serendipitously detect previously undiagnosed GD homozygotes, we analyzed our experience in screening over 8000 individuals who reported that both parents and all grandparents were of AJ descent.

METHODS

RESULTS

DISCUSSION

For over two decades, physician experts have suggested that up to two-thirds of AJ N370S homozygous Type 1 GD patients were asymptomatic throughout life and eluded medical attention.^6,20 This presumption was based on studies that estimated the heterozygote frequency of the common N370S mutation in the AJ population to be ~1 in 14.5 to 17.5 which predicted that ~1 in 840 to 1225 AJ individuals were N370S homozygotes.^12,13 However, experts in North America and Israel noted that the frequency of N370S homozygotes in their clinics was less than expected.^6,12,20 Is homozygosity for N370S “benign’” or “low penetrant”?²⁴ Are “asymptomatic” GD homozygotes essentially “normal” throughout life²¹ or do they have clinical manifestations that progress? Here, we addressed these issues by identifying asymptomatic N370S homozygotes diagnosed by prenatal carrier screening programs and evaluating their GD manifestations.

First, the heterozygote and homozygote frequencies were determined by testing for six common AJ Type 1 GD mutations in over 8000 AJ individuals who sought prenatal genetic carrier screening. The overall heterozygote frequency in this population was 1 in 15.4; the heterozygote frequency for the most common mutation, N370S, was 1 in 16.8, representing 92% of the GD alleles detected. Notably, the predicted homozygote frequency for the common mutations of 1 in 949 was, in fact, essentially that observed, nine previously undiagnosed GD homozygotes were detected or 1 in 897. Of these, eight were N370S homozygotes, an observed frequency (1 in 1009) which also was close to that expected (1 in 1129), based on the carrier frequency (p = 0.94). Thus, our ascertainment of AJ GD homozygotes in this population was complete.

Second, what proportion of GD homozygotes detected by prenatal carrier screening were “symptomatic or asymptomatic”? Of the 37 GD homozygotes detected by our or other prenatal carrier screening programs, 35% (13 of 37 reported symptoms consistent with Type 1 GD, including bruising, bone pain, and fatigue. As might be expected, the two previously undiagnosed N370S/L444P patients, whose genotype typically manifests in childhood,³⁶ were both symptomatic reporting easy bruisability, had significant disease manifestations (Table 2) and were referred for enzyme replacement therapy. Notably, 65% (24 of 37) were asymptomatic for GD on review of systems, consistent with previous expert predictions of the proportion of asymptomatic patients.^6,20,21 However, clinical, laboratory, and/or imaging studies of all 37 patients revealed GD manifestations ranging from mild to relatively severe at a mean age of 30 years, with the youngest, and mildest, a 17 year old N370S/N370S female who had only marrow infiltration (Table 2). Even the three patients identified with the mildest N370S/R496H genotype had disease manifestations.

Third, what are the manifestations in these previously undiagnosed homozygotes? All 37 patients had bone marrow infiltration which is evidence of GD and is the precursor of future irreversible bone involvement. Notably, of the 29 patients who had imaging studies, 43% (10 of 23) had the Erlenmeyer flask deformity, 14% (4 of 28) had a bone infarction, and 22% (5 of 23) had bone lucencies on imaging. Among the others, the manifestations ranged from splenomegaly, thrombocytopenia, anemia, marrow infiltration, bone infarctions, osteoporosis and pathologic fractures.

In post-menopausal women, osteopenia is a well-established risk factor for fracture, particularly of the hip.³⁷ However, the relationship between osteopenia and fracture risk in young premenopausal women and in men has not been defined. The guidelines for treatment of osteopenia/osteoporosis in post-menopausal women with low BMD values are not applicable to pre-menopausal osteopenic women.³⁸ Moreover, there are limited data on the use of standard pharmacologic therapy (e.g., bisphosphonates) in women who are planning a pregnancy. These findings highlight the need to recognize and closely monitor young GD patients with low bone mass. Bone density should be monitored annually and supplementation with calcium and Vitamin D for GD patients who are osteopenic is recommended. Early detection of disease manifestations in these patients will facilitate monitoring and therapeutic intervention prior to the development of irreversible complications.

Of the 29 GD homozygotes who had complete GD evaluations, imaging studies revealed that 97% (28 of 29) had splenomegaly and 49% (18 of 37) had abnormal hematologic values. Two women needed treatment during pregnancy due to progressive thrombocytopenia. There was no correlation between the hematologic and/or visceral involvement and the presence or severity of bone manifestations among AJ Type 1 GD patients, as previously noted.³⁹ In fact, of our 19 patients with normal blood counts, 79% (15 of19) had imaging studies which showed varying degrees of bone involvement including two (13%) patients with bone infarcts and five (42%) with osteopenia/osteoporosis based on DEXA.

Finally, do they remain mostly “asymptomatic” throughout life or do they become the patients who are diagnosed and treated later in life as the disease progresses? Of this cohort, 21 patients continue to be followed by our center and have been evaluated annually or biannually. The oldest patient in our group was 50 years old at her last evaluation, which showed no evidence of disease progression since her initial evaluation 10 years earlier revealed mild splenomegaly and minimal bone involvement. Her plasma chitotriosidase activity was low at 564 nmol/hr/ml. Contrary to the variable progression observed over years in the other N370S/N370S homozygotes, this patient’s experience suggests that a small subset of N370S homozygotes may remain stable for years with minimal disease progression. In contrast, enzyme replacement therapy has been recommended for 13 (42%) of the N370S homozygotes who had significant disease at diagnosis or evidence of disease progression over a 9 year observation period. These results indicate that this disease genotype is not low penetrant, but manifests in adulthood and is often progressive. In fact, the later-onset of manifestations in N370S homozygotes may contribute to its diagnostic delay which may be associated with irreversible complications.⁴⁰

Our findings are in contrast to those reported by Azuri et al. who suggested asymptomatic/mild patients do not require frequent monitoring.⁴¹ The fact that all 31 N370S homozygotes detected by carrier screening had GD manifestations at mid-life solves the perception about the genotype being benign. Clearly, these patients should be identified as early as possible, a likelihood if future genome (exome) sequencing becomes commonplace. A comprehensive baseline evaluation based on GD consensus recommendations should be performed to assess disease involvement for these homozygotes on an annual or biannual basis.²⁷ Based on our experience; at least 40% will become candidates for enzyme replacement or future therapies.^42,43

In summary, the evaluation of AJ GD homozygotes identified serendipitously by prenatal carrier screening revealed that these young to middle aged homozygotes actually had disease manifestations, particularly bone involvement that progressed with age in many homozygotes, often requiring therapeutic intervention. Our findings indicate that homozygosity for the common N370S mutation does not result in a benign or low penetrant disease, and emphasizes the importance of early recognition and appropriate management to minimize or prevent future irreversible disease complications.